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Thesis for master degree (MD)
2010-09-13
Impact of vehicle exhaust emitted by the combustion of
biofuels on human health
by
Luiza Panosyan
A thesis submitted in partial fulfilment of the requirements for the degree
of
Master Sciences
in
Environmental Science
under the supervision of
Professor Lars-Gunnar Franzén
Final Version
Halmstad University
Halmstad 2010
- - 2
Abstract of the Thesis
INTRODUCTION: Significant changes in the global ecosystem, together with a potential shortfall in
oil resources, have stimulated intense interest in the development of other sources of energy, and
most particularly biofuels since these are basically considered to be less harmful to human health
than petroleum-based fuels. However, information about the impact of biofuel-derived vehicle
emissions on human health is limited and incomplete.
AIM OF THE STUDY: To identify those biofuels that are less detrimental to human health on the basis
of published results from toxicological and chemical studies of vehicle emission products.
TASKS OF THE STUDY: To review systematically all conventional and alternative fuels used in
internal combustion engines, to identify all known toxic emission products formed by such fuels, to
review their toxic effects on human health, and to analyse the data collected in order to develop
conclusions concerning the possible health benefits deriving from the use of alternative fuels.
MATERIALS AND METHODS: In order to fulfil the requirements of a complete, comprehensive and up-
to-date review of the toxic effects of automotive exhaust, an extensive search of official scientific
data sources has been performed. Relevant publications were retrieved from public domain
databases with a toxicological focus such as Toxcenter and CAplus, as well as from the websites of
the US Environmental Protection Agency and the US Agency for Toxic Substances and Disease
Registry. Keywords employed in the literature search were: petrol, gasoline, diesel exhaust,
emission, biofuel, biogas, biodiesel, bioethanol, bioalcohol, toxicity, methanol and ethanol. A total
of 295 references were initially selected relating to the period 1962 to 2008, and 142 of these
presented titles and abstracts that met the main inclusion criteria, i.e. describing toxicological and
epidemiological studies in humans. In cases where eligible studies relating to the goals and tasks of
the review were limited or not available, some in vitro or in vivo toxicological studies involving
animal models were included.
RESULTS: In comparison with petroleum diesel, the emissions derived from biodiesel contain less
particulate matter, carbon monoxide, total hydrocarbons and other toxic compounds including
vapour-phase C1-C12 hydrocarbons, aldehydes and ketones (up to C8), selected semi-volatile and
particle-phase polycyclic aromatic hydrocarbons (PAHs). Whilst sulphur-containing compounds
appear to be undetectable in biodiesel, nitrogen oxide and a soluble organic fraction comprising
unregulated pollutants including the ―aggregated toxics‖ (i.e., formaldehyde, acetaldehyde, acrolein,
benzene, 1,3-butadiene, ethylbenzene, n-hexane, naphthalene, styrene, toluene and xylene) are
present at elevated levels. Toxicological studies have shown that the mutagenicity of exhaust
particles from biodiesel is normally lower than those obtained from petroleum diesel, however,
rapeseed oil-derived biodiesel exhibits toxic effects that are 4-fold greater than petroleum diesel.
Such enhanced toxicity is probably caused by the presence of carbonyl compounds and unburnt
fuel. The toxicity of highly volatile components of biofuel exhaust has not yet been evaluated
accurately. A substantial portion of these compounds was apparently lost in the process of preparing
the test samples used for the assays (during the evaporation). The overall recoveries of these
compounds have not been evaluated and the accuracy of the sample preparation method has not
been validated. Hence, it could be that the cytotoxic effect of biodiesel exhaust is higher than that
- - 3
reported. Moreover, compared with fossil diesel, fuel derived from rapeseed oil emits particulate
matter with increased mutagenic effects. Epidemiological investigations of the effects of biofuels on
humans are very sparse but have revealed dose-dependent respiratory symptoms following exposure
to rapeseed oil biodiesel, although the observed differences between this fuel and petroleum diesel
are not significant. Such data, however, give rise to serious concerns about the future usage of this
plant material as a replacement for established diesel fuels. Combustion of alcohol-based fuels leads
to a reduced formation of photochemical smog in comparison with gasoline or diesel, however, the
emission of aldehydes (officially classified as carcinogenic or potentially carcinogenic) is several
times higher. The toxicity of the exhaust emissions of gasoline-fuelled engines is generally
significantly greater than that of alcohol-burning engines. However, some harmful effects from
ethanol blends might be expected, such as enhanced emissions of carcinogenic PAHs and increased
ozone-related toxicity associated with the high level of aldehydes emitted. The use of ethanol–diesel
fuel blends gives rise to increases in regulated exhaust emissions and, possibly, to greater emissions
of aldehydes and unburnt hydrocarbons. The most promising fuels, in terms of reduced toxicity and
genotoxicity of exhaust emissions, are methanol-containing blends. However, the emission from
these fuels still contains formaldehyde, which is a carcinogen. The use of biogas can significantly
reduce emissions of total PAHs and formaldehyde and, consequently, the risk of lung toxicity. On
the other hand, the emissions of particulate matter by compressed natural gas, and the mutagenic
potencies of the exhaust, are similar to those associated with gasoline and diesel fuels.
CONCLUSIONS: The use of biofuel is currently viewed very favourably and there are suggestions that
the exhaust emissions from such fuel are less likely to present risks to human health in comparison
with gasoline and diesel emissions. However, the expectation of a reduction in health effects based
on the chemical composition of biodiesel exhaust is far from reality. Thus, although toxicological
evidence relating to the effects of biofuels on humans is sparse, it is already apparent that emissions
from the combustion of biofuel and blends thereof with petroleum-based fuels are toxic. In addition
to the regulated toxic compounds, such as total hydrocarbons, carbon monoxide, nitrogen oxides,
particulate matter and polycyclic aromatic hydrocarbons, biofuel emissions contain significant
amounts of various other harmful substances that are not regulated, e.g. carbonyls (including
formaldehyde, acetaldehyde, benzene, 3-butadiene, acrolein, etc.). Whilst biofuels may be
potentially less damaging to human health than petroleum fuels, considerable harmful effects must
still be expected. Substitution of conventional fuel by biofuel decreases the concentration of
regulated toxic pollutants in vehicle exhaust, but increases the concentration of some unregulated
toxic pollutants emitted from on-road engines. Generally, the toxicity of biofuels decreases in the
order biodiesel>biogas>ethanol>=methanol. In this respect, methanol produced by the oxidation of
biogas appears to represent an alternative fuel that exhibits the least potential for damage to human
health, however, this alcohol represents a source of formaldehyde pollution and is carcinogenic. .
- 4 -
Table of Contents
Abstract of the thesis 2
Table of contents 4
1. Abbreviations 6
2. Introduction 7
2.1. Background 7
2.1.1. Definition of the problem 7
2.1.2. Research questions of the study 7
2.2. Automotive fuels 7
2.2.1. Conventional fuels 7
2.2.2. Alternative fuels 8
2.2.2.1. Vegetable oils 9
2.2.2.2. Biodiesel 10
2.2.2.3. Bioalcohols 10
2.2.2.4. Biogas 11
3. Aim of the study 12
4. Tasks of the study 12
5. Materials 12
5.1. Research strategy 12
5.2. Literature sources 13
6. Methods 14
6.1. Inclusion and exclusion criteria 14 6.2. Study selection 14
6.3. Data abstraction 15
6.4 Data synthesis and analysis 15
7. Results 15
7.1. Toxic compounds emitted from the exhaust systems of motor vehicles 16
7.1.1. Regulated pollutants 17
7.1.1.1. Particulates 19
Total hydrocarbons 20
Polyaromatic hydrocarbons 21
7.1.1.2. Carbon monoxide 22
7.1.1.3. Nitrogen oxides 22
7.1.1.4. Sulphur oxides 23
7.1.1.5. Heavy metals 23
7.1.1.6. Ozone 24
7.1.2. Unregulated pollutants 24
7.1.2.1. Carbonyl compounds 24
Formaldehyde 24
Acetaldehyde 25
Acrolein 25
7.1.2.2. Other oxygenated hydrocarbons in exhaust emissions 26
Methanol 26
Ethanol 26
Formic acid 26
7.1.2.3. Hydrocarbons in exhaust emissions 27
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Benzene. 27
1,3-Butadiene. 27
7.2. Acute and chronic adverse health effects of vehicle exhaust produced by
the combustion of petrofuels, gasoline and diesel 27
7.2.1. Acute and chronic adverse health effects 31
7.2.2. Carcinogenic effects 31
7.2.3. Mutagenic effects 32
7.3. Chemical composition and toxic effects of vehicle exhaust produced by the
combustion of biofuels 32
7.3.1. Biodiesel and vegetable oils 32
7.3.1.1. Chemical composition of the emissions from biodiesel engines 32
7.3.1.2. Biodiesel exhaust emissions and human health 37
7.3.2. Bioalcohol 41
7.3.2.1. Composition of bioalcohol exhaust. 41
7.3.2.2. Toxicological studies of exhausts from bioethanol and alcohol-
blended fuels. 45
Ethanol-based blends 45
Methanol-based blends 47
7.3.3. Biogas 48
7.3.4. Other biofuels 49
8. Discussion and overall conclusions 50
9. Acknowledgements 52
10. References 52
- 6 -
1. Abbreviations
ALA-D – 9-aminolevulinic acid dehydratase
ANOVA – analysis of variance
ATSDR - US Agency for Toxic Substances and Disease Registry
BaPeq – benzo(a)pyrene equivalents
BFC – brown-fat-cell
CNG - compressed natural gas
CNS – central nervous system
DEP - diesel exhaust particles
DPM - diesel particulate matter
GTL- gas-to-liquid
GWP - global warming potential
HSDB – Hazardous Substances Data Bank
IARC – International Agency for Research on Cancer
LFG - lead-free gasoline
LPG - liquefied petroleum gas
LS-PDD – low sulphur content petroleum-derived diesel
MCA – methanol-containing additive
NOx – nitrogen oxides
PAC - polynuclear aromatic compounds
PAH - polyaromatic (polycyclic) hydrocarbons
PAN - peroxyacyl nitrates
PDD - petroleum-derived diesel
PLG - premium leaded gasoline
PM - particulate matter
RFG - reformulated gasoline
RME - rapeseed oil methyl esters
RSO - rapeseed oil
SME - soybean oil methyl esters
SOx – sulphur oxides
TCDD – 2,3,7,8-tetrachlorodibenzo-p-doxin
US EPA - US Environmental Protection Agency
WHO – World Health Organization
- 7 -
2. Introduction
2.1. Background
2.1.1. Definition of the problem
There are several significant problems associated with the current interest in biofuels:
Problem 1. Significant changes in the global ecosystem first became apparent in the 20th
century
and are mainly related to the enormous impact of the petrochemical industry on the ecological
balance.
Problem 2. Oil resources are limited and the depletion mid-point (the date at which roughly
half of anticipated global oil resources have been exploited) will be reached within the next 10
to 20 years). Possible solutions to the future potential energy shortfall involve the greater use of
biofuels together with wind, solar, tidal, geothermal and hydro-power, and other renewable
energy sources. If alternative energy sources are not exploited effectively, it is likely that the
price of crude oil will increase exponentially with a doubling time of 11 years (Munack and
Krahl, 2007).
Problem 3. Permanent dependence on supplies of oil and gas from countries such as Iran, Iraq,
Russia, Venezuela, Libya etc could generate shifts in the balance of world power that may be
unacceptable to the main energy consumers.
The benefits that could derive from the use of biofuels include:
reduction in greenhouse gas emissions,
reduction in the amounts of fossil fuel consumed,
increased rural development,
decreased dependence on politically sensitive regions for supplies of energy,
a sustainable fuel supply for the future.
Although such ideas currently dominate public thinking, the hard information available concerning
the impact of biofuel-derived vehicle emissions on human health is limited and incomplete.
2.1.2. Research questions of the study
What is the impact of biofuel-derived vehicle emissions on human health?
It is generally believed that biologically-derived products are less harmful to the human organism
than industrially-generated chemicals. Similarly, biofuels are believed to be less toxic than gasoline
or diesel and, therefore, biofuels derived vehicle emissions should considered to be less damaging
than those derived from petroleum. The validity of this generally-held belief still remains open to
question, however, and a systematic assessment of the comparative safety/toxicity of biofuel-
derived emission is urgently required.
2.2. Automotive fuels
2.2.1. Conventional fuels
Currently, the main source of fuel for road vehicles is petroleum oil. Oil is a naturally occurring,
fossil liquid comprising a complex mixture of hydrocarbons of various molecular weights together
- 8 -
with various other organic compounds. These components are separated by fractional distillation in
an oil refinery in order to produce gasoline, diesel fuel, kerosene and other products, namely:
hydrocarbons containing from 5 to13 carbon atoms are refined into gasoline (petrol),
hydrocarbons with 9 to 16 carbons are processed to yield diesel fuel and kerosene (the
primary component of many types of jet fuel),
hydrocarbons with 17 to 25 carbons form the basis of fuel oil and lubricating oil,
higher molecular weight hydrocarbons (typically with 35 or more carbons) can either be
used to produce asphalt or may be cracked in modern refineries to provide more valuable
products,
hydrocarbons with less than 5 carbon atoms are normally gaseous and are processed as
natural gas (Speight, 2006).
Gasoline, or petroleum spirit (petrol), is a complex mixture consisting of mainly aliphatic
hydrocarbons [C5-C13 n-alkanes (17.7%) and C4-C13 branched alkanes (32%), C6-C8 cycloalkanes
(5%), C6 olefins (1.8%), aromatics (benzene, toluene, xylene, ethylbenzene, C3-benzenes, C4-
benzenes (30.5%)] and other possible components such as octane enhancers (MTBE, TBA, ethanol,
methanol), antioxidants, metal deactivators, ignition controllers, icing inhibitors, detergents and
corrosion inhibitors (Harper and Liccione, 1995).
Diesel, or diesel fuel, refers in general to any fuel that can be used in a diesel engine. Petroleum-
derived diesel (PDD) is composed of ca. 64-75% saturated hydrocarbons (primarily paraffins
including n-, iso-, and cyclo-paraffins), l-2% olefinic hydrocarbons and 25-35% aromatic
hydrocarbons (including naphthalenes and alkylbenzenes) (Agency for Toxic Substances and
Disease Registry, 1995; Briker et al., 2001). Although the average chemical formula for common
diesel fuel is C12H23, components range from approximately C10H20 to C15H28.
2.2.2. Alternative fuels
Biofuels are defined as solid, liquid, or gaseous fuels derived mainly from recently dead biological
materials. This distinguishes biofuel from fossil fuel, which is derived from long dead biological
material. Biofuel can theoretically be produced from any biological carbon source, however, the
most common sources of biofuels are photosynthetic plants and plant-derived materials. There are
three main strategies to obtain biofuels (Olsson and Ahring, 2007; Worldwatch Institute, 2007;
Mousdale, 2008):
to grow plants that naturally produce oils, such as corn, switchgrass and soya (in the United
States), rapeseed, wheat and sugar beet (in Europe), sugar cane (in Brazil), palm oil and
Miscanthus (in South-East Asia), sorghum and cassava (in China), and Jatropha (in India).
