IncinerationIncineration is a method of disposing of waste by burning it.

Incineration usually functions as an

alternative to other disposal methods,

especially landfilling. Incineration reduces the

overall volume of the waste stream and,

especially for hazardous wastes, is intended to

reduce the wastes' toxicit and other hazardous

characteristics. It is particulary popular in

countries such as Japan where land is a scarce

resource. Incineration is often described as

Thermal Treatment.

Incineration and Waste Management

For the Environmental engineer today Incineration may be used as one of a variety

of tools to design and implement a waste management strategy.

The basic aim of pro-environmental waste management policies is generally to

reduce the impact on the environment by reducing the sum of energy used and

emissions to the environment. In the waste management policies of the EU,

Incineration comes in step four and five of the hierarchy of waste strategies:

1: Reduce waste at source

2: Reuse products (no or minimal processing)

3: Recyle

4: Dispose with energy recovery

5: Dispose without energy recovery

Aspects of Incineration

Incineration does not divide neatly into separate categories but rather several

factors influence the design of incinerators and how harmful they are to the

environment. These aspects are discussed in greater depth below:

Scale

The scale of incineration can vary hugely from burn barrels for individual houses

to large municipal waste incinerators catering for whole cities.

Small scale describes people burning household waste in small quantities. It is

used most in areas with regular municipal waste collection. The low temperatures,

lack of control and lack of montitoring mean that this form of incineration is

generally the most polluting per kilo of waste burned.

Medium scale incineration is usually for commercial, industrial or institutional

waste. For example large hospitals may use an on-site incinerator to dispose of

medical waste. The type of waste such incinerators burn is often highly specific.

Large scale incineration is usually for municipal waste. Such incinerators deal

with 100 to 1000 tons of waste per day. The waste they burn is mixed and its

characteristics (e.g. water content and calorific value) may vary significantly.

Waste Types

Most waste generated by human activity can be burnt in an incinerator.

Municipal Waste

Municipal Waste is the largest source of incinerator fuel. It may be sorted or

unsorted waste.

Sorted municipal waste, has as the name suggests, been sorted to remove some

forms of waste. In developed countries, particularly the EU and Japan, this usually

means that recyclable material is removed by residents or by a company before

waste is burnt.

Valuable materials, like steel, aluminum, paper and some types of plastics are

removed, materials that are not economically recyclable, such as polystyrene,

paper towels and wax-coated paper are sent for incineration.

Garden waste and compostable material is also often removed prior to

incineration. The high water content of compostable material means that it reduces

the efficiency of the incinerator.

Unsorted municipal waste on the other hand has not had potentially valuable

material removed from it.

Medical waste

Medical waste incineration entails the combustion of a waste stream with special

biological contamination risks. One of the major problems with medical waste is

that there are many relatively-small and under-regulated, under-supervised

combustion sites.

Chemical Weapons

Chemical weapons are also incinerated. For example, the U.S. has thousands of

tons of sarin stockpiled during the Cold War era, as a countermeasure against even

larger stockpiles of the Warsaw pact. Having signed the international ban, U.S.

Government has been trying to destroy that sarin by incineration, but the

destruction is hampered by the environmental concerns about incineration.

Furnace Types

An incinerator may utilise any of a number of furnace types. The selection of

furnace type will depend on the quantity and type of waste the incinerator will be

dealing with.

To select a furnace type the calorific value of the waste to be burned needs to be

ascertained. This will vary with the type of waste to be burned (medical vs

municipal)and also with the particular city and the level of sorting carried out.

Rotary Kiln

A rotary kiln, which comprises a rotating oven and a post-combustion chamber,

may be specifically used to burn chemical wastes, and is also suited for use as a

regional health-care waste incinerator. Main characteristics of rotary kilns are

summarized as follows:

Adequate for the following waste categories:

- Infectious waste (including sharps) and pathological waste.

- All chemical and pharmaceutical wastes, including cytotoxic waste.

Inadequate for the following wastes:

- Non-risk health-care wastes - incineration in rotary kilns would represent a waste

of resources.

- Radioactive waste - treatment does not affect radioactive properties and may

disperse radiation.

Wastes should not be incinerated:

- Pressurized containers - may explode during incineration and cause damage to

the equipment.

