Factors Affecting Indoor Air Quality 5

2The indoor environment in any buildingis a result of the interaction between thesite, climate, building system (originaldesign and later modifications in thestructure and mechanical systems), con-struction techniques, contaminant sources(building materials and furnishings,moisture, processes and activities within thebuilding, and outdoor sources), andbuilding occupants. The following four elements are involvedin the development of indoor air qualityproblems:

Source: there is a source of contaminationor discomfort indoors, outdoors, or withinthe mechanical systems of the building.

HVAC: the HVAC system is not able tocontrol existing air contaminants and ensurethermal comfort (temperature and humidityconditions that are comfortable for mostoccupants).

Pathways: one or more pollutant pathwaysconnect the pollutant source to the occu-pants and a driving force exists to movepollutants along the pathway(s).

Occupants: building occupants are present.

It is important to understand the role thateach of these factors may play in order toprevent, investigate, and resolve indoor airquality problems.

SOURCES OF INDOOR AIRCONTAMINANTS

Indoor air contaminants can originatewithin the building or be drawn in fromoutdoors. If contaminant sources are notcontrolled, IAQ problems can arise, even ifthe HVAC system is properly designed andwell-maintained. It may be helpful to thinkof air pollutant sources as fitting into one of

the categories that follow. The examplesgiven for each category are not intended tobe a complete list.

Sources Outside Building

Contaminated outdoor air■ pollen, dust, fungal spores■ industrial pollutants■ general vehicle exhaust

Emissions from nearby sources■ exhaust from vehicles on nearby roads

or in parking lots, or garages■ loading docks■ odors from dumpsters■ re-entrained (drawn back into the

building) exhaust from the buildingitself or from neighboring buildings

■ unsanitary debris near the outdoor airintake

Soil gas■ radon■ leakage from underground fuel tanks■ contaminants from previous uses of the

site (e.g., landfills)■ pesticides

Moisture or standing water promotingexcess microbial growth■ rooftops after rainfall■ crawlspace

Equipment

HVAC system■ dust or dirt in ductwork or other

components■ microbiological growth in drip pans,

humidifiers, ductwork, coils■ improper use of biocides, sealants, and/

or cleaning compounds■ improper venting of combustion

products■ refrigerant leakage

Factors Affecting Indoor Air Quality

Four elements—sources, the HVACsystem, pollutantpathways, andoccupants—areinvolved in thedevelopment of IAQproblems.

6 Section 2

Non-HVAC equipment■ emissions from office equipment (vola-

tile organic compounds, ozone)■ supplies (solvents, toners, ammonia)■ emissions from shops, labs, cleaning

processes■ elevator motors and other mechanical

systems

Human Activities

Personal activities■ smoking■ cooking■ body odor■ cosmetic odorsHousekeeping activities■ cleaning materials and procedures■ emissions from stored supplies or trash■ use of deodorizers and fragrances■ airborne dust or dirt (e.g., circulated by

sweeping and vacuuming)

Maintenance activities■ microorganisms in mist from improp-

erly maintained cooling towers■ airborne dust or dirt■ volatile organic compounds from use of

paint, caulk, adhesives, and otherproducts

■ pesticides from pest control activities■ emissions from stored supplies

Building Components and Furnishings

Locations that produce or collect dust orfibers■ textured surfaces such as carpeting,

curtains, and other textiles■ open shelving■ old or deteriorated furnishings■ materials containing damaged asbestos

Unsanitary conditions and water damage■ microbiological growth on or in soiled

or water-damaged furnishings■ microbiological growth in areas of

surface condensation■ standing water from clogged or poorly

designed drains■ dry traps that allow the passage of

sewer gas

Given our presentknowledge, it isdifficult to relatecomplaints ofspecific healtheffects to exposuresto specific pollutantconcentrations,especially since thesignificant exposuresmay be to low levelsof pollutant mixtures.

Chemicals released from buildingcomponents or furnishings■ volatile organic compounds or■ inorganic compounds

Other Sources

Accidental events■ spills of water or other liquids■ microbiological growth due to flooding

or to leaks from roofs, piping■ fire damage (soot, PCBs from electrical

equipment, odors)

Special use areas and mixed use buildings■ smoking lounges■ laboratories■ print shops, art rooms■ exercise rooms■ beauty salons■ food preparation areas

Redecorating/remodeling/repair activities■ emissions from new furnishings■ dust and fibers from demolition■ odors and volatile organic and inorganic

compounds from paint, caulk, adhesives■ microbiologicals released from demoli-

tion or remodeling activities

Indoor air often contains a variety ofcontaminants at concentrations that are farbelow any standards or guidelines foroccupational exposure. Given our presentknowledge, it is difficult to relate com-plaints of specific health effects to expo-sures to specific pollutant concentrations,especially since the significant exposuresmay be to low levels of pollutant mixtures.

HVAC SYSTEM DESIGN ANDOPERATION

The HVAC system includes all heating,cooling, and ventilation equipment servinga building: furnaces or boilers, chillers,cooling towers, air handling units, exhaustfans, ductwork, filters, steam (or heatingwater) piping. Most of the HVAC discus-sion in this document applies both to centralHVAC systems and to individual compo-nents used as stand-alone units.

Factors Affecting Indoor Air Quality 7

A properly designed and functioningHVAC system:■ provides thermal comfort■ distributes adequate amounts of outdoor

air to meet ventilation needs of allbuilding occupants

■ isolates and removes odors and con-taminants through pressure control,filtration, and exhaust fans

Thermal Comfort

A number of variables interact to deter-mine whether people are comfortable withthe temperature of the indoor air. Theactivity level, age, and physiology of eachperson affect the thermal comfort require-ments of that individual. The AmericanSociety of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE)Standard 55-1981 describes