Since the viscosities of these oils diminish with heating they can be burnt directly in a diesel
engine, although the oils can be treated chemically in order to produce biodiesel,
to grow crops containing high levels of sugar (sugar cane, sugar beet and sweet sorghum) or
starch (corn/maize) and subsequently ferment the biomass with yeast in order to produce
bioalcohols (mainly ethanol),
to harvest biomass and submit it to anaerobic digestion in order to produce biogas (mainly
methane). Such biomass can, in fact, derive from almost any form of waste organic material.
The world leaders in the development and use of biofuels are, at the time of writing, Brazil, USA,
France, Sweden and Germany.
- 9 -
First generation biofuels refers to biofuels made from:
seeds that are pressed to yield vegetable oil for use as biodiesel,
grains that yield starch for fermentation into bioethanol,
natural waste materials for anaerobic degradation to biogas.
The most common first generation biofuels are listed below.
2.2.2.1. Vegetable oils. The use of a plant oil as a fuel for a compression ignition (diesel) engine
dates back to the very early years of the invention of the devise itself. Indeed the first engine of this
type presented by Diesel at the world exhibition in Paris in the year 1900 was fuelled by peanut oil.
Vegetable oil alone is, however, too viscous for continuous use in the diesel engine (Knothe, 2005),
although it can be used in many older diesel engines (equipped with indirect injection systems) but
only in warm climates.
The composition of vegetable oil is quite different from that of petrol-derived fuels, the major
constituents being lipid triglycerides (triacylglycerols). The triglycerides in native fats or oils
comprise a glycerol backbone, the –OH groups of which are esterified with palmitic acid, oleic acid,
α-linolenic acid, etc (Kates, 1972). The triglycerides of sunflower oil comprise predominantly
linoleic acid (68%) together with palmitic, stearic and oleic acids (Table 1) (Somerville, 1993;
Krahl, 2009). It should be noted, however, that there can be significant variation, strongly
influenced by both genetics and climate, in the unsaturated fatty acid profile of the oil. The
vegetable oil also contains minor amounts of lecithin, tocopherols, waxes (long chain alkanes,
alcohols and their esters), carotenoids, phytosterols (e.g. stigmasterol, β-sitosterol, campesterol etc),
acyl-phytosterols, diacylglycerols, monoacylglycerols, glycolipids, glyceroglycolipids, free fatty
acids and other minor non-polar lipid fractions (Kates, 1972).
Table 1. Main fatty acids present in the triglycerides of vegetable oils used as biofuels.
(Table from Krahl, 2009).
a Very small amounts of 20:2 (0.08%), 22:0 (0.34%) and 22:1 (0.29%) are also present (Krahl,
2009)
Vegetable
oil
Fatty acid composition (number of carbon atoms; number of double bonds)
6:0 8:0 10:0 12:0 14:0 16:0 18:0 20:0 16:1 18:1 20:1 18:2 18:3
Rape seed a 0.06 4.62 1.7 0.6 0.25 61 1.3 20.4 9.2
Soybean 11 4 34 54 7
Corn 11 2 28 58 1
Cotton seed 1 22 3 1 19 54 1
Palm 1 45 4 40 10
Peanut 11 2 1 1 48 2 32
Olive 13 3 1 1 71 10 1
Canola 4 2 62 22 10
Sunflower 7 5 19 68 1
Coconut 1 8 6 47 18 9 3 6 2
Palm kernel 3 4 48 16 8 3 15 2
Cocoa 26 34 1 34 3
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For all of the vegetable oils mentioned, each lipid fraction comprises several homologues
containing different numbers of C=C bonds (usually from 0 to 3) separated by methylene (CH2)
groups. In the presence of high temperatures or light, these moieties readily oxidize to yield
hydroperoxides which, in turn, are transformed into a plethora of autoxidation products that are
potentially toxic to living organisms (Kates, 1972).
2.2.2.2. Biodiesel. Combustion of crude vegetable oils causes severe damage to normal diesel
engines, but such oils can be used to manufacture biodiesel that is compatible with most diesel
engines when blended with conventional diesel fuel. Currently biodiesel and PDD blends are used
in unmodified diesel-engined vehicles. Typically, biodiesel comprises fatty acid methyl or ethyl
esters produced by trans-esterification of vegetable oil (Figure 1).
Figure 1. Transformation of triglycerides of vegetable oil into biodiesel and glycerol.
In this process the plant oil is reacted with an alcohol, generally methanol, in the presence of a
catalyst such as potassium hydroxide to produce the corresponding ester and glycerol (Schuchardt
et al., 1998). Methyl esters are less dense than glycerol and can be readily separated from the latter
on the basis of differential density. The glycerol by-product is removed from the bottom of the
reaction vessel and can be further refined for use in the pharmaceutical or cosmetic industries. Feed
stocks for the production of biodiesel include animal fats, algae and various vegetable oils such as
soy, mustard, flax, sunflower, palm, hemp, field pennycress, Jatropha and Madhuca.
2.2.2.3. Bioalcohols. Alcohol fuels can be obtained from petroleum-based or biological sources,
although when they are derived entirely from the latter they are referred to as bioalcohols (e.g.
bioethanol). Methanol, ethanol, propanol and butanol (alcohols with the general formula CnH2n+1OH
where n = 1, 2, 3 or 4) are of interest as fuels because they can be synthesised biologically and they
have characteristics that allow them to be used in unmodified engines. Bioalcohols are produced by
the fermentation of sugars or starch present in wheat, corn, sugar beet, sugar cane, molasses or, in
fact, any carbohydrate-containing material (such as potato and fruit waste, etc) from which
alcoholic beverages can be formed. Typically, the production of ethanol employs enzymatic
digestion in order to release sugars from stored starches, followed by fermentation of the sugars,
distillation and drying (Worldwatch Institute, 2007; Mousdale, 2008). The presence of trace
amounts of many other fermentation products including alcohols with 3 to 12 carbon atoms as well
as numerous fusel oils (ethyl esters of short chain fatty acids; Table 2) varies appreciably depending
on the feedstock employed as well as different processing factors.
BIODIESEL
VEGETABLE OIL COOCH3
COOCH3
COOCH3CH2OH
CH2OH
CHOH+
CH3OH KOH
- 11 -
glucose pyruvate acetaldehyde ethanol
Table 2. Composition of volatile compounds found in the drinking ethanol. (Utsunomiya et al.,
2010)
Component Component
Methanol Diethyl Succinate
1-Propanol Ethyl-9-decenoate
Isobutanol Ethyl Undecanoate (C11Ethyl)
Isoamylalcohol 1-Decanol
Ethyl acetate Phenylethyl acetate
Isoamyl acetate Ethyl Dodecanoate (C12Ethyl)
Ethyl Hexanoate (C6Ethyl) 2-Phenylethanol
Ethyl Octanoate (C8Ethyl), 1-Dodecanol
Isoamyl Hexanoate (C6Isoamyl) Ethyl Myristate (C14Ethyl)
Furfural Octanoic acid (C8)
Ethyl Nonanoate (C9Ethyl) 1-Tetradecanol
1-Octanol Ethyl Palmitate (C16 Ethyl)
Ethyl Decanoate (C10Ethyl) Ehtanol
The combustion of ethanol follows the following equation: C2H5OH + 3O2 → 2CO2 + 3H2O + heat.
Ethanol can be used in petrol engines as a replacement for gasoline, and can also be mixed with
gasoline in a range of percentages. Most existing automotive petrol engines can run on blends, E85
(15% gasoline, 85% ethanol) is the highest ethanol/gasoline blend that existing flex-fuel vehicles in
the United States can use today [Ginnebaugh et al., 2010].
Methanol is currently produced from natural gas, but it can also be produced from biomass as
biomethanol. Methanol has been proposed as a future biofuel since, compared with ethanol, it has a
primary advantage in that it provides very high ―well-to-wheel‖ efficiency when produced from
synthetic gas. The combustion of methanol follows the equation: 2CH3OH + 3O2 → 2CO2 + 4H2O
+ heat. However, both propanol and butanol are considerably less toxic and less volatile than
methanol. Butanol, for example, has a high flashpoint of 35°C, which is a benefit for fire safety but
may present a difficulty for starting engines in cold weather.
2.2.2.4. Biogas. Biogas is the gas resulting from the breakdown of organic matter by
microorganisms in the absence of oxygen. Biogas produced by anaerobic fermentation of manure,
sewage, municipal waste or energy crops normally contains methane (50-75%) and CO2 (25-50%),
with nitrogen (0.1-10%) as the principal minor constituent. Varying amounts of hydrogen, hydrogen
sulphide, oxygen, ethane, propane, iso-butane, n-butane, and other hydrocarbons can be detected as
is shown in the chromatogram displayed in Figure 2 (Schumacher and Thompson, 2005).
Polysaccharide
- 12 -
Figure 2. Gas chromatogram of natural gas. Each of the peaks represents one (or possibly more)
compound, and the intensity of the peak provides a rough indication of the relative amount. (Figure
taken from Schumacher and Thompson, 2005)
3. Aim of the study
The aim of the present study was to determine which biofuels are less harmful to humans by
comparing the published results of chemical studies and toxicological investigations of the emission
products of road vehicles.
4. Tasks of the study
Task 1 was to review all conventional and alternative fuels presently in use in internal
combustion-engined vehicles together with those likely to be used in the future (see section
2.2),
Task 2 was to identify all known toxic compounds present in fuel emission,
Task 3 was to review the toxic effects of such compounds on human health,
Task 4 was to analyse the collected data in order to draw conclusions concerning the
possible health benefits of using alternative fuels.
5. Materials
5.1. Research strategy
The following steps were undertaken in order to perform the thorough search of the scientific
literature required to meet the stated objective:
Step 1. An initial literature survey was conducted in order to understand:
the background of the problem – why conventional fuels need to be replaced by alternative
fuels,
the chemical nature of fossil-derived oils and alternative biofuels,
the chemical nature of compounds generated and emitted into the atmosphere as a result of
the combustion of fuels in the automobile engine.
Dete
cto
r
resp
on
se
- 13 -
Step 2. A literature search was performed in order to define the nature of the harmful effects
generated by the principal toxic substances identified in vehicle exhausts.
Step 3. A review of the findings of toxicological and epidemiological studies on the effects of
vehicle exhausts on human health was conducted in order to establish possible differences in
harmful effects between petroleum-derived fuels and biofuels, and among the different types of
biofuels.
The bibliographies of the articles highlighted by the initial computer-assisted search were studied in
detail and any relevant studies cited were considered for inclusion in the data base assembled for the
present review.
5.2. Literature sources
In the introductory step 1, Google and the Google book engine, e.g.
http://books.google.com/books?cd=1&q=&btnG=Search+Books, websites were surveyed in order
to gain a rapid understanding of the current classification of fuels, their chemical compositions, and
the recent trends in the development of new energy sources. The keywords used were: petrol,
gasoline, biofuel, biogas, biodiesel, bioethanol and bioalcohol. As a result, a number of books on
chemistry, analysis and technology of fuels, oils and biofuels were used (Speight, 2006; Kates,
1972; Yinon, 2004, 2008; Olsson and Ahring, 2007; Worldwatch Institute, 2007) together with
various articles concerning the chemical composition of fuels that were located on the internet using
the keywords: fuel oil, chemical composition, GC analysis, diesel and gasoline (Agency for Toxic
Substances and Disease Registry – ATSDR, 1995a; Schumacher and Thompson, 2005).
In order to fulfil the requirements of a complete and comprehensive up-to-date review of the toxic
effects of automotive exhaust, an extensive search of official scientific data sources was performed.
Relevant publications were retrieved from public domain data bases with a toxicological focus,
namely, Medline, Toxcenter (http://toxnet.nlm.nih.gov/) and CAplus
(http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed; http://stneasy.fiz-
karlsruhe.de/html/english/login1.html). The keywords used in this stage of the literature search
were: petrol, gasoline, diesel exhaust, emission, biofuel, biogas, biodiesel, bioethanol,
bioalcohol, toxicity, methanol and ethanol.
The website of the US Environmental Protection Agency (US EPA)
(http://www.epa.gov/epahome/quickfinder.htm) provided facile access to relevant information and
official summaries on toxicity and regulatory issues associated with toxic compounds. Thus the
health effects of formaldehyde (one of the major toxins of biogas and biomethanol) can be found at
this URL, whilst complete information on its health assessment and a comprehensive review of
chronic toxicity data can be viewed at http://www.epa.gov/iris/subst/0419.htm. A further sources of
information concerning the toxicity of auto-exhaust compounds and their carcinogenic activity are
the websites of the US Agency for Toxic Substances and Disease Registry (ATSDR;
http://www.atsdr.cdc.gov/toxprofiles/index.asp; http://www.atsdr.cdc.gov/substances/index.html)
and International Agency for Research on Cancer (IARC) - http://monographs.iarc.fr/index.php
where all monographs released by WHO expert committees summarising the results of evaluation
carcinogenic activity of substances are described in details. The US Department of Health and
Human Service Report on the Toxicological Profile of Polyaromatic Hydrocarbons published in
- 14 -
1995 was accessed at http://www.atsdr.cdc.gov/toxprofiles/tp69.pdf (Agency for Toxic Substances
and Disease Registry, 1995b).
The key words employed in these searches were: methanol, ethanol, formaldehyde, carbon
monoxide, acetaldehyde, acrolein and polyaromatic hydrocarbons.
6. Methods
6.1. Inclusion and exclusion criteria
The only articles included in the present review were those describing: (i) the results of
toxicological studies of biofuel and petrofuel emissions, (ii) health effects of toxic compounds
found in biofuel and petrofuel emissions, (iii) the content of potentially toxic compounds in
petrofuel and biofuel emissions. Numerous publications covering many other issues related to
petrofulel emission and air pollution were excluded. Articles in Chinese, Russian and any other
languages except of English were excluded.
6.2. Study selection
A total of 218 articles published between 1962 and 2009 were initially selected from the Medline
database, although a number of these were duplicated in other databases. A more specific search in
the Toxcenter database using the keywords: biofuel, exhaust emission, biogas and bioethanol
produced 25 references published between 1984 and 2008. A search of the CAPlus database using
the keywords: biofuel, exhaust emission, biogas and bioethanol yielded 42 references published
between 1975 and 2008, whilst a similar search in the Hazardous Substances Data Bank (HSDB)
database (http://toxnet.nlm.nih.gov/cgi-bin/sis/search) gave 12 references dated between 2002 and
2008.
Flow chart of the study selection process
Total number of citations identified through database searching: n=297
Additional references selected for the reviewing: n=16
Total number of references included
in the review, n=146
Number of articles assessed for eligibility: n=113, including:
articles selected for the review of health effects of individual toxic compounds emitted
from petrofuels (sections 7.1 and 7.2) : n = 46, including full text articles - 27
articles selected for the systematic review of biofuels (section 7.3) : n=67, including full
text articles - 27
Citations excluded after screening of titles and abstracts as
well as after removal duplicates: n = 167
- 15 -
In order to ensure the validity of the information provided in the articles selected, only official
reports released by governmental regulatory organisations and papers published in peer-reviewed
journals were considered. Of the references selected, 142 presented titles and abstracts that met the
main inclusion criteria, i.e. toxicity and epidemiological studies in humans, etc. In cases where
eligible studies relating to the goals and tasks of the review were limited or not available, some in
vitro or in vivo toxicological studies involving animal models were also included. All selected
articles were incorporated into the next step of the selection process.