- Wastes with high-metal content -

incineration will cause emission of

toxic metals (e.g. leads, cadmium,

mercury) into the athmosphere

Incineration temperature: 1200-

1600°C, which allows

decomposition of very persistent

chemicals such as PCBs

(polychlorobiphenyls)

Incinerator capacity: Available

capacities range from 0.5 to 3

tonnes/hour.

Exhaust-gas cleaning and ash treatment equipment: Likely to be needed, as the

incineration of chemical waste produces exhaust gases and ashes that may be

loaded with toxic chemicals.

Additional remarks: Equipment and operation costs are high, as is energy

consumption. Wases and incineration by-products are highly corossive, and the

refractory lining of the kiln often has to be repaired or replaced. Well trained

personnel are required.

The axis of a rotary kiln is inclined at a slight angle to the vertical (3-5% slope).

The kiln rotates 2 to 5 times per minute and is charged with waste at the top.

Ashes are evacuated at the bottom end of the kiln. The gases produced in the kiln

are heated in high temperatures to burn off gaseous organic compounds in the

post-combustion chamber and typically have a residence time of 2 seconds.

Rotary kilns may operate continuously and are adaptable to a wide range of

loading devices. Those designed to treat toxic wastes should preferably be

operated by specialist waste dispocal agencies and should be located in industrial

areas or "parks".

Fluidised Bed

Cement kilns

A versatile type of incinerator is the cement kiln, whose main product is Portland

cement. A cement kiln is a rotating cylinder, almost horizontal but slightly

inclined, with the upper end continuously fed with a mixture of clay and lime or

limestone, and the lower end fed with burning fuel. The temperature of thousands

of degrees causes the lime and clay react chemically, and a continuous stream of

the white-hot molten portland cement flows from the lower end of the cylinder.

Cement kilns benefit from this fact in their auxiliary role of incinerators, since by

their nature, they combine incineration with scrubbing those inorganic gases.

Kilns convert and lock those gases and ashes into mineral products. This is done

by the molten cement and lime covering the entire walls of the rotating kiln. The

alkaline properties of the hot, molten mixture neutralize those gases. A

conventional incinerator would produce sulfur dioxide, or when equipped with a

scrubber, harmless but cumbersome by-products. A cement kiln converts that

sulfur dioxide into the mineral of gypsum, later locked in the portland concrete.

The chlorine becomes less toxic calcium chloride, the phosphorus becomes the

mineral of apatite, fluorine becomes the mineral of spat, and other similar

reactions occur. The only remaining gaseous contaminants that leave a cement

kiln are small amounts of nitrogen oxide, and residues of dioxins.

Energy Recovery

A modern incinerator will nearly always encompass some form of energy

recovery. The two principle methods of energy recovery used are electricity

generation and municipal heating. One incinerator may combine the two methods

depending on the demands of the local area.

Electricity Generation

This is where the furnace of the incinerator is used to power an electricity

generation plant (power plant).

Municipal Heating

This is where the furnace of the incinerator is used to generate steam and hot water

which is pumped directly to businesses/institutions and houses in the vicinity of

the incinerator. The hot water is then used to heat the buildings.

Emissions from Incineration

Incineration generates several forms of waste itself, such as the emission of

unburned gases and metals in, the hazardous secondary products of combustion,

and leftover ash. The health hazards associated with these emissions and

incinerator wastes are the subject of intense controversy.

Solids

The volume of solids or ash left after incineration is usually from 30% to 10% of

the original quantity of waste. The ash is far more concentrated with pollutants

than the original waste. The ash is often regulated as a hazardous waste itself and

must be landfilled.

This concention of pollutants can allow otherwise unrecoverable metals to be

recycled. The portion of metals that could not be separated prior to combustion are

periodically removed from the boilers and sent to foundries for recycling.

Gases

The exhaust gases produced as by products of incineration are a major source of

concern. Among the unintended pollutants caused by incineration are dioxins and

furans, which are the subject of ongoing study and debate.

The quantity of hazardous substances in Incinerator exhaust gases is reduced by a

technique known as scrubbing

Incinerator Toxic

Emissions:The province of Ontario, Canada has banned construction of solid waste

incinerators since 1992 (1). The Clinton administration may soon do the same (2).