6.3. Data abstraction
Data, including the details of the chemical composition, the content of potentially toxic compounds
from petrofuel and biofuel emissions, and results of toxicological studies were extracted using
predefined criteria. For each eligible article, information related to the task and aim of the study was
abstracted, with particular attention being given to any possible benefits of a biofuel in terms of
reduced toxicity to human health. For this comparison, comprehensive reviews entitled ―Health
Assessment Document for Diesel Engine Exhaust‖ (US EPA, 2002a), ―A Comprehensive Analysis of
Biodiesel Impacts on Exhaust Emissions‖ (US EPA., 2002b), ―Health Assessment Document for
Diesel Engine Exhaust‖ (US EPA, 2002a), and ―Recent Advances in Investigations of Toxicity of
Automotive Exhaust” (Stupfel, 1976), in which the toxic effects of gasoline and diesel engine
exhausts on human health were systematically evaluated, were considered to be of particular value.
A number of references cited in these articles are included in the present treatise by virtue of their
importance in the assessment of comparative toxicity of alternative and conventional biofuels.
At an early stage in the review process it became clear that, regardless of the origin of the fuel (i.e.
fossil or biological), combustion produced essentially the same toxic compounds but in different
ratios and amounts, particularly when the fuels were blends of diesel and biodiesel or gasoline and
ethanol. This finding influenced further data abstraction and analysis of publications, and led to a
focus on the toxicity of regulated and unregulated pollutants. Thus, several of the most important
studies concerning the mutagenic effect of biofuels are considered in some detail in the foregoing
discussion.
6.4. Data synthesis and analysis
The most relevant parts of selected articles containing specific information comprising the complete
body of evidence described in Section 7 (Results), were analysed and summarized in tables as well
as in the form of conclusions at the end of every sub-section, and finally at the end of Section 8
(Discussion and overall conclusions). The final conclusions are unexpectedly alerting, and certainly
not in accord with current public opinion regarding the harmless nature of biofuels.
7. Results
The main fuels currently used in road vehicles are petrol, diesel and fossil gas. The strategy of
substituting these fuels by alternative fuels from renewable sources such as biodiesel, bioalcohols
and biogas in order to reduce traffic emissions is very attractive, but needs to be supported by
rigorous scientific evidence verifying the reduced effect of such emissions on human health.
- 16 -
Since biofuel is chemically quite different from petrol it is possible that the chemical compositions
of the emissions derived from these two fuels vary and, consequently, present diverse impacts on
human health. This was the basic hypothesis of the present study, the objective of which was to
identify fuels that exhibit the least harmful effects on human health. However, in the course of the
review process it became obvious that the combustion of petrofuel and biofuel produced essentially
the same toxic compounds but in different ratios and amounts (sections 7.2.-7.4), particularly when
the fuels were blends of diesel and biodiesel or gasoline and ethanol.
There follows a short description of the major pollutants emitted from the engine and exhaust
system of internal combustion engines.
7.1. Toxic compounds emitted from the exhaust systems of motor vehicles
Primary emissions are a complex mixture containing hundreds of organic and inorganic constituents
Table 3. Chemical composition of diesel exhaust. (Table modified from US EPA, 2002a).
Particulate phase Gas phase
Heterocyclics, hydrocarbons (C14-C35), and Heterocyclics, hydrocarbons (C1-C10), and
PAHs and derivatives: derivatives:
Acids Cycloalkanes Acids Cycloalkanes, Cycloalkenes
Alcohols Esters Aldehydes Dicarbonyls
Alkanoic acids Halogenated cmpds. Alkanoic acids Ethyne
n-Alkanes Ketones n-Alkanes Halogenated cmpds.
Anhydrides Nitrated cmpds. n-Alkenes Ketones
Aromatic acids Sulphonates Anhydrides Nitrated cmpds.
Quinones Aromatic acids Sulphonates
Quinones
Elemental carbon Acrolein
Inorganic sulphates and nitrates Ammonia
Metals Carbon dioxide, carbon monoxide
Benzene
1,3-Butadiene
Formaldehyde, formic acid
Hydrogen cyanide, hydrogen sulphide, methane, methanol
Nitric and nitrous acids
Nitrogen oxides, nitrous oxide, sulphur dioxide
Toluene
- 17 -
Table 4. Major components of gas phase diesel engine emissions, their known atmospheric
products and the biological impact of such products. (Table modified from US EPA., 2002a).
Gas phase Atmospheric reaction product Biological impact
CO2 - Global warming
CO - Highly toxic, blocks oxygen
uptake
NOx Nitric acid Irritates respiratory tract
SO2 Sulphuric acid Irritates respiratory tract
C1-C17 Alkanes Aldehydes, ketones, alkyl nitrates Irritates respiratory tract
C1-C3 Alkenes Aldehydes, ketones Carcinogenic and mutagenic
Formaldehyde Carcinogenic
Higher aldehydes (e.g.
acetaldehyde, acrolein)
CO, hydroperoxyl radical Irritates respiratory tract
Monocyclic aromatic
compounds (benzene,
toluene, xylene)
Hydroxylated and
hydroxylated nitro derivatives
Carcinogenic and mutagenic
PAH Nitro-PAH Carcinogenic and mutagenic
Nitro-PAH Quinones and hydroxylated nitro
derivatives
Mutagenic
Some compounds that are initially emitted in the exhaust gases may be wholly or partially
transformed in the atmosphere into other toxic substances, as outlined in Tables 4 and 5 (US EPA,
2002a).
Table 5. Major components of particulate phase diesel engine emissions, their known atmospheric
products and the biological impact of such products. (Table modified from US EPA., 2002a).
Particle phase Atmospheric reaction product Biological impact
Elemental carbon - Absobs deep into the lung
Sulfate and Nitrate - Irritation of respiratory tract
C14-C35 Hydrocarbons Aldehydes, ketones, alkyl nitrates ?
PAH Carcinogenic and mutagenic
Nitro PAH Carcinogenic and mutagenic
7.1.1. Regulated pollutants
In the USA, the Clean Air Act (US EPA, 2006). requires the US EPA to set National Ambient Air
Quality Standards for the six principal pollutants defined in Table 6. These so-called "criteria"
pollutants are considered to be harmful to public health and to the environment, and their levels are
restricted according to the following criteria:
Primary standards set limits to protect public health, especially the health of "sensitive"
populations such as asthmatics, children, and the elderly.
Secondary standards set limits to protect public welfare, including protection against
decreased visibility, damage to animals, crops, vegetation, and buildings (US EPA, 2006).
- 18 -
Table 6. The National Ambient Air Quality Primary Standards (Table modified from National
Ambient Air Quality Standards (NAAQS, 1990)
Pollutant Level Averaging Time
Carbon monoxide 9 ppm (10 mg/m3) 8 h
35 ppm (40 mg/m3) 1 h
Lead 1.5 µg/m3 Quarterly average
Nitrogen dioxide 0.053 ppm (100 µg/m3) Annual (arithmetic mean)
0.100 ppm 1 h
Particulate matter (PM10) 150 µg/m3 24 h
Particulate matter (PM2.5) 15.0 µg/m3 Annual (arithmetic mean)
35 µg/m3 24 h
Ozone 0.075 ppm (2008 std) 8 h
0.08 ppm (1997 std) 8 h
0.12 ppm 1 h
Sulphur dioxide 0.03 ppm Annual (arithmetic mean)
0.14 ppm 24 h
Figure 3. Typical diameters of particulate matter in relation to the size of grains of fine beach sand
and the diameter of a human hair. (Figure taken from US EPA, 2006).
- 19 -
7.1.1.1. Particulates. Both Otto and Diesel engines emit large quantities of solid or liquid
material suspended in particulates. Such particulates, which are often referred to as particulate
matter (PM), are of micron size and can coagulate rapidly. As shown in Figure 3, PM typically
comprises ―fine particles" with diameters ≤ 2.5 µm (PM2.5), and ―inhalable coarse particles" with
diameters in the range 2.5 – 10 µm (PM10). The larger particles are less harmful than their smaller
counterparts since they present a reduced surface area for the absorption of toxic compounds.
Additionally, in the less likely event that larger particles were to enter the human body, they would
be eliminated more rapidly than particles with a smaller diameter. US EPA regulations refer only to
exhalable particles with diameters < 10 µm.
Core PMs (i.e. soot and black smoke) are formed by nucleation and coagulation of primary
spherical particles comprising solid carbonaceous material (mainly elemental carbon) and ash (trace
metals and other elements). Organic and sulphur-containing compounds (i.e. sulphate) may add to
these particles through coagulation, adsorption and condensation, and ultimately combine with
other condensed materials (Figures 4 and 5). In this manner, black carbon cores may adsorb known
or suspected mutagens and carcinogens, e.g. polycyclic aromatic compounds (PAC) as organic
phase (US EPA 2002a). Indeed, organic solvent extracts of particulate matter have been shown to
exhibit mutagenic activity in various in vitro bacterial and mammalian assays including the
Salmonella typhimurium mammalian microsome assay. The direct mutagenic effect of extracts of
particulate matter has been reported in many studies), and is mainly associated with substituted
polyaromatic hydrocarbons (PAH) and particularly with nitrobenzanthrones (US EPA 2002a).
Figure 4. Schematic diagram of typical PMs derived from diesel engine exhaust. (Figure taken
from US EPA, 2002a).
- 20 -
Figure 5. Chemical composition of 2.5 μm diameter PMs derived from diesel engine exhaust.
(Figure taken from US EPA, 2002a).
The region of the respiratory tract where the inhaled particles rest depends on the diameter of the
PMs. The smallest particles (< 100 nm) may be extremely harmful to the cardiovascular system
Moreover, it has been shown that PM0.1 (and smaller) can pass through cell membranes and migrate
into other organs including the arteries, where deposition of plaque can induce vascular
inflammation and atherosclerosis (Pope et al., 2002), and the brain, where damage similar to that
found in Alzheimer patients has been detected. PM2.5 and PM10 materials also tend to penetrate into
the gas-exchange regions of the lungs, while smaller particles may be absorbed by the bronchi and
the lungs, enter the bloodstream, and hence cause serious disorders, including:
increased respiratory symptoms, such as irritation of the airways, coughing, or difficulty
breathing,
decreased lung function,
aggravated asthma,
development of chronic bronchitis,
irregular heartbeat,
non-fatal heart attacks,
premature death in people with heart or lung disease.
Typically, children and older adults are more sensitive to exposure to particle pollution than the
general population (US EPA, 2006).
Total hydrocarbons. These toxic substances are derived from unburnt or partially burnt fuel and are
considered to be a major contributor to the toxicity of urban smog, which can cause liver damage
and cancer. Many of these hydrocarbons are regulated in the USA by the Occupational Safety and
Health Administration. Thus, the typical composition of gasoline hydrocarbons (% volume) is as
follows: 4-8% alkanes; 2-5% alkenes; 25-40% isoalkanes; 3-7% cycloalkanes; l-4% cycloalkenes;
and 20-50% total aromatics (0.5-2.5% benzene) (IARC 1989).) (Harper and Liccione, 1995). It is
accepted that concentrations of alicyclic hydrocarbons in excess of 500 ppm are toxic, although
levels detected in the ambient atmosphere are generally 100 – 1000 fold lower than this (Stupfel,
- 21 -
1976). Concentrations greater than 1% have been found to produce narcosis and convulsions,
supposedly due to a hypoxic effect, whilst eye irritation caused by these hydrocarbons has been
attributed to their chemical structure (Stupfel, 1976). The potential danger of hydrocarbons found in
car exhaust is not limited, however, to their nature per se. As outlined in Tables 4 and 5, alicyclic
hydrocarbons interact in the presence of UV irradiation with nitrogen oxides (NOx) and oxygen-
containing free radicals giving rise to other toxic substances, including aldehydes and ketones that
are characteristic of photochemical smog. One group of such products, the peroxyacyl nitrates
(PANs), are formed from a hydrocarbon-derived peroxyacyl radical and nitrogen dioxide (NO2) in
the presence of UV light according to the equation (US EPA 2002a)
Hydrocarbons + O2 + NO2 + light
Peroxyacetyl nitrate (CH3COOONO2) is known to cause skin cancer (Jacobson, 2007).
Polyaromatic hydrocarbons. Polynuclear aromatic compounds (PACs), or PAHs (syn.), are by-
products of the incomplete combustion of fuel (fossil or biomass). More than 100 different PAHs
have been characterised, and the chemical structures of some of these are shown in Table 7. PAHs
are mainly solids, and occur as complex mixtures that are carried in the air on soot and other
particles onto which they have been absorbed. For this reason it is technically very difficult to
measure PAH concentrations in urban air. As pollutants, PAHs are of great concern because many
have been found to possess carcinogenic, mutagenic and teratogenic activities (ATSDR, 1995; EPA
2002a; IARC, 1983).
Table 7. Polycyclic aromatic hydrocarbons identified in PM from diesel engine exhaust (US EPA,
2002a).
Compound Concentration
ng/mg extract
Concentration
ng/mg extract
Acenaphthylene 30 Pyrene 3,532–8,002
Trimethylnaphthalene 140–200 Ethylmethyl anthracene 590–717
Fluorene 100–168 Methyl(fluoranthene/pyrene) 1,548–2,412
Dimethylbiphenyl 30–91 Benzo[a]fluorene/benzo[b]fluorene 541–990
C4 -Naphthalene 285–351 Benzo[b]naphtho[2,1-d]thiophene 30–53
Trimethylbiphenyl 50 Cyclopentapyrene 869–1,671
Dibenzothiophene 129–246 Benzo[ghi]fluoranthene 217–418
Phenanthrene 2,186–4,883 Benzonaphthothiophene 30–126
Anthracene 155–356 Benz[a]anthracene 463–1,076
Methyldibenzothiophene 520–772 Chrysene or triphenylene 657–1,529
Methylphenanthrene 2,028–2,768 1,2-Binapthyl 30–50
Methylanthracene 517–1,522 Methylbenz[a]anthracene 30–50
Ethylphenanthrene 388–464 3-Methylchrysene 50–192
4H-Cyclopenta[def]phenanthrene 517–1,033 Phenyl(phenanthrene/anthracene) 210–559
Ethyldibenzothiophene 151–179 Benzo[j]fluoranthene 492–1,367
2-Phenylnaphthalene 650–1,336 Benzo[b]fluoranthene 421–1,090
Dimethyl(phenanthrene/anthracene) 1,298–2,354 Benzo[k]fluoranthene 91–289
Fluoranthene 3,399–7,321 Benzo[e]pyrene 487–946
Benzo[def]dibenzothiophene 254–333 Benzo[a]pyrene 208–558
Benzacenaphthylene 791–1,643 Benzo[ah]anthracene 50–96
- 22 -
Seventeen PAHs have been identified as most harmful to human health, namely, acenaphthene,
acenaphthylene, anthracene, benzo[ah]anthracene, benzo[a]pyrene, benzo[e]pyrene,
benzo[b]fluoranthene, benzo[ghi]perylene, benzo[j]fluoranthene, benzo[k]fluoranthene, chrysene,
dibenzo[a,h]anthracene, fluoranthene, fluorene, indeno[1,2,3-cd]pyrene, phenanthrene and pyrene
(ASTDR, 1995).