Alternatives are being actively explored, including recycling, waste-reduction, and

re-use of materials (3-6). Nonetheless, some municipalities still consider waste

incineration as an option. Definitive studies on the effects of some of the

substances and amounts emitted are reported to be lacking in the relevant

scientific literature

Incinerator-specific studies of human effect are rare. A significant correlation is

reported between the purchase of bronchodilators, expectorants, and cough

medicines and closeness to a municipal incinerator (7). Winter deaths in London,

England, varied significantly with air levels of total particulate matter, SO2, and

acidic aerosol SO2 and particulate matter are common incinerator emissions (6,

7). Elevated levels of NO2 emitted from an industrial facility caused a significant

increase in bronchitis among children exposed for 2 and 3 years; illness varied

significantly with distance from the plant (9). NO2 is also found in incinerator flue

gases, and is a major problem worldwide due to its automobile and industrial

emissions (9-14). There is a report of a four-fold increase in lung ailments in

Canada since 1970 (15). Childhood asthma rates near the Toronto Western

Hospital, which imports hazardous waste from other hospitals to burn, are reported

among the highest in the world (8). A significant correlation has been shown

between car ownership and leukaemia (16), and the authors speculate that

background exposure to benzene is an explanation for the large increase in

leukaemias of recent decades. Benzene is among the most common incinerator

toxic emissions

  

SPECIFIC INCINERATOR POLLUTANTS

Hydrocarbon Emissions: The U.S. Environmental Protection Agency (EPA) has

reportedly done field tests of eight incinerators, nine industrial boilers, and six

industrial kilns, and showed measurable amounts of 55 hazardous chemicals from

their list (“Appendix VIII”) of ~400 known hazardous compounds (6). Most

common were benzene, toluene, carbon tetrachloride, chloroform, methylene

chloride, trichloroethylene, tetrachloroethylene, chlorobenzene, naphthalene,

phenol, and bis(2-ethylhexy)phthalate. Concentrations ranged 5 orders of

magnitude between chemicals and between different measurements at the same

facility (6), so predictions of average or total output are problematic. Other

investigators are reported finding multiple emissions from MSW incinerators

including PAHs (polycyclic aromatic hydrocarbons), PCBs (polychlorinated

biphenyls), PHAHs (polycyclic halogenated aromatic hydrocarbons), halogenated

organic acids, phthalates, aldehydes, ketones, organic acids, alkanes, and alkenes.

Examples noted include chlorobenzenes and chlorophenols, naphthalene,

benzo(a)pyrene, anthracene, flouranthene, pyrene, and phenanthrene, as well as

highly toxic PCDDs and PCDFs (polychlorinated dibenzo-p-dioxins and -furans)

The presence of these compounds intact would indicate the incinerators were

operated under conditions inadequate for their complete combustion (7). Many are

formed in the stack and are thus termed PIC (products of incomplete combustion)

(6). PICs include as-yet unidentified compounds with unknown toxicity. The EPA

has focused largely on identification of known, Appendix VIII compounds only

(6). Since only a small percentage of the hydrocarbon emissions have been

identified (6), this leaves a great deal unknown about PAH emission effects.

Dioxins and furans can be formed by burning PCBs (17). Dioxins are believed

formed in incinerators at a temperature of ~500º C and destroyed at a temperature

of at least 900º C (6, 7). But the incinerator must be running at maximum

efficiency; dioxin survival is favoured by low combustion temperature, wet refuse,

insufficient or excess oxygen and inadequate residence time (7). MSW

incinerators have been measured as consistently producing far more dioxin and

furans than hazardous waste incinerators; almost 1000 times as much (6).

One study is reported to have found the concentration of PAHs in stack gases of

an MSW incinerator increased more than 1000-fold during cold start-up of the

plant (7), so a large load of plastics or solvents could result in a huge surge of

toxic emissions (3). Another reported that the highest concentrations of PAHs are

on the smallest atmospheric particulates and therefore in the respirable range (7).

Thus a wide range of hazardous materials are possibly being emitted in a form ripe

for absorption by the human breathing system. Performance data on incinerator

efficiency is usually conducted at new facilities operating at peak performance (2).

More than half of the 221 hazardous waste incinerators in the United States

employ no pollution control equipment at all (6).

Hydrocarbon Effects:

Light and Noncyclic Hydrocarbons: Benzene is an established human carcinogen

(18, 19), induces DNA strand breaks (20), and may have no lower threshold of

effect (16). Chloroform, carbon tetrachloride, and many other PHAHs are

carcinogens and liver and kidney toxic (21). Methylene chloride, trichloroethylene

(TCE), and tetrachloroethylene are strongly suspected carcinogens (21). TCE

damages the fatty acid components of brain cell membranes (22). Many

halogenated alkanes, alkenes, and alkynes are cytotoxic and/or carcinogenic (21).

Toluene induces DNA strand breaks (20) and can induce CNS symptoms,

including brain damage, with chronic or acute use (23). Mixtures of solvents used

industrially often include toluene, methylene chloride and chloroform and have

been implicated in inducing fatigue, loss of appetite, loss of memory, and other

symptoms in chronic use (23, 24). Recent work shows that toluene, TCE, and

benzene may interact synergistically with ethanol, and each other, to inhibit or

amplify effects (25). A 67-fold increase in liver toxicity of carbon tetrachloride

occurs in the presence of chlordecone (26-28), and a mechanism has been

described. This raises the question: is other chemical damage similarly multiplied

by the presence of small amounts of a second chemical? We need more data on

multiple interactions (28).

Polycyclic Aromatic Hydrocarbons: Many of the hundreds of congeners of

PHAHs, PCBs, dioxins, furans, and other PAHs appear to use the same

mechanism to create human cancer and toxicity (29, 30); the parent binds to a

special aromatic hydrocarbon receptor in cell membranes, and a resulting complex

interacts with cell DNA (29, 31, 32) to cause multiple gene alterations, including

genes of drug metabolism enzymes and cellular growth (29). The induction of

P450 enzymes may be important in the formation of toxic metabolites (21, 33).

Some toxic effects, particularly from PCBs, may operate through other

mechanisms (33). Effects of PAHs reported in humans include: severe

disturbances in Vitamin A metabolism (34); neurological changes including

altering dopamine concentrations in the CNS (35); retarded child cognitive and

motor development (35), lowered birth weight, growth rate, and child activity

levels (36), poorer child memory (37); cancer (38); bronchial and liver damage

(39); and immune changes (40, 41). Animal studies are reported to show dermal,

immune, and liver toxicity; cancer; and teratogenic and neurobehavioral effects

(30, 42). According to one review (42), toxic effects of TCDD, the most toxic

dioxin isomer, found to recur in four or more human studies each are: chlorachne;

liver damage; elevated liver enzymes; disorders of carbohydrate metabolism;

cardiovascular disorders; neurological damage; peripheral neuritis; sensory

impairments (sight, hearing, taste, smell); and depressive psychological

syndromes.

Although amounts of many PAHs found in incinerator emissions are very small,

many PAHs are toxic in tiny quantities; parts per billion or trillion, as opposed to

parts per million for most other toxins studied (42). A review of reproductive

animal studies on PCB effects concluded that a No Observable Adverse Effect

Level could not be formulated since effects were present at the lowest levels

studied; the background contamination of the control diets would interfere with

testing of lower amounts (43). This is especially troubling since many PAHs are

not excreted from the body; they build up in fat tissue; this is well established in

humans, other mammals, fish, and insects (44-46). Virtually all humans are now

carrying a load of TCDD at more than 3 parts per trillion; in the U.S.A. the range

is from 1.4 to 20.2 ppt for non-occupationally exposed individuals (47). Human

and animal studies show that, in PCBs, the unmetabolized body burden can be

passed in mother’s milk to the infant (35-37, 48). Particularly troubling are two

independent studies showing PCB-related developmental impairment in children

at levels now encountered in the general population (35-37).

Metals and Heavy Metals: Over thirty-five metals are reported from MSW

incineration; most are found in all of bottom ash, fly ash, and suspended

particulate, and undergo enrichment in the fine ash (6, 7). Several are reported as

possible human carcinogens or toxins, including Cd (49, 50), Cr, Ni, Pb, Hg, As,

Ba, and Be (7). Aluminum, Cu, Fe, Pb, Ti and Zn are found largely in slag, while

more volatile elements such as Cd, Pb, Sb, Se and Sn are vaporized and condense

on fine particles, which are either trapped or escape to the atmosphere as

suspended particulates (7). Volatile chlorides of elements including As, Cd, Ni,

Pb, Sb, and Zn are formed, which greatly increases their presence in fly ash and

suspended particulates (7). Over 80% of inputed Hg, largely from Hg batteries, is

estimated to be released into gas phase as halides (7). Other metals are also used in

batteries and deserve attention (7). Small boilers employing hazardous waste as a

fuel, including waste oil, are a serious concern since 50 to 60% of the inputted Pb

is emitted from the stack (6).