7.1.1.2. Carbon monoxide. CO is a product of the incomplete combustion of fuel in an internal-
combustion engine and is formed according to the equation O2 + 2 C → 2 CO. Even very small
amounts of CO can induce hypoxic injury, neurological damage, and possibly death. When CO is
inhaled, it strongly binds to hemoglobin and blocks oxygen binding, resulting in oxygen starvation
throughout the body. Prolonged exposure to fresh air (or pure oxygen) is required for the CO-bound
hemoglobin to clear.
The specific effects of CO at various concentrations are listed below (Goldstein, 2008):
35 ppm (0.0035%) headache and dizziness within 6 to 8 h of constant exposure,
100 ppm (0.01%) slight headache in 2 to 3 h,
200 ppm (0.02%) slight headache within 2 to 3 h,
400 ppm (0.04%) frontal headache within 1 to 2 h,
800 ppm (0.08%) dizziness, nausea and convulsions within 45 min: unconscious within 2 h,
1,600 ppm (0.16%) headache, dizziness and nausea within 20 min: death in less than 2 h,
3,200 ppm (0.32%) headache, dizziness and nausea in 5 to 10 min: death within 30 min,
6,400 ppm (0.64%) headache and dizziness in 1 to 2 min: death in less than 20 min,
12,800 ppm (1.28%) unconsciousness after 2-3 breaths: death in less than 3 min.
The earliest symptoms of CO poisoning, particularly at low level exposure, are often non-specific
including depression, chronic fatigue syndrome, chest pains, and migraine or other headaches (Ilano
and Raffin, 1990). CO poisoning can also promote myocardial ischemia, arterial fibrillation,
pneumonia, pulmonary oedema, hyperglycemia, muscle necrosis, acute renal failure, skin lesions,
visual and auditory problems and respiratory arrest (Choi, 2001). Chronic exposure to CO may
increase the severity of cardiovascular symptoms, persistent headaches, light-headedness,
depression, confusion and nausea (Fawcett et al., 1992). Severe neurological manifestations may
occur days or even weeks after acute CO poisoning and are associated with cognitive functions,
such as impaired short-term memory, dementia, irritability, gait disturbance, speech disturbances,
Parkinson-like syndromes, cortical blindness and depression (Roohi et al., 2001). A recent report
concluded that exposure to CO can lead to a significant reduction in lifespan owing to damage to
the heart muscle (Henry et al., 2006). Additionally, teratogenic effects of CO have been
demonstrated in chick embryos (Baker and Tumasonis, 1972).
7.1.1.3. Nitrogen oxides. NOx [typically nitric oxide (NO) together with nitrogen dioxide (NO2)]
are generated when nitrogen in the air reacts with oxygen under the high temperature and pressure
conditions present inside the engine. During combustion, nitrogen bound in the fuel is released as a
free radical and forms free nitrogen or NO, the latter being potentially further oxidised to NO2.
Acute exposure to NOx for 0.5 - 24 h induces inflammatory symptoms in the respiratory system of
healthy subjects and aggravates existing symptoms in asthmatic patients. Atmospheres containing 5
ppm or more of mixtures of NO and NO2, as encountered in the occupational exposure of welders,
- 23 -
cause a decrease in maximum breathing capacity and an increase in expiratory resistance
accompanied by methemoglobinemia (Stupfel, 1976). Exposure to NOx at levels between 25 to 75
ppm induces bronchitis and bronchopneumonia, while concentrations in the region of 500 ppm
provoke pulmonary oedema and, ultimately, death. Moreover, a positive association has been
detected between short-term exposure to elevated NO2 concentrations and increased visits to
emergency departments and hospital admissions (US EPA, 2006). In this context, children, elderly
people and asthmatic patients are more sensitive to NO2 exposure than the general population.
Additionally, it should be noted that concentrations of NO2 within about 50 meters of a roadway are
typically between 30 and 100% greater than levels measured in the same area but away from the
roadway.
Interaction of NOx with ammonia in the presence of water vapour and other compounds can
generate fine particles that may be absorbed in the lungs inducing respiratory disorders, such as
bronchitis, and the exacerbation of existing heart disease, leading to increased hospital admissions
and early death. (US EPA 2006). Recent studies have raised concern over the use of oxidative
catalytic converters with diesel engines. It is suggested that such converters can boost the formation
of direct-acting mutagens under certain conditions by reaction of NOx with PAH resulting in the
formation of toxic nitrated-PAHs (Bünger et al., 2006).
7.1.1.4. Sulphur oxides. Sulphur dioxide (SO2) is emitted at concentrations in the range 0.2 to 1.66
ppm by coal-burning heaters and power sources, and by those using sulphur-contaminated fuel.
Intoxication with SO2 mainly affects the upper airways, and a strong association between SO2 levels
and the incidence of lung cancer has been demonstrated (Abbey et al., 1999). Fortunately, man can
detect the odour of SO2 at concentrations between 1 - 3 ppm (Stupfel, 1976). Short-term exposure
(from 5 min to 24 h) to SO2 induces bronchoconstriction and asthmatic symptoms.
Sulphur oxides (SOx) can react with other compounds in the atmosphere resulting in the formation
of aggressive particles that readily penetrate into the lungs and stimulate respiratory diseases such
bronchitis and emphysema (US EPA, 2006).
7.1.1.5. Heavy metals. Small amounts of various heavy metals, including platinum, palladium, lead,
cadmium, zinc and nickel, are emitted following the combustion of gasoline in automobile engines.
Roadside soils and vegetation are often found to be contaminated by deposits of the last four of the
metals listed. Lead, in particular, is easily absorbed into the blood and subsequently distributed
throughout the whole body. The metal interacts with the oxygen transportation system of the
blood. (US EPA, 2006), and affects the nervous, immune, cardiovascular and reproductive systems
as well as the kidneys. The diagnostic criteria of lead intoxication in man are anaemia,
coproporphyrinuria, urinary excretion of 9-aminolevulinic acid dehydratase (ALA-D), and
decreased levels of ALA-D in red blood cells (Stupfel, 1976). Around 90% of absorbed lead is
accumulated in the bones, but long-term experiments involving both man and animals continuously
inhaling high concentrations of lead (3.5 to 20 g/m3) revealed increased blood lead and decreased
blood ALA-D, but an absence of apparent modifications to health (Stupfel, 1976). Infants and
children are especially sensitive to very low levels of lead, which can induce mental disorders
affecting behaviour , learning capacity and cognitive functions. (US EPA, 2006).
- 24 -
Currently, the gasoline sold in most of developed countries is free of lead. That is beneficial for
human health not only because of elimination of lead toxicity. The absence of the lead in gasoline
is associated with reduced emission of PAH, which are highly carcinogenic. Thus, it has been
shown that if premium leaded gasoline is replaced by 92 octane lead-free gasoline, the emissions of
total PAHs and total benzo(a)pyrene equivalents (BaPeq) would be decreased (Mi et al., 2001). The
lead containing gasoline still is widely used in developing countries. That is the problem not only
for people living in these countries, but also for people living in all other countries, where the lead
containing gasoline is forbidden for sale. The reason is that the lead emitted from cars can be
absorbed in plants and disseminated all over the world via the food imported to other countries.
Meat of animals feed with lead contaminated grass will also contain the lead which is easily
accumulated in animal tissue.
It has been found that cadmium accumulates in the kidneys causing functional disturbances, whilst
nickel may increase the risk of bronchial cancer (Stupfel, 1976).
7.1.1.6. Ozone. Ozone is formed when NOx and volatile organic compounds interact at high
temperature in the presence of sunlight. High levels of ozone in the atmosphere induces
coughs, airway irritation, breathing difficulties, lung impairment and pain,
inflammation, similar to sunburn on the skin,
aggravation of asthma and increased susceptibility to respiratory diseases (pneumonia,
bronchitis etc.).
Patients with respiratory tract disorders, children and older adults are especially sensitive to ozone
exposure (US EPA, 2006).
7.1.2. Unregulated pollutants
So called ―aggregated toxics‖, namely, acetaldehyde, acrolein, benzene, 1,3-butadiene,
ethylbenzene, n-hexane, naphthalene, styrene, toluene, and xylene are currently unregulated.
7.1.2.1. Carbonyl compounds. These represent an important class of pollutants emitted from
vehicles fuelled by oxygenated fuels (Sharp et al., 2000; Grosjean et al., 2001; Poulopoulos et al.,
2001; Cardone et al., 2002; Magnusson et al., 2002). Included in this group of contaminants are:
formaldehyde, acetaldehyde, acrolein, acetone, propionaldehyde, crotonaldehyde, methylacrolein,
butyraldehyde, o-tolualdehyde, m-tolualdehyde, valeraldehyde, hexaldehyde and
dimethylbenzaldehyde (Pang et al., 2008). These compounds are precursors of free radicals, PANs
and ozone in the atmosphere (Carter, 1995; Gaffney et al., 1997), while some of them are toxic,
mutagenic and/or carcinogenic (Carlier et al., 1986). Various aldehydes are potent eye irritants:
thus, the eye irritation threshold of formaldehyde is estimated to be 0.01-1.0 ppm, the olfaction and
eye irritation threshold of acrolein is 0.25 ppm, whilst the irritant threshold of acetaldehyde is 50
ppm. Many of the unpleasant odour and irritant properties of diesel and gasoline exhausts are
caused by the presence of aldehydes. Moreover, the upper airways are very sensitive to these
products, and concentrations as low as 0.20 - 1.20 ppm of total aldehydes, or 0.05 - 4.12 ppm of
formaldehyde, are physiologically active in the air (Stupfel, 1976). The ciliary activity of rat and
sheep trachea is inhibited in vitro by very small amounts of aldehydes, and this could inhibit
resistance to air particles penetrating into the lungs (Stupfel, 1976)..
- 25 -
Formaldehyde
Acute effects - the main toxic effects caused by acute exposure to formaldehyde via
inhalation are irritation to the eye, nose and throat, and inflammation in the nasal cavity. In
humans, exposure to high levels of formaldehyde may result in spasms of coughing and
wheezing, and to chest pains and bronchitis. (US EPA, 1988; WHO, 1989).
Chronic effects (non-cancer) - chronic inhalation exposure of humans to formaldehyde is
associated with various respiratory symptoms and irritation of the eye, nose and throat.
Animal studies have revealed chronic effects of formaldehyde exposure on the nasal
respiratory epithelium and lesions in the respiratory system (US EPA, 1988; WHO, 1989;
Calabrese and Kenyon, 1991).
Reproductive/developmental effects - an increased incidence of menstrual disorders and
problems in pregnancy have been reported in women workers using urea-formaldehyde
resins, although possible confounding factors were not evaluated in the study (US EPA,
1988; WHO, 1989).
Cancer risk – WHO and INTERNATIONAL AGENCY FOR RESEARCH ON CANCER
(IARC) consider formaldehyde to be a human carcinogen (cancer-causing agent) and it has
been ranked in EPA group B1 (IARC, 2006; US EPA, 1993). There is sufficient evidence in
humans for the carcinogenicity of formaldehyde.There is sufficient evidence in experimental
animals for the carcinogenicity of formaldehyde. (IARC, 2006).
Acetaldehyde
Acetaldehyde is the major metabolite of ethanol and is also released into the environment by
combustion and photo-oxidation of hydrocarbons (US EPA, 1994a). In air, acetaldehyde reacts with
singlet oxygen, OH radicals, NO3 and NO2 to form, amongst other products, the highly toxic
peroxyacetyl nitrate together with methyl nitrate, methyl nitrite and nitric acid (US EPA, 1994a).
The acute effects of acetaldehyde in humans include irritation of the eyes and respiratory tract and
altered respiratory function. According to the International Agency for Research on Cancer (IARC,
1994a), sub-chronic and chronic dermal exposure to acetaldehyde can cause erythema and burns in
humans, whilst repeat contact may give rise to dermatitis resulting from irritation or sensitisation
(IARC, 1985). In animals, repeat doses of acetaldehyde, administered in high concentrations by
inhalation, caused adverse respiratory tract effects. Additionally, direct administration of
acetaldehyde to rats established alcohol dependency, and it is claimed that many of the adverse
effects of ethanol itself may be attributable to acetaldehyde (US EPA, 1994a).
Cancer risk – There is inadequate evidence in humans for the carcinogenicity of acetaldehyde.
There is sufficient evidence in experimental animals for the carcinogenicity of acetaldehyde. (IARC,
1987). Overall evaluation: acetaldehyde is possibly possibly carcinogenic to humans (Group B2)
(IARC, 1987; US EPA, 1994a). Additionally, acetaldehyde has been shown to possess neurotoxic
activity since it is able to penetrate the human blood-cerebrospinal fluid barrier and can cause
depression of the central nervous system (IARC, 1985; 1987).
Acrolein
Acrolein is a strong dermal irritant and causes skin burns in humans. Exposure to acrolein in the
range 0.17 - 0.43 ppm induces irritation of the upper respiratory tract and congestion, whilst at the
level of 10 ppm it can cause death. The provisional reference dose calculated by US EPA for
- 26 -
acrolein is 0.02 mg/kg body weight/day (ATSDR, 2007). No information is available concerning
the embryotoxic or carcinogenic effects of acrolein in humans. However, a toxic effect on the
reproductive system was observed when the aldehyde was injected directly into embryonic tissue of
rats, and a tumour promotion effect on the respiratory tract and adrenocortical tissue could also be
demonstrated in the murine model.
7.1.2.2. Other oxygenated hydrocarbons in exhaust emissions
Methanol
Acute methanol intoxication is manifested by symptoms of narcosis followed by accumulation of
formic acid in the body causing metabolic acidosis (US EPA, 1994b). Symptoms of lesions of the
central nervous system (CNS) include headache, dizziness, severe abdominal, back and leg pain,
delirium (that can lead to nausea and coma) and visual degeneration that can bring about blindness.
Chronic exposure to methanol vapour (time of exposure and dose not stated) caused conjunctivitis,
headache, giddiness, insomnia, gastric disturbances and bilateral blindness, whilst vision loss
reportedly occurred after exposure to 1200 to 8000 ppm vapour for 4 years (US EPA, 1994b). The
lethal dose of methanol when administrated orally to humans is in the range of 80 - 150 mL, whilst
in the vapour phase a dose of 4000 -13,000 ppm (equivalent to 1140 - 3705 mg/kg) over 12 h is
fatal (US EPA, 1994b).
A genotoxic effect of methanol, in the form of increased sister chromatid exchange, was observed
in Syrian hamster embryo cells in vitro, whilst aneuploidy and chromosome aberrations were
detected in Neurospora crassa (US EPA, 1994b). Methanol also exhibits a neurotoxic effect in
humans and animals, causing depression of the central nervous system as well as degenerative
changes in the brain and visual system.
In polluted air, methanol reacts with NO2 to yield methyl nitrite, and with photochemically-formed
hydroxyl radicals to produce formaldehyde. The half-life of methanol in the atmosphere is 17.8
days, whilst levels of methanol in the air around Stockholm were reported to be in the range 3.83 -
26.7 ppb in 1994 (US EPA, 1994b).
Ethanol
Ethanol exhibits the lowest degree of acute toxicity (category IV) for all effects tested including
acute oral and inhalation toxicity, and primary eye and skin irritations (US EPA, 1995). Ethanol is
generally recognized as a human developmental neurotoxicant causing foetal alcohol syndrome. In
an industrial environment, however, the risk from ethanol appears to be minimal (US EPA, 1995).