The emissions of Cadmium and Mercury may be serious cause for concern since

Cd and Hg both preferentially displace Zinc in human metallo-enzymes (51); Cd

10,000-fold (52). Zn is increasingly known as crucial for many living processes

including enzyme DNA transcription (53), immune system activation (54, 55), and

membrane stabilization (56), and the average western diet is deficient in it (57).

Cd body burdens are rising (58) and Cd has a 30-year half-life in the human body

Data for metal emissions and for air pollution control device effectiveness for

metals are limited and incomplete (6). And more than half of the 221 hazardous

waste incinerators in the United States are reported as employing no pollution

control equipment at all (6).

Gases: Among others (HCL, CO, HF, Bromine), S02 and nitrogen oxides are

reported released from incinerators (6, 7). S02 and N02 have been studied

extensively due to their release from other sources and their toxic effects on the

human respiration system; both are now known to reduce the body’s ability to

fight lung infection (9-11, 60). Ozone, also toxic to the human lung anti-bacterial

system, is not reported released from incinerators per se but is formed in the

presence of sunlight on oxides of nitrogen (11). Mouse studies with ozone at

concentrations corresponding to the .7 ppm measured in the suburbs of Los

Angeles (11) show impairment of bacterial killing in the lung in only 3 to 4 hours

(61). No incinerator real-time monitors exist for measuring destruction and

removal efficiency of these or other stack emissions (6).

Bottom Ash and Washwater Pollutants: Cd, Pb, and Mn, among 20-odd metals,

and a variety of organics including chlorinated benzenes, alcohols, phenols,

aldehydes, ketones, esters, amines, amides, hydrocarbons, and dioxins and furans

also appear in wastewater and bottom ash (6, 7). Wastewater (7), and to an

increasing extent under EPA regulations, ash (6), are considered hazardous waste.

Storage in ordinary land-fills carries an undetermined level of threat of leaching

into the water table (6).

Effects On Fish: Data specifically incinerator-linked are scarce. The 1,2,8,9-

TCDD isomer has been found as a contaminant in fly ash, river water, and

sediments surrounding several MSW and industrial incinerators (62). Several

studies are reported establishing that fish retain the most toxic dioxins and furans

(2,3,7,8-TCDD and 2,3,7,8-TCDF) preferentially when exposed to them from fly

ash. Effect levels were observed as low as 38 parts per trillion (rainbow trout) for

TCDD (7). An EPA study is reported to state that TCDD is bioaccumulating in

fish and low-level contamination of fish is widespread (47). A recent study

correlates human levels of Pb and Se with fish consumption (63); both Pb and Se

are incinerator emissions (6).

Effects On Soils and Plants: A Finnish study reportedly found 11 elements on

birch leaf samples, including Pb, Ni, Pb, Zn, Ti, Cl, and Cd, showing a strong

correlation between concentration and closeness to an incinerator (7). Experiments

using incinerator fly ash as a partial soil additive report increased uptake of a

number of heavy metals, including Cd and Pb (5). Cabbage grown on 20% ash-

amended soil contained 146 times the concentration of Cd of controls (7). The

concentration of TCDD on fruits and vegetables consumed by humans has been

estimated to be 60% from air-to-leaf transfer, 33% deposition, and 8% root uptake

(47). The main mode of human intake of dioxin is by food (47, 64). Multiple

studies are reported showing incinerator and other acid gas contributions to acid

rain (7).

 

DISCUSSION

Toxic Burden: A study is reported in which human fat concentration of the most

toxic isomer of TCDD increases directly proportionally with age (7). Another

reported study shows levels of dioxins and furans higher in cow’s milk sampled

near an incinerator than elsewhere (7). If this pattern of body-burden increase

holds for even a fraction of the other organic contaminants such as PCBs and for

metals such as Cd and Hg emitted from incinerators, the detrimental effect of an

incinerator on human health may outweigh its benefits in energy retrieval and

waste-stream reduction. In addition, the synergistic effects of combinations of

chemicals are only beginning to be explored. Although one review calculates,

using a theoretical dispersion model, that only 4% of the total all-source emissions

of TCDD into the U.S. environment comes from MSW incinerators (47), their

total from known sources accounts for only 11% of total all-source emissions (47).

As a result, in the data they present, MSW incinerators can equally well be said to

account for 36% of the known sources of TCDD; or 45% if hospital incinerators

are included.

Regulatory Control: Even if technology can be shown to exist that could curb all

avenues of toxic effluent from incinerators (gas, particulate, water and ash) are

there sufficient political and regulatory safeguards in place to assure that such an

incinerator will operate at this level on a daily basis? It’s been reported that after

recent confidential EPA tests conducted for a report to Congress, involving

samples from 15 of the 114 cement kilns in the United States – eight that burn

hazardous waste and seven that do not – the EPA expressed great concern over

unacceptably high levels of As, Cr, Pb, dioxins and furans, as well as a variety of

radioactive compounds, it found in the waste dust (2). Both concrete products and

kiln emissions were affected.

Although the EPA has standards in place for some emissions, and has done some

testing to see if those standards are met (6), no satisfactory explanation has been

demonstrated for the effects of the unknown products of incineration, nor for the

rationale behind a particular level being set as a standard. Risk assessment studies

have been performed, but the EPA Science Advisory Board has criticized their

data base and recommended a more complete assessment (6). Meanwhile, waste

generators, faced with the Hazardous and Solid Waste Amendments Act and with

possible future environmental damage settlements over contaminated

groundwater, look to incineration as a viable alternative (6). In 1981 figures,

industrial boilers and furnaces disposed of twice as much hazardous waste as

incinerators did. A principal attraction to this approach is exemption from

incineration emissions performance standards (6).

In Canada, St. Lawrence Cement, in densely populated Mississauga near Toronto,

disposes of about 3.5 million litres a year of highly chlorinated waste solvents and

plastic residues drawn from industries in Ontario and the United States, and the

plant has never been required to test for toxic residues (2). Several years ago the

Ontario Environment Ministry, responsible for control of St. Lawrence Cement,

was the target of an intense many-year struggle of a group of concerned Toronto

citizens who asked, then legally forced, the Ministry to take action concerning Pb

emissions from Toronto Pb smelters. A lawyer involved who later wrote a

description of the social and legal battles (65) observed that the common law has

traditionally favoured after-the-fact compensation of victims and has never

developed adequate concern for prevention of harm. He speculates that in

environmental health the burden of proof ought to be removed from the shoulders

of the potential victim and placed upon the alleged polluter (65).

Concluding Opinion: Incinerators take waste that is concentrated and partly toxic,

destroy some of it, produce new waste that is partly toxic, and spread the product

extremely thinly throughout the environment. That the effects are hard to measure

is understandable; perhaps that is one reason, even in some cases the most

important reason, why incineration is done. Although an efficiently-operating,

technologically-advanced incinerator is theoretically capable of destroying

complex hydrocarbons, it would still produce metals, sometimes conjugated with

other elements, in ash and gas. And to what degree such an incinerator has been

successfully built or is operating is an open question, due to our poor regulatory

policies. Until a major change in the political organization of incinerator control is

clearly established, the building of any further incinerators would seem unwise.

An appropriate course might be to begin careful monitoring of the existing units,

and combine this with continued exploration of other options of waste disposal,

such as reduction of waste generation at source and re-use of materials.

 

References

1. The Case Against Municipal Solid Waste Incineration. The Allergy And Environmental Health Association Quarterly. 14(3/4):25-27 (1992).

2. Ferguson J. Toxins Linked To Waste Burning. The Globe and Mail. Toronto. 1993. (As reprinted in The AEHA Quarterly, 15(2), p.16).

3. Ministry of the Environment and Energy. The Case Against Municipal Solid Waste Incineration. Government of Ontario. 1992. Booklet.

4. Riha CA. Energy efficient home made of recycled items. The Citizen. Ottawa. August 21, 1993.

5. Elliott J. The Last Straw in alternative housing. The Citizen. Ottawa. September 4, 1993. p: E1.

6. Oppelt ET. Air emissions from the incineration of hazardous waste. Toxicol Ind Health. 6(5):23-51 (1990).

7. Lisk DJ. Environmental implications of incineration of municipal solid waste and ash disposal. Sci Total Environ. 74:39-66 (1988).

8. Ito K, Thurston GD, Hayes C, Lippmann M. Associations of London, England, Daily Mortality with Particulate Matter, Sulfur Dioxide, and Acidic Aerosol Pollution. Arch Environ Health. 48(4):213-220 (1993).

9. Pearlman ME, Finklea JF, Creason JP, Shy CM, Young MM, Horton JM. Nitrogen Dioxide and Lower Respiratory Illness. Pediatrics. 47(2):391-398 (1971).