Formic acid
Formic acid presents very low toxicity when ingested via the oral route, but in the vapour phase it
causes severe eye and skin irritation. Inhalation of the vapour at 100 ppm is immediately dangerous
to life and health in humans causing respiratory irritation, lachrymation, coughing and headache,
followed within 6 - 8 h by pulmonary oedema, dizziness, frothy expectoration and cyanosis (bluish
skin discoloration caused by lack of oxygen in the blood). Breathing lower concentrations over time
can lead to erosion of the teeth, local tissue death in the jaw, bronchial irritation with chronic cough,
frequent attacks of bronchial pneumonia and gastrointestinal disturbances (Federal Register, 1997).
- 27 -
7.1.2.3. Hydrocarbons in exhaust emissions
Benzene.
Exhaust benzene is produced either from unburned benzene or benzene formed during the use of
other aromatic and nonaromatic compounds found in gasoline. Brief exposure (5–10 minutes) to
very high levels of benzene in air (10,000–20,000 ppm) can result in death. Lower levels (700–
3,000 ppm) can cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, and
unconsciousness. Long-term exposure to benzene cam can lead to anemia, suppress immune
system, cause .acute myeloid leukemia (AML). The International Agency for Cancer Research and
the EPA have determined that benzene is carcinogenic to humans (Group A). Exposure to benzene
may be harmful to the reproductive organs and induce irregular menstrual periods. (Wilbur et al.,
2007)
1,3-Butadiene.
Brief exposure to 1,3-butadiene exposure may cause nausea, dry mouth and nose, headache, and
decreased blood pressure and pulse rate in humans. In laboratory animals, 1,3-butadiene induces
inflammation of nasal tissues, changes to lung, heart, and reproductive tissues, neurological effects,
blood changes, reduced fetal body weight, and birth defects. Long term exposure to 1,3-butadiene in
humans may have an increased risk for cancers of the stomach, blood, and lymphatic system. The
International Agency for Research on Cancer (IARC), National Toxicology Program (NTP), and
EPA all classify 1,3-butadiene as a human carcinogen. EPA has set a reference concentration in
breathing air of 0.9 ppb for 1,3-butadiene. (Ashizawa et al., 2009).
7.2. Acute and chronic adverse health effects of vehicle exhaust produced by the
combustion of petrofuels, gasoline and diesel
More than three decades ago, Stupfel published a comprehensive review of numerous in vitro and in
vivo studies on animals, and epidemiological and experimental studies on humans, in which the
toxic effects of automotive exhaust, and individual components thereof, had been reported (Stupfel,
1976). Table 8 presents a summary of the levels of the major components of the exhausts from
gasoline and diesel-fuelled engines and reveals some differences in their chemical compositions.
Overall, diesel cars emit less hydrocarbons, CO and lead than petrol cars, but produce more noxious
gases and significantly more particulates. (Stupfel, 1976). Stupfel (1976) has pointed out that the
acute toxicity of gasoline exhaust emission is mostly due to CO, and describes one of the first
experiments carried out in 1921-1922 by Henderson and his colleagues to confirm this assumption.
These researchers exposed themselves, together with several other volunteers, to the exhaust gas of
a Ford car containing levels of CO in the range 200-300 ppm, as a result of which the participants
suffered dizziness, headaches, fainting and vomiting. These symptoms were then compared with
those observed following exposure to similar levels of CO, from which it was concluded that CO
was the main toxic constituent of exhaust gas derived from the combustion of gasoline.
- 28 -
Table 8. Concentrations of the main components in automotive exhausts. (Table modified from
Stupfel, 1976).
Gases and particles Gasoline exhaust Diesel exhaust Biological activity
threshold
Air quality
standards
Nitrogen (%) 76-90
Hydrogen (%) 2-6 0.05-8
Carbon dioxide (%) 5-15 1-14 5 0.5
Carbon monoxide 2-6% 0-0.1% 200-300 ppm 30-120 ppm
NOx (ppm) 30-4000 30-2000 3-10
NO2 (ppm) 0.5-40 0.5-5 0.2-0.5
SO3 (ppm) 0-80 100-300 1-5 0.03-0.1
Total aldehydes (ppm) 40-300 10-120 0.06-0.1
Formaldehyde (ppm) 10-300 5-30 0.5-16
Total hydrocarbons 0.03-1.5% 0.01-0.10% 0.2-5 ppm 0.24 ppm
Methane 200-800 ppm 1-20% 1000 ppm
Benzopyrene 1-10 μg/m3 0.01-100 μg/m
3
Lead 79-80% of the lead
of gasoline
0 15.5 μg/m3
Oils (mg/m3) 200-900
Particles 0.2-3 mg/g of
burned gasoline
150-450 mg/m3 60-75 μg/m
3
Data shown in table 8 demonstrate the main differences in gasoline and diesel exhaust from old cars
used more than 40 years ago. In my review, the results obtained in the last two decades will be used
for the comparisons of the effects of biofuel and petrofuel exhausts on human health.
Nowadays, the amounts of many toxic compounds in gasoline and diesel exhaust is significantly
lower, due to invention and implementation of catalytic converters (table 9), particle filters used in
cars working on diesel fuel (Schifter et al., 2001; Franklin et al., 2001; US EPA, 2002a) and many
other innovations in engine technology resulting both in decreased emission of regulated toxics
(table10) and significant changes in a smoke standards for on-road diesel engines (US EPA, 2002a),
figures 6 and 7. For example, table 10 shows US EPA emission standards from 1970 to 2007 for
heavy duty highway diesel engines, wherefrom is obvious that the upper limit for particulates (PM)
and NOx is reduced in 50-60 (!!!) times. Figure 6 shows changes in engine certification data for
NOx, PM, aldehydes, soluble organic fraction emissions reported in the many studies that have
employed the transient test over the 25 years (US EPA 2002a). Figure 7 shows real-world, in-use
emissions measurements of PM, total hydrocarbons, CO and NOx and therefore more accurately
reflects emission factors than engine test data during this period (US EPA, 2002a).
In the past, organic lead compounds were usually used as anti-knock/isooctane booster agents in
gasoline, however, they have been almost completely replaced by methyl-tertiary-butyl ether as an
anti-knock agent in the production of unleaded gasoline. Leaded gasoline has been produced in
much smaller quantities for use in engines not equipped with catalytic converters (Harper and
Liccione J, 1995). For old engines not equipped with catalytic converters other additives in some
extent fulfil the same function as lead.
Comparison of tailpipe and exhaust emission studies showed that reactions in the catalytic converter
are quite effective in destroying most hydrocarbons, methyl tertiary-butyl ether by-product species,
including CO, table 9 (Schifter et al., 2001; Franklin et al., 2001).
- 29 -
Figure 6. Engine certification data for NOx, PM, aldehydes, soluble organic fraction emissions
reported in the many studies that have employed the transient test over the 25 years (Modified from
US EPA 2002a).
Figure 7. Model year trends in PM, NOx, THC, and CO emissions from HD diesel vehicles (Taken
from US EPA 2002a).
- 30 -
Table 9. Emissions of particle-bound PAHs (µg/ml), carbone monooxyde (g/km), nitrogen oxides
(g/km), total hydrocarbons (g/km) and ozone (g of ozone produced by g of nonmethane organic
gases emitted) in gasoline exhaust from vehicles with oxidative catalyst, tree-way catalytic
converter and without catalyst. Effect of ethanol (10% in gasoline-ethanol blend) is shown as well
(Table from data in Schifter et al., 2001 and from US EPA, 2002a).
Gases No catalyst Oxydative catalyst Tree-way catalytic
converter
Gasoline Gasoline-
ethanol
Gasoline Gasoline-
ethanol
Gasoline Gasoline-
ethanol
Carbon monoxide 18 13 13 8 3 3
NOx (ppm) 1.73 1.75 1.08 1.23 0.41 0.48
Total hydrocarbons 1.56 1.45 0.7 0.56 0.22 0.22
Ozone 4.19 4.27 3.95 3.66 3.12 3.08
Pyrene 45 4
Fluoranthene 32 3.6
Benzo[a]pyrene 3.2 3.0
Benzo[e]pyrene 4.8 3.6
Table 10. US EPA emission standards for HD highway diesel engine, (modified from US EPA
2002a).
It is noteworthy that filtered diesel exhaust or the exhaust from the gasoline-catalyst engines are less
harmful than diesel engine emissions (Brightwell et al., 1989). There was no significant changes in
the incidence of lung tumours in the group of rats exposed to filtered diesel exhaust or to the
exhaust from the gasoline or gasoline-catalyst engines for two years (16 hours per day, 5 days per
week), while a statistically significant increase in the incidence of lung tumours was observed in the
group of rats exposed to diesel engine emissions (Brightwell et al., 1989).
Multivariate statistical methods have been applied in order to identify factors that affect the
biological potency of car exhausts. The physical and chemical characteristics of 10 diesel fuels were
determined, and extracts of the particulate exhaust emissions derived from the combustion of these
fuels were assayed chemically and for mutagenic activity using the Ames test and a test involving
Model year Pollutant (g/b hp-hr THC CO NOx Particulate
(PM), 1970 - - - - 1974 - 40 - - 1979 1.5 25 - - 1985 1.3 15.5 10.7 - 1988 1.3 15.5 10.7 0.6 1990 1.3 15.5 6 0.6 1991 1.3 15.5 5 0.25 1993 1.3 15.5 5 0.1-0.25 1994 1.3 15.5 5 0.07-0.1 1996 1.3 15.5 5 0.05-0.1 1998 1.3 15.5 4 0.05-0.1 2004 1.3 15.5 - 0.05-0.01 2007 15.5 0.2 0.01
- 31 -
the binding of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to the Ah receptor (Sjögren et al.,
1996). The strongest associations between the biological effects of the extracts and the chemical
characteristics of the fuels related to the levels of sulphur, PACs and naphthalenes. Fuel density and
flash point were positively correlated with genotoxic potency, whilst octane number and upper
distillation curve points were associated negatively with mutagenicity and Ah receptor affinity. The
mass of emitted particles and the levels of 1-nitropyrene, particle bound nitrate, and indeno[1,2,3-
cd]pyrene were highly correlated with both mutagenicity and Ah receptor affinity. Exogenous
metabolic activation (S9)-dependent mutagenicity was highly associated with certain PACs, while
S9-independent mutagenicity was more related to the concentrations of nitrates and 1-nitropyrene
present. The results of the study revealed that fuels presenting low levels of biological hazard were
those that contained the least PAC and sulphur (Sjögren et al., 1996).
7.2.1. Acute and chronic adverse health effects
Increases in hospital admissions and daily mortality have been observed in patients with chronic
obstructive pulmonary disease, chronic bronchitis, asthma and cardiovascular disease during short-
term exposure to high levels of PM in the ambient air (Dockery and Pope 1994; Samet et al., 1995;
Katsouyanni et al., 1997). It has been reported that exposure to diesel exhaust particles (DEP)
increases many symptoms of systemic and pulmonary inflammation in healthy humans (Salvi et al.,
1999). In several observational studies, a correlation between mortality and chronic exposure to air
pollution from PM10 has been reported (Dockery et al., 1993; Schwartz 1993; Pope et al., 1995;
Abbey et al., 1999).
7.2.2. Carcinogenic effects
Lung tumours were reproducibly observed in rats, but not in other experimental rodents, following
chronic inhalation of diesel engine exhausts at high concentrations (US 2002a; Mauderly et al.,
1987). In a more detailed study of the potential carcinogenic effects of inhaled automobile exhaust,
rats and hamsters were subjected to the emissions from (i) a gasoline engine, (ii) a gasoline engine
fitted with a three-way catalytic converter, (iii) a diesel engine and (iv) a diesel engine fitted with
particle filtration (US EPA 2002a). Following exposure over a 2 year period (5 days per week and
16 h per day), a statistically significant increase in the incidence of lung tumours in rats exposed to
diesel engine emissions was observed compared with controls. There were, however, no increases
in the incidence of lung tumours in rats exposed to filtered diesel exhaust or to the exhausts from
gasoline engines with or without catalytic converters.
Diesel engine emissions generate strong carcinogenic effects in animals and potential effects in
humans according to a number of extensive reports from scientific and governmental institutions in
USA and Europe (IARC, 1989; US EPA, 2002a).
A prospective environmental cohort study demonstrated a strong correlation between incidence of
lung cancer in non-smokers and the concentration of PM10 in the air (Abbey et al., 1999). The
increased risk of lung cancer following long-term occupational exposure to high concentrations of
diesel emissions has been revealed in several epidemiological studies (Mauderly,; 1994; EPA,
2002a). Similar results were obtained in other studies involving workers in the potash mining
industry and operators of heavy equipment who had been exposed to diesel engine emissions
(Bruiske-Hohlfeld et al., 1999; EPA, 2002a).
- 32 -
7.2.3. Mutagenic effects
Strong mutagenic activities of DEP extracts in the Salmonella typhimurium/mammalian microsome
assay is associated with the presence of substituted PACs, in particular nitro-PAC, while
unsubstituted PACs exhibited a mutagenic effect in the experimental model with metabolic
activation by S9 (ATSDR, 1995; EPA, 2002a; IARC, 1983).
7.3. Chemical composition and toxic effects of vehicle exhaust produced by
combustion of biofuels
In Sweden, there is intense interest in the use of biofuels, particularly for city buses, delivery trucks
and light duty vehicles, with the intent of reducing emissions of the main toxic pollutants. In 1991
the Swedish Government launched an extensive research program, administrated by the Swedish
Transport and Communications Research Board, in order to demonstrate the feasibility of using
ethanol in Flexible Fuel Vehicles and biogas in both light and heavy-duty buses and delivery trucks.
Several Swedish Universities and Institutes of Technology have conducted projects in support of
this program (Egeback and Foxbrook, 1998).
7.3.1. Biodiesel and vegetable oils
7.3.1.1. Chemical composition of the emissions from biodiesel engines. Assuming that the
chemical compositions of petrofuel and biofuel are very different [section 2.2., e.g. RME contains
almost no aromatic compounds whereas fossil diesel fuels comprise 3–40 % aromatics including
PACs at levels up to 340 mg/l (Sjögren et al., 1995)], it might, perhaps, be expected that the
emissions from oil and biodiesel-fuelled engines would be quite dissimilar and, in the case of the
latter, relatively harmless. Despite the compositional differences, however, the combustion products
and exhaust emissions of both types of diesel still contain the same irritant gases, including NOx,
aldehydes and a wide range of organic compounds, many of which can induce oxidative stress in an
organism (Krahl et al., 1996a,b; Mauderly, 1997). An initial comparative study of biodiesel and
rapeseed oil exhaust demonstrated that emissions from biodiesel-fuelled engines were qualitatively
similar to those running on PDD (Krahl et al., 1996b) in spite of significant quantitative differences
in the content of various groups of compounds, such as PAHs, aldehydes, ketones, hydrocarbons
and particulates (Figure 8).
Thus biodiesel emission contains amounts of particulates that are 2-3 times higher compared with
PDD, but 3-4 times less PAHs, while rapeseed oil emission is extremely rich in carbonyl
compounds (with ≤ 4.5 fold higher levels of aldehydes/ketones) and hydrocarbons compared with
PDD and biodiesel. This is most probably associated with the glycerol moiety in the triglycerols,
which can be transformed into acrolein and other aldehydes at high temperatures during the
incomplete burning of rapeseed oil (Graboski and McCormick, 1998), and the residual levels of
methanol or ethanol that could suffer partial combustion to form formaldehyde or acetaldehyde,
which are considered to possess actual or potential carcinogenic properties, respectively (IARC,
1985). Thus, the quality of biodiesel depends substantially on the content of free glycerol in the fuel
after refining, and on the amount of free alcohol (employed in the transesterification procedure)
remaining in the fuel following down-stream processing.
- 33 -
RSO RME0
100
200
300
400
500Smoke Nr
Particulate
Hydrocarbons
CO
NOx
PAH
Aldehydes
Emission from RSO and RME
Rela
tive v
alu
es i
nco
mp
ari
sio
n t
o d
iesel
(10
0%
)
Figure 8. Emissions from engines fuelled with rapeseed oil (RSO) and rapeseed diesel (RME) in
comparison with levels emitted by a diesel-fuelled engine (taken as 100%). Figure modified from
Krahl et al., 1996b.
Further studies have revealed that, in comparison with petroleum diesel, the combustion of
biodiesel produces less CO, solid material (in the form of total particulate matter), total
hydrocarbons and other toxic compounds, including vapour-phase C1-C12 hydrocarbons, aldehydes
and ketones (up to C8), and certain semi-volatile and particle-phase PAHs (Krahl et al., 1998;
Mauderly, 1997; Sharp et al., 2000; Graboski et al., 2003; McCormick, 2007). Thus, the amounts of
the toxic aromatic hydrocarbons benzofluoranthene and benzopyrenes in biodiesel exhaust are,
respectively, 56 and 71% less than in petroleum diesel exhaust, whilst sulphur-containing
compounds appear to be undetectable in biodiesel emissions.
Typically, a significant inverse correlation between the emissions of PM, CO and total
hydrocarbons with increasing content of biodiesel in blends with PDD has been observed in a
number of studies. The US EPA has released details of a comprehensive study of the emission
impact of biodiesel in which statistical regression analysis was employed to correlate the
concentration of biodiesel in admixture with conventional diesel fuel with changes in regulated and
unregulated pollutants emitted from on-road heavy-duty diesel engines (Durbin and Norbeck, 2002;
US EPA 2002a; Zou and Atkinson, 2003). It was concluded that a significant reduction in total
hydrocarbons, CO and PM can be achieved using biodiesel instead of PDD (Figure 9).
- 34 -
Figure 9. Average emission impacts of biodiesel on emission of regulated pollutants [PM, CO,
hydrocarbons (HC) and NOx] compared with those of diesel exhaust from heavy-duty highway
engines. (Figure taken from US EPA. 2002b).
In another study, biodiesel manufactured from canola oil was blended, prior to combustion, with
petroleum diesel in various proportions ranging from 0 to 100%, and the DPMs and gaseous
emissions were determined with particular attention to the levels of PAHs (Zou and Atkinson,
2003). It was found that the concentration of particulate matter was reduced by up to 33% when the
engine burnt 100% biodiesel compared with 100% PDD, and that the diminution in particulate
emissions was related to the increased percentages of biodiesel in the fuel blends. Among 16
targeted PAHs, 9 were detected in the particulate phase of biodiesel exhaust at much lower levels
than in the gaseous phase, but the particulate PAH emissions, which have greater importance with
regard to adverse health effects, were virtually unchanged and did not show a statistically
significant reduction with increased percentage of biodiesel. Some marked reductions were
observed in the levels of the less toxic gaseous PAHs, such as naphthalene, with 100% biodiesel
fuel.
In contrast, the combustion of biodiesel in a diesel engine typically leads to an increase in the
release of NOx if the fuel is burnt without additives (Figure 8; US EPA 2002a; Hansen et al., 2006;
McCormick 2007). For example, the tail-pipe emission of NOx from pure biodiesel (B100) is about
10-12% higher than that of PDD but, since the former has a very low sulphur content, it has been
suggested that NOx could be readily reduced through the use of a catalytic converter (Beer et al.,
2004; McMillen et al., 2005; Hansen et al., 2006).
- 35 -
Figure 10. Typical changes in the levels of NOx emissions according to percentage of biodiesel in
admixture with PDD (A) together with examples of variations according to the source of the
biodiesel (B). (Figure taken from US EPA, 2002b).
The release of NOx depends significantly on the source of the biodiesel (see Figure 10b).
Detailed analyses have shown that molecular the structure of the fatty acids in biodiesel has a
substantial impact on the emission of NOx, which increases with increasing numbers of C=C
double bounds (measured in terms of iodine number, figure 11a), decreases with hydrocarbon chain
length (data obtained using 18, 16, and 12 carbon chain molecules), and increases with cetane
number, which is correlated with un-branched open chain alkanes, figure 11b; (McCormick et al.,
2001).
Figure 11. Effects of iodine number (a) and cetane number (b) on emissions of NOx from biofuels.
(Figures taken from McCormick et al., 2001 with permission of the author).
A B
a b
- 36 -
The direct tailpipe-emission of particulates by biodiesel engines fitted with particulate filters is
reduced by as much as 20% compared with low-sulphur (< 50 ppm; LS) diesel and by ca. 50%
compared with fossil-sourced diesel (Beer et al., 2004), whilst the ranges of size of particles emitted
by PDD and biodiesel engines are roughly the same. However, in comparison with fossil fuel,
biodiesel emits 40-49% less of the sub-micron particles that are believed to deposit deeply in the
respiratory tract and cause respiratory diseases such as lung cancer (Chen and Wu, 2002). In this
respect, it might be considered that the emissions from biodiesel are potentially less detrimental to
humans than those of PDD.
In contrast to the above, it was found that the particles emitted from biodiesel engines typically
contain in the order of 40% more soluble organic material than petroleum diesel soot (Sharp, 1998;
Durbin et al., 1999). This increase in soluble organic fraction most probably results from the
incomplete combustion of fatty acid methyl esters in the plant oil, a situation that is exacerbated by
the fact that the tested engines were optimised for the combustion of PDD fuel (Syassen, 1998). A
lower production of particles with a higher concentration of soluble organic mater may impact on
the biologic effect and toxicity of biodiesel exhaust emissions.
In addition to impacts of regulated pollutants in biodiesel emissions, those of unregulated hazardous
air pollutants, such as the ―aggregated toxics‖ (i.e. acetaldehyde, acrolein, benzene, 1,3-butadiene,
ethylbenzene, n-hexane, naphthalene, styrene, toluene and xylene), have also been investigated. The
results of one study revealed that the mass ratio of total toxics to hydrocarbons actually increased
with the addition of biodiesel, thus, although the total hydrocarbon content decreased, the
concentration of unregulated pollutants in biodiesel emission increased (US EPA, 2002a). Quite
different results were reported in another study, however, where the content of these unregulated
aldehydes was found to be significantly lower in biodiesel exhaust compared with that of PDD
(Figure 12; Krahl et al., 2007a).
Figure 12. Specific emission rates of (a) aromatic hydrocarbons and (b) aldehydes and ketones in
particle extracts of, Swedish low sulphur diesel fuel (MK1), German biodiesel (RME), fossil diesel
fuel (DF), low sulphur diesel fuel with a high aromatic compound content and flatter boiling
characteristics (DF05). (Figures taken from Krahl, et al., 2007a).
a b
- 37 -
However, it should be mentioned that such a big discrepancy in the results is due to different
combustion conditions (as suggested by Krahl el al. 2007a), and because of systematic
methodological errors. The present reviewer (LP) further suggests that there are problems with the
methods of analysis used in these studies since they were apparently not validated for accuracy and
the recoveries of the rather volatile compounds were not estimated. In fact, no data relating to the
validation of the analytical methods employed can be found in the published articles (Krahl et al.,
2007, 2009; Buenger et al., 2000, 2006, 2007; Song et al., 2007). In these studies, all gases emitted
during combustion were collected in dichloromethane and the solution was concentrated by vacuum
evaporation prior to analysis using the Ames test. However, the boiling points of some of the
analytes (e.g. acrolein, 53°C) are very close to the boiling point of the organic solvent
(dichloromethane, 40°C,) whilst others [e.g. formaldehyde (-15°C), acetaldehyde (20°C)] are
actually lower. Obviously they will all evaporate to some extent together with the solvent and their
residual content in the final test sample will be very different from that of the original sample prior
to evaporation. This can lead to large deviations and discrepancies in the results. Consequently the
final results related to toxicity of emitted gases might be distorted and final conclusions might
underestimate the real mutagenic and toxic potential of the constituents of the original sample,
section 7.3.1.2 below.
7.3.1.2. Biodiesel exhaust emissions and human health. Overviews of recent studies of the impact
of biodiesel on air pollutant emissions and health effects are available (McCormick, 2007; Swanson
et al., 2007). However, more than two decades ago it was shown that most of the mutagenic activity
of biodiesel exhaust may be attributed to minor components of the soluble organic fraction,
particularly the PAHs (Mauderly 1987). Sub-chronic exposure of rats to emissions from a diesel
engine burning biodiesel derived from soybean oil induced a dose-related increase in particle-
containing alveolar macrophages (Mauderly, 1994; Finch et al., 2002). However, no, or little, lung
neutrophilia or centriacinar fibrosis were observed in the majority of the exposed rats, and no
significant differences between this group of animals and an air-exposed control group were
detected in respect of most of the health parameters evaluated (Finch et al., 2002).
In an earlier in vitro study of the mutagenicities of the soluble organic fractions of diesel fuel
exhausts, it was observed that the mutagenic effect, as determined by the traditional Ames assay,
was lower for RME biodiesel than for fossil diesel (Eckl et al., 1997). Ames genotoxicity test is
normally used to detect whether a substance is mutagenic and eventually carcinogenic. The
bacteria Salmonella used in this test can not normally grow in the absence of histidine, but after
exposure to a mutagenic substance inducing DNA changes , no longer need histidine in order to
grow. Usually two Salmonela strains are used (TA98 or TA100) with or without rat liver-derived
metabolic activation system (S9). Additionally, the mutagenic (Ames genotoxicity test) and
cytotoxic (assayed against mouse lung fibroblast line L929) effects of diesel engine exhausts from a
modern passenger car burning RME biodiesel or PDD fuel have been compared directly (Bünger et
al., 1998). In comparison with the control, particle extracts of both fuels induced significant
increases in the number of mutations, although for PDD fuel the revertants were significantly higher
compared with RME. The results of the study suggested a higher mutagenic potency of PDD
compared with RME, which was probably due to the lower content of PAC in RME exhaust,
although the emitted masses of RME were higher in most of the test procedures applied in this
study.
- 38 -
In a further detailed comparative study of the toxic potentials of biodiesel and PDD, Bünger et al.
(2000) assayed the particulate matter of diesel engine exhausts from four different fuels (RME,
SME, PDD and LS-PDD for their PAC content, and mutagenic and cytotoxic effects (Table 11).
Parameter PDD LS-PDD SME RME
Mutagenic effect 100 <100 <100 <100
Cytotoxic effect 100 400
PAH 100 <100 <100 <100
Particulate matter 100 ~50 100 ~50
Insoluble matter 100 <100 <100 <100
Sulphur 100
Table 11. Comparison of the main characteristics of PDD and LS-PDD fuels with those of plant-
derived biodiesels (SME and RME). (Table modified from Bünger et al., 2000).
The results indicated that diesel exhaust particles from RME, SME and LS-PDD contained less
carbon black and total PACs, indicating that they should be significantly less mutagenic in
comparison with PDD. Indeed, when the soluble organic fractions were assayed for mutagenic
effect using the Ames assay, the extract from PDD showed an activity that was 4-fold higher with
tester strain TA98 (and 2-fold with TA100) than that of the RME extract. It may be deduced,
therefore, that the lower mutagenic potency of biodiesel exhaust particles compared with fossil
diesel exhaust is likely due to lower emissions of PAC. High sulphur content of the fuel and high
engine speeds (rated power) and loads are also believed to be related to an increase in mutagenicity
of the diesel exhaust particles (Bagley et al., 1998; Bünger at al., 2000b). Despite higher total
particle emission, the solid particulate matter (or soot) in the emission from RME was less than in
those from PDD. While the size distributions and the numbers of emitted particles at "rated power"
were nearly identical for the two fuels, at "idling" speed PDD fuel emitted substantially higher
numbers of smaller particles than RME. However, Bünger et al. (2000a) reported that RME extracts
exhibit a 4-fold stronger toxic effect on mouse fibroblasts at "idling" speed, but not at "rated
power", than PDD extracts. The higher toxicity observed is probably caused by increased levels of
carbonyl compounds and unburned fuel. Results such as these illustrate the benefits, as well as the
disadvantages, for humans and the environment arising from the use of RME as a fuel in, for
example, light and heavy duty trucks and tractors.
Although combustion of the biodiesel blend gave rise to small reductions in the emission of most of
the aromatic and polyaromatic compounds in comparison with PDD, such differences were not
statistically significant at the 95% confidence level. On the other hand, the increase of 18% in the
amount of formaldehyde produced by the biodiesel was statistically significant. However, in vitro
toxicological assays showed that the overall mutagenic potencies and genotoxic profiles of the two
fuels were similar (Turrio-Baldassarri et al., 2004).
A number of economic incentives have recently led to an increase in the use of crude unmodified
vegetable oil as a fuel in Germany. In response to this growing tendency, a systematic comparison
of the mutagenic activity of cold-pressed crude rapeseed oil (RSO) with RME, natural gas derived
synthetic fuel (gas-to-liquid; GTL) and reference PDD has been performed on the basis of the
emissions from a heavy-duty diesel truck engine running the European Stationary Cycle, (Bünger et
- 39 -
al., 2007; Krahl et al. 2007c, 2009). PMs from the exhausts were sampled onto polytetrafluoro-
ethylene-coated glass fibre filters and extracted with dichloromethane in a Soxhlet apparatus, while
the gas phase constituents were sampled as condensates. The mutagenicities of both extracts and
condensates were assayed using Ames assay with tester strains TA98 and TA100. Compared with
PDD, the mutagenic effects of the RSO fuel extracts were significantly larger, showing a 9.7-fold
increase with strain TA98, and a 5.4-fold increase with strain TA100 (Figure 13; Bünger et al.,
2007; Krahl et al. 2007c, 2009).
Figure 13. Numbers of revertants in tester strains TA98 (upper panel) and TA100 (lower panel)
with or without rat liver-derived metabolic activation system (S9) (mean ± standard deviations of
quadruple tests) induced by particle extracts and the corresponding exhaust condensates per L of
exhaust following the combustion of diesel fuel (DF), natural gas derived fuel (GTL), rapeseed
methyl ester (RME), rapeseed oil (RSO) and preheated rapeseed oil (mRSO). TA98-S9" and
"TA98+S9" - tester strain TA98 with or without rat liver-derived metabolic activation system (S9)
Significance of the differences between mean values for DF compared with the other fuels were
analysed using two-tailed Student’s t-test for independent variables (* P < 0.01, ** P < 0.001, *** P
< 0.0001). (data taken from the table in Bünger et al., 2007).
Additionally, the condensate obtained from RSO fuel exhibited a 13.5-fold higher mutagenic
activity compared with the reference fuel. The mutagenicities of RME extracts were also
significantly higher than those of PDD in both TA98 assays with metabolic activation and in TA100
assays without metabolic activation, but the increases were more moderate than those observed with
RSO fuel. No significant differences in mutagenicities were observed between GTL samples and
those derived from PDD fuel. As was mentioned above, there was a big difference in
aldehydes/ketones emitted from rape seed oil compared with biodiesel, and this is apparently due to
the glycerol moiety that can be transformed into acrolein and other carbonyl-containing volatile
compounds. It might be speculated that some of these volatile compounds can be lost during rotary
TA98-S9
DF GTL RME RSO mRSO0
500
1000
1500Extracts
Condensates
*
*****
***
Fuel
Revera
nts
, m
S
D
TA98+S9
DF GTL RME RSO mRSO0
500
1000
1500Extracts
Condensates
**
***
***
***
**
Fuel
Revera
nts
, m
S
D
TA100+S9
DF GTL RME RSO mRSO0
500
1000
1500Extracts
Condensates
**
***
***
Fuel
Revera
nts
, m
S
D
TA100+S9
DF GTL RME RSO mRSO0
500
1000
1500Extracts
Condensates
**
***
***
Fuel
Revera
nts
, m
S
D
- 40 -
evaporation of the dichloromethane solutions prior to bioassay, since their recovery has not been
estimated in the course of preparation of analytical samples and the method was not validated for
the accuracy.
It should be mentioned that some unregulated volatile pollutants, such as formaldehyde,
acetaldehyde [lethal concentration 50 (LC50) 37 g/m3 in rats, lowest-observed-adverse-effect-level
(LOAEL) 728 mg/m3 in rats), and acrolein (LC50 3750 mg/m
3 in rats, LOAEL 4.6 mg/m
3 in rats),
although present in small amounts, are known to exhibit mutagenic and/or carcinogenic activities
(US EPA 2002b). Acetaldehyde forms peroxyacetyl nitrate (via formation of peroxyl radicals and
reaction with NO2), which has been shown to be a direct-acting mutagen toward S. typhimurium
strain TA100 (Kleindienst et al., 1985; Shepson et al., 1986).
To date, only one epidemiological study is available concerning the acute effects on health resulting
from exposure to biodiesel exhaust fumes (Hasford et al., 1997). In this investigation, a
questionnaire was applied to groups of workers who might be constantly exposed to diesel fumes,
i.e. delivery truck drivers, road-maintenance workers and industrial fork lift truck drivers. The
results revealed a range of dose-dependent respiratory symptoms following exposure to RME and
PDD fumes, but no significant differences between the two fuels could be established.
It conclusion it should be emphasised that:
the substitution of PDD by biodiesel fuel requires greater consideration than has been given
to date in respect of a number of important factors relating to human health,
biodiesel is usually employed in the form of a blend (containing between 2 – 20%) with
petroleum diesel, and this is somewhat unlikely to affect significantly the quantity and
composition of emissions and the potential biologic effects of the exhaust, since a negative
effect of blends has been observed with respect to the level of exhaust gas emission, whilst
an increase of mutagenic potency was been reported for such blends showing the maximal
mutagenic effect using B20 (20% of biodiesel and 80% of PDD), which must be considered
as a critical blend (Krahl et al., 2008),
numerous additives are present in biodiesel fuel and these may have significant effects on
human health,
low-quality biodiesels (for example, those that have received inadequate post-
transesterification refining) emit larger quantities of aldehydes and their exhaust emissions
may be associated with varying impacts on the indices of human health,
the use of pesticides on the crop plants with subsequent contamination of feedstocks could
possibly affect human health and this requires further investigation (Van Gerpen et al.,
2004).
Compared with emissions from PDD, those from biodiesel have been shown to contain less PM,
CO, total hydrocarbons and other toxic compounds including vapour-phase C1-C12 hydrocarbons,
aldehydes and ketones (up to C8), certain semi-volatile and particle-phase PAHs. Whilst sulphur-
containing compounds appear to be undetectable in biodiesel exhausts, NOx and soluble organic
fraction comprising unregulated pollutants including formaldehyde, acetaldehyde, acrolein,
benzene, 1,3-butadiene, ethylbenzene, n-hexane, naphthalene, styrene, toluene, and xylene, are
elevated. The mutagenic potential of these unregulated pollutants may have been underestimated
and should be carefully and extensively evaluated.
- 41 -
Toxicological studies have demonstrated the lower mutagenicity of exhaust particles from biodiesel
in general, although fuel derived from rapeseed oil presented toxic activity that was 4-fold higher
than that of PDD. Such increased toxicity is probably caused by the presence of carbonyl
compounds and unburnt fuel. The single epidemiological investigation carried out in humans
demonstrated dose-dependent respiratory symptoms after exposure to RSO or to PDD, but no
significant differences between the two fuels could be detected. The available data represent a cause
for deep concern regarding the future application of rapeseed oil as a replacement for fossil fuels.
7.3.2. Bioalcohol
7.3.2.1. Composition of bioalcohol exhaust.
Initial studies regarding the use of bioalcohol as an alternative fuel were rather encouraging and
provided evidence that the addition of alcohol to gasoline or diesel could reduce the formation of
regulated toxic compounds in the exhaust emissions (Schifter et al., 2001). Indeed, Hansen et al.
(2006) found that NOx emission was suppressed by the addition of 5% ethanol to biodiesel-PDD
blends. It was concluded that ethanol could act as an effective additive to reduce NOx emissions
from biodiesel (Hansen et al., 2006).
Arapatsakos et al. (2003) measured gaseous emissions (CO and hydrocarbons) and fuel
consumption when different ethanol-gasoline mixtures (containing 10, 20, 30 or 40% alcohol) were
burnt in an engine functioning at idle speed and under full load (1 kW). Significant decreases in
gaseous emissions were obtained with blended fuels, and these were roughly proportional to the
percentage of ethanol in the fuel mixtures. It has been shown that blending gasoline with ethanol
leads to a reduction in tailpipe emissions of CO by as much as 30%, volatile organic compounds by
up to 12%, and particulate matter by more than 25%. Since volatile organic compounds readily
form ozone in the atmosphere, the use of ethanol could play an important role in smog reduction.
Ethanol 85 (E- 85) is a commercially available alternative fuel that contains 85 percent ethanol and
15 percent unleaded gasoline. Sheehan et al. (2004) demonstrated that burning E-85 instead of
gasoline reduced the emissions of CO, NOx and total hydrocarbons. However, whilst some authors
support the view that ethanol reduces total hydrocarbon emissions (typically by between 2 and
17%) (Hsieh et al., 2002; Al-Hasan, 2003), others have reported increases in total hydrocarbon
emissions by using ethanol (Poulopoulos et al., 2001; Magnusson et al., 2002).
As for the unregulated pollutants, the results of recent studies are rather frustrating. Thus, the
tailpipe emissions of aldehydes (including formaldehyde, acetaldehyde, acrolein , acetone,
propionaldehyde, crotonaldehyde, methylacrolein, mutyraldehyde, benzaldehyde , o-tolualdehyde,
m-tolualdehyde, valeradehyde, hexaldehyde, dimethylbenzaldehyde) from E10 ethanol-blended
gasoline (containing 10% of ethanol) and ethanol-biodiesel were compared with those from
gasoline and diesel (Pang et al., 2008). Total aldehydes from E-10 varied in the range of 9.2-
20.7 mg/kW.h, values that were 3.0 – 61.7% greater than those from gasoline emissions. The total
aldehyde emissions from ethanol-biodiesel were 1–22% higher than those from PDD under
different engine conditions. Compared with fossil fuels, E-10 presented a slight reduction in CO
emission whilst ethanol-biodiesel showed a substantial decrease in PM emission. However, both
alternative fuels exhibited slight increases in NOx and acetaldehyde emissions.
- 42 -
The problem is that the incomplete combustion of alcohols (oxidative transformations below) in an
internal-combustion engine generates a number of intermediate toxic compounds, e.g. acetaldehyde
and formaldehyde, which have been found in the exhaust emissions [Pang et al., 2008].
[O] [O] [O] [O]
CH3OH HCHO HCOOH CO CO2
Methanol formaldehyde formic acid carbon monoxide carbon dioxide
[O] [O] [O] [O]
C2H5OH H3CCHO H3CCOOH CO CO2
Ethanol acetaldehyde acetic acid carbon monoxide carbon dioxide
Ethanol and methanol are aldehyde precursors and could be partially combusted to form
formaldehyde or acetaldehyde, which are considered to possess actual or potential carcinogenic
properties, respectively (International Agency for Research on Cancer IARC, 1985). Moreover, in
the atmosphere, acetaldehyde increases formation of highly toxic ozone, which can significantly
increase mortality, hospitalization, emergency room visits (Jacobson 2007) and peroxyacetyl nitrate
(PAN), which is known as a potent carcinogen.
Winebrake et al. (2001) estimated toxic emissions of vehicles operating on a variety of fuels,
including conventional gasoline (CG), reformulated gasoline (FRFG), conventional diesel (CD),
conventional natural gas (CNG), ethanol (E85) , methanol (M85), liquefied petroleum gas (LPG)
and electricity (EV). A version of Argonne National Laboratory’s Greenhouse Gas, Regulated
Emissions, and Energy Use in Transportation(GREET) model was used for evaluation of a total
fuel-cycle analysis that calculates emissions of the four most common unregulated mobile air toxics
(acetaldehyde, benzene, 1,3-butadiene, and formaldehyde) from both downstream(e.g., operation of
the vehicle) and upstream (e.g., fuel production and distribution) stages of the fuel cycle. For almost
all of the fuels studied (except of ethanol) , emissions of volatile organic compounds (VOC) were
reduced in comparison with conventional gasoline (Figure 14 c). On the other hand, the addition of
ethanol to gasoline (as in E85) or to RFG was found to lead to a significant increase in acetaldehyde
and formaldehyde emissions (Figure 14 a, b), and the use of methanol or compressed natural gas
(CNG) may result in increased formaldehyde emissions. Considering the emission levels of the four
air toxics (i.e. benzene, 1,3-butadiene, acetaldehyde, and formaldehyde), together with their cancer
risk factors, the authors concluded that all of the alternative fuels and vehicle technologies
presented emission reduction benefits (Winebrake et al., 2001).
In a similar study Jacobson (2007) used a combination of US inventory field data and modeling to
conclude that the high acetaldehyde, formaldehyde, ozone and PAN can be significantly higher in
USA due to the use of ethanol-gasoline blend (E85) instead if conventional gasoline (figure 15 )
- 43 -
Figure 14. Changes in fuel-cycle of: (a) - VOC, (b) - formaldehyde, and (c) - acetaldehyde
emissions for varieties of mobile fuels {reformulated gasoline (FRFG), conventional diesel (CD),
conventional natural gas (CNG), ethanol (E85) , methanol (M85), liquefied petroleum gas (LPG)
and electricity (EV)} relative to conventional gasoline (CG) vehicles. Figures taken from
Winebrake et al., 2001with permission of the author.
.
a b
c
- 44 -
Figure 15. Modeled differences in the August 24-hour average near-surace mixing ratios of (b)
acetaldehyde, (c) formaldehyde, (f) ozone, and (g) PAN in Los Angeles and the United States when
all gasoline vehicles (in2020) were converted to E85 vehicles. Figures taken from Jacobson 2007
with permission of the author.
In conclusion, whilst substantial reductions in PM from ethanol-biodiesel and in CO from E-
gasoline could be achieved, the significant augmentation of carbonyl emissions from the alternative
fuels should arouse public concern because of their potential effects on human health and the
environment
- 45 -
7.3.2.2. Toxicological studies of exhausts from bioethanol and other alcohol-blended fuels.
In fact, very few comparative investigations have focused on the toxic emissions from vehicles
running on alternative fuels.
Comparative acute and chronic toxicity studies have been carried out in which rodents were
maintained for periods of 5 weeks in chambers into which exhaust fumes from gasoline and ethanol
engines were introduced (Massad et al., 1985, 1986). The results showed that the acute toxicity (in
terms of LC50) and the chronic toxicity (assessed in terms of various biological parameters
including pulmonary function and mutagenicity along with haematological, biochemical and
morphological examinations) of gasoline exhaust was significantly higher than that of ethanol.
Ethanol-based blends
The first attempt to evaluate the toxicities of the particulate phases of exhaust emissions from
vehicles fuelled with gasoline and alcohol-mixed fuels was based on the cellular test system known
as the brown-fat-cell (or BFC) test (Bernson, 1983). The results were evaluated by analysis of
variance (ANOVA) with multiple comparisons, and revealed that the exhaust extracts from cars
running on commercially available gasoline were significantly more toxic to cellular oxygen
consumption than equivalent extracts derived from alcohol-mixed fuels. The lowest toxicities were
found in extracts obtained from exhaust gases that had been submitted to catalytic processing.
However the results of two latest studies are somewhat confusing (Song et al., 2007; Jacobson,
2007). One of them is an experimental study (Song et al., 2007), while the second one is based on
field data, simulation of mathematical/ computerized models and cancer risk factors values for the
variety of pollutants (Jacobson, 2007). U.S. Environmental Protection Agency (EPA) estimated
Cancer Unit Risk Estimate (CURE) values are for acetaldehyde 2.7 x 10–6 (µg/m3)–1; benzene 2.9
x 10–5 (µg/m3)–1; 1,3-butadiene1.7 x 10–4 (µg/m3)–1; and formaldehyde 6.0 x 10–6 (µg/m3)–
1;Diesel PM - 3.0 x 10–4 (Winebrake et al., 2001).
In has been shown that the gradual addition of ethanol to diesel (from 0 to 20% ethanol) increases
mutagenic activity of the exhaust emissions (Song et al., 2007). The experiments were conducted
using a heavy-duty diesel engine, and the five fuels tested were E0 (pure diesel fuel), E5 (diesel +
5% ethanol), E10 (+ 10% ethanol), E15 (+ 15% ethanol) and E20 (+ 20% ethanol). Regulated
emissions (total hydrocarbons, CO, NOx and PM) as well as PAH emissions were monitored, and
the Ames and comet assays were used to investigate the mutagenicity and genotoxicity of these
particulate extracts, respectively. The comet assay is a method of detection of DNA fragments
(visible like comets when analyzed by gel electrophoresis) indication on DNA damages in isolated
cells. It was shown that total hydrocarbons and CO emissions significantly increased in parallel
with the ethanol content in the blends up to 53.1% and 70.0% respectively (E20 vs E0). Similarly,
a significant increase of PAH (Figure 16) in the exhaust emitted from E20 was detected by GC-MS
in comparison to E0. Additionally, E20 showed the highest mutagenic effect with both TA98 and
TA100 tester strains (with or without metabolic activation) while E5 was the least mutagenic
(Figure 17). Data relating to DNA damage revealed the lower genotoxic potencies of E10 and E15
overall (Song et al., 2007).
- 46 -
Figure 16. The content (µg/kWh) of various PAH and their total amount in exhaust emissions from
ethanol-gasoline blends. Data from table 3 published in the article Song et al., 2007.
Figure 17. Numbers of brake specific revertants (1000 reverants/kWh) in tester strains TA98 and
TA100 with rat liver-derived metabolic activation system (S9) (mean ± standard deviations of
quadruple tests) induced by extracts (0.1 mg/plate) of exhaust emissions from ethanol-diesel blends
(E0 (pure diesel fuel), E5 (diesel + 5% ethanol), E10 (+ 10% ethanol), E15 (+ 15% ethanol) and
E20 (+ 20% ethanol). Significance of the differences between mean values for DF compared with
the other fuels were analysed using two-tailed Student’s t-test for independent variables (* P < 0.01,
** P < 0.001, *** P < 0.0001). Figure modified from Song et al., 2007.
It should be mentioned, however, that such an erratic concentration-effect curves might be due to
different losses of volatile mutagenic compounds (presumably aldehydes) during the evaporation of
dichlormethane solutions in the course of preparation of analytical samples for Ames test.
In a model stimulation study, assuming that E85 will partially replace gasoline in 2020, Jacobson
(2007) estimated that this replacement may be associated with an increase in asthma and O3-related
E0 E5 E10 E15 E200
50
100
150E0
E5
E10
E15
E20
Ethanol, %
To
tal
PA
H,
g/(
kW
h)
0 5 10 15 20 250
5
10
15Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Anthracene
Phenanthrene
Pyrene
Fluoranthene
Benzo(a)anthracene
Chrysene
Benzo(a)pyrene
Benzo(b)fluoranthene
Benzo(k)fluoranthene
Benzo(g,h,i)perylene
Dibenz(a,h)anthracene
Indeno(1,2,3-cd)pyrene
Ethanol, %
PA
H,
g/(
kW
h)
0 5 10 15 200
100
200
300
400 TA98 + S9 TA100 + S9
Ethanol, %
BS
R,
revera
nts
10
3/k
W h
- 47 -
hospitalisation and mortality. In Los Angeles, E85 was apparently responsible for a 9% growth in
O3-related effects, but the percentage increase for the US as a whole was only 4% since the rises in
the northeast were partially offset by decreases in the southeast. Moreover, it has been shown that
no benefits can be expected in reductions of population-weighted number of cancer cases relative to
gasoline, suggesting that E85 may result in cancer rates similar to those of gasoline
("40/yrinLosAngelesand"430/yr in the U.S. E85 also led to an increase in the emissions of
peroxyacetyl nitrate, but it was estimated that this would cause little change in cancer risk. Because
of its ozone effect, the use of E85 may present a greater overall public health risk than gasoline.
Additionally, the emission of unburnt ethanol from E85 may result in a global-scale source of
acetaldehyde that is greater than that derived from direct emissions. However, since future emission
regulations relating to E85 are currently uncertain, it could only be concluded with confidence that
E85 is unlikely to improve air quality over future gasoline vehicles.
In conclusion, it would seem that harmful effects as well as distinct benefits must be expected to
accrue from the use of ethanol fuel blends.
Methanol-based blends
The exhaust from gasoline-fuelled engines induced DNA and chromosomal damage, thus indicating
a distinct genotoxicity, while emissions from vehicles burning methanol did not exhibit such
genotoxic potential (Zhang et al., 2005b). Gasoline exhaust showed mutagenicity in the Ames assay
(with strain TA98) with or without metabolic activation (S9), although when S9 was used
mutagenicity increased and a good dose-response relationship could be established. In contrast,
methanol exhaust presented no mutagenic potential in Ames assay (with strains TA98 or TA100)
with or without S9. With respect to in vitro A549 cell micronucleus mutagenicity tests, no
significant differences were detected between exhausts from methanol-fuelled vehicles and the
control, whilst the test was positive for gasoline exhausts. The results strongly suggested that
gasoline-fuelled vehicle exhaust was potential mutagenicity whilst that from methanol-fuelled
engines was not (Zhang et al., 2005a).
Detailed study of the genotoxicity and cytotoxicity of the organic extracts of condensate, PM and
semi-volatile organic compounds emitted by gasoline and absolute methanol-fuelled engines were
examined using Ames test and methyl-thiazol-tetrazolium, micronucleus and Comet assays (Zhang
et al., 2007). The results revealed that gasoline engine exhaust increased micronucleus formation in
human lung carcinoma A549 cells, induced DNA damage in A549 cell lines and increased TA98
revertants in a concentration-dependent manner in the presence of metabolic activating enzymes. In
contrast, methanol engine exhaust did not to exhibit these adverse effects suggesting that methanol
may represent a cleaner automobile fuel.
The effects of diesel fuel blended with methanol-containing-additive (MCA) on the toxicities and
genotoxicities of the exhaust emissions have been investigated by Lin et al. (2002). Five blends of
diesel fuel containing 0, 5, 8, 10 or 15% by volume of MCA were tested, and the results of
Microtox assays indicated that fuels with additive presented moderately lower toxicities of particle-
associated samples, but generally higher vapour-phase associated toxicities. Fuels containing 5 or
8% of additive showed relatively lower genotoxicities according to the Mutatox test in comparison
with those of base diesel.
- 48 -
A recently conducted health assessment on rodents of the combustion emissions of a diesel fuel
emulsion comprising diesel-water-methanol was consistent with results obtained with PDD exhaust
(Reed et al., 2006). Animals were exposed to exhaust atmospheres for 61 to 73 days at the rate of 6
h/day for 5 days/week for the first 11 weeks and for 7 days/week thereafter. Indicators of general
toxicity (body and organ weight, and clinical, pathological and histopathological examinations),
neurotoxicity (glial fibrillary acidic protein assay), genotoxicity (Ames assay, micronucleus assay
and sister chromatid exchange) and reproduction and development were monitored. The outcomes
of the combustion emissions of the emulsion were mild, and no significant reproductive or develop-
mental toxicity, neurotoxicity or in vivo genotoxicity effects were observed. However, some
emission sub-fractions did exhibit positive mutagenic responses in several strains of S.typhimurium.
The presence of methanol in a fuel blend has no significant effect on the rates of emission of
regulated exhaust components (organic material, CO or NOx) or on the composition of total
hydrocarbons, with the exception of methane which is increased significantly (Gabele, 1990).
However, as the percentage of methanol in the blend rises, formaldehyde and methanol account for
increasingly larger proportions of the organic material while hydrocarbons comprise less.
In conclusion, the combustion of alcohol-containing fuels leads to reduced formation of
photochemical smog in comparison with gasoline or diesel, although the emission of potentially
carcinogenic aldehydes may be several-fold higher. The toxicity of the emission from a gasoline-
fuelled engine is significantly higher than that from an engine fuelled by alcohol, however, some
harmful effects from alcohol blends must be expected including increased emissions of
carcinogenic PAHs and an increase in O3-related toxicity associated with the emission of high
levels of aldehydes. Moreover, ethanol-diesel fuel blends could increase the emission of unburnt
hydrocarbons and aldehydes.
The most preferable fuels in terms of reduced toxicity and genotoxicity in comparison with PDD
exhaust emissions are those containing methanol, however, this alcohol represents a source of
formaldehyde pollution and is potential carcinogenic.
7.3.3. Biogas
The methane component of biogas would normally suffer incomplete combustion in an internal-
combustion engine and, hence, the following toxic compounds could be present in the exhaust:
[O] [O] [O] [O] [O]
CH4 CH3OH HCHO HCOOH CO CO2
Methane methanol formaldehyde formic acid carbon monoxide carbon dioxide
In addition, nitrogen can be converted into nitric oxides and hydrogen sulphide into sulphur oxides:
[O] [O]
N2 NO NO2
[O] [O] [O]
H2S SO SO2 SO3
- 49 -
Very few publications are available concerning the toxic effects of biogas-derived emissions.
Although CNG is often regarded as a clean-burning fuel, characterisation of the emissions from
vehicles fuelled by CNG has revealed that the exhaust contains potentially toxic materials (Lapin et
al., 2002; Ristovski et al., 2004; Seagrave et al., 2005). In comparison with diesel-fuelled vehicles,
those burning CNG appear to produce less particulate matter but more total hydrocarbons
(McCormick et al., 2000). In contrast, Ristovski et al. (2004) demonstrated that conversion of an
engine from gasoline fuel to CNG led to a significant reduction in the emissions of total PAH and
formaldehyde but had no effect on the emission of particles.
The potential health risks of particulate exhaust emissions from natural gas fuelled vehicles have
been investigated by Lapin et al. (2002). Organic solvent extracts of PMs from natural gas fuelled
heavy-duty engine tested positive against Salmonella strains TA98 and TA100. The mutagenic
activity of natural gas-derived particulate matter was similar to that of the PM from a light-duty
diesel vehicle.
Seagrave et al. (2005) compared the composition, mutagenic potential and acute lung toxicity of the
collected emissions from heavy-duty vehicles fuelled by CNG and PDD. One important outcome of
the study was that it confirmed and extended previous data (Lapin et al., 2002) relating to the
organic extracts of the PM fraction of a CNG-powered heavy-duty truck and showed that such
emissions contained substantial amounts of direct-acting mutagens. Vapour-phase semi-volatile
organic compounds and PMs were collected from three CNG-fuelled buses, powered by three
different engines including a new technology engine with an oxidative catalytic converter. The
emissions were analysed chemically and their mutagenicities were assessed using the Salmonella
reverse mutation assay. The emission samples were also compared with those collected from
normal and ―high-emitter‖ gasoline and diesel engines. While the range of mutagenic potencies of
the CNG emission samples was similar to those determined for the gasoline and diesel emission
samples, lung toxicity potency factors were generally lower than those for gasoline and diesel
samples (Seagrave et al., 2005). Among the engines tested, those with oxidative catalytic converters
showed lower mutagenic effects.
It may be concluded that the use of biogas, and particularly CNG, can significantly reduce the
emissions of total PACs and also diminish lung toxicity potency. However, the emission of PM and
the mutagenic potencies of the exhaust gases are similar to those found in gasoline and diesel
emission samples. These findings require follow-up studies in order to develop a database relating
to natural gas fuelled vehicles that would allow informed decisions to be made on the use of
alternative fuels to reduce air pollution health risks.
7.3.4. Other biofuels.
Handling cellulosic (second generation) biofuels such as straw, wood chips etc may release dust
particles that contain high concentrations of hazardous micro-organisms, thus representing a
significant potential occupational health problem. It has been found that the storage of such biofuels
outdoors over summer months increases the microbiological contamination (or dustiness) and
should therefore be avoided. The bacterial dustiness of straw is much greater than of wood chips,
whereas the fungal dustiness does not differ much (Sebastian et al., 2006). Moreover, ecological
straw generally contains less microbe-laden dust than conventional straw and is hence the preferred
- 50 -
feedstock. Samples taken from the inner part of biofuel materials are typically dustier than samples
taken from the surface, except for fungal and bacterial biomass in wood chips and total fungi and
fungal biomass in ecological straw.
8. Discussion and overall conclusions
It is very good idea to use a renewable energy source, but it does not necessarily mean that such
fuels would necessarily be "healthy for humans‖. At present, the use of biofuels is favourably
viewed and there are suggestions that its emissions are less hazardous to human health compared
with petroleum gasoline and diesel. As shown by an advertisement in Arlanda airport in Stockholm
(Figure 18), biofuel is currently believed to be ―clean enough to drink‖.
However, the less harmful effects of biodiesel on human health is speculative and far from reality.
Indeed, the combustion of biofuel alone or blended with petroleum fuel produces toxic emissions
that are, in many ways, similar to those present in the exhaust of petrofuels. Thus, alongside the
regulated toxic emissions, such as total hydrocarbons, CO, NOx, PM and PAH, other non-regulated
emissions, especially carbonyls (i.e. formaldehyde, acetaldehyde and acrolein), are also present in
the exhaust from burnt biofuel and these compounds are potentially detrimental to human health.
Figure 18. An advertisement in Arlanda airport in Stockholm
It might be reasonable to classify biofuels into different categories depending on their toxicity:
A. crude or refined vegetable oil, which should not be recommended for use as fuel due to the
extremely high mutagenicity of the exhaust.
B. biodiesel, which has substantially lower amounts of carcinogenic and mutagenic PAH than
PDD,
C. biogas, which can significantly reduce the emissions of PAH, but not the emission of PM
and the mutagenic potencies of the exhaust gases which is comparable to those found in
gasoline and diesel emission.
D. ―pure‖ bioalcohol fuels, which do not emit highly toxic PAH and particulates, but still
contain mutagenic and carcinogenic non-regulated pollutants
- 51 -
A serious problem of biodiesel is associated with NOx levels, which are normally enhanced (up to
15%), and especially with carbonyls (e.g. acetaldehyde), which can rise substantially.
In general, the safety/toxicity of highly volatile components of biofuel exhaust has not yet been
evaluated accurately. A substantial portion of these compounds can be lost during the treatment of
filters and condensates for the non regulated pollutants assay and the Ames test. Moreover, the
overall recoveries of these compounds have not been evaluated and the accuracy of the sample
preparation method has not been validated. Hence, it could be that the cytotoxic effect of biodiesel
exhaust is higher than that reported in several in vitro and animal studies (Buenger et al., 2000,
2006, 2007; Krahl et al., 2009; Song et al., 2007).
There is a great deal of additional research required in relation to these components, as well as on
the fuel blends (i.e. mixtures of PDD with up to 20% biofuels) that are currently promoted by
policies in Europe and the USA.
Additionally, toxicological studies in humans are lacking. According to the knowledge of the
present reviewer only one epidemiological study, involving a very small cohort of road workers, is
available and this has reported irritation of mucosal membranes and an unpleasant odour (similar to
old cooking oil) arising from the emissions of vehicles fuelled with biodiesel (Hasford et al., 1997).
Unfortunately, much less information is available concerning the other biofuels.
Finally, it can be concluded that:
Biofuels are potentially less harmful to human health than petroleum fuels, but may be
considerably more toxic than expected (Figure 19),
substitution of conventional fuels for biofuels in on-road engines decreases the
concentration of regulated toxic pollutants in car exhaust emissions but increases the
concentration of some non-regulated pollutants especially with carbonyls (e.g.
acetaldehyde), which can rise substantially (Figure 19),
the safety/toxicity of highly volatile components of biofuel exhaust has not been yet
accurately evaluated, and detailed toxicological studies of biofuels in humans are lacking,
from the information that is available, it appears that the toxicity of biofuels decreases in the
order biodiesel>biogas>ethanol=methanol,
on this basis, methanol (produced by the oxidation of biogas) is the alternative fuel that
appears to present the least potential for damage to human health, however, this alcohol
represents a source of formaldehyde pollution and is potential carcinogenic.
With respect to the future availability of resources for the production of biofuels (e.g. biodiesel), it
should be mentioned that these are rather limited as well. Thus, an area of about 20 000 square km
(i.e. two million ha), if cropped with the plant sources currently available, would provide a
contribution of just 25% of the fuel actually used in Germany: in this context, the total territory of
Germany comprises 357 021 square km) (Munack and Krahl, 2007).
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Figure 19. A simplified and presumably more realistic advertisement of the effect of petrofuel and
biofuel emission on human health, partially reflecting my conclusions above.
This opinion/conclusion is in line with the conclusion of Swedish scientists from Chalmers
University in Gothenburg, where the authors stated that "Hitherto, the disadvantages of renewable
products have been neglected in research and development. The advantages of renewable products
are advocated strongly by their proponents urging for a quick and subsidized market introduction."
(Pedersen et al., 1999).
9. Acknowledgements
The author wishes to thank Halmstad University, and especially Professor Lars-Gunnar Franzén for
helpful critical comments and Professor Stefan Weisner for the opportunity to carry out further
study and to research the subject area of this thesis.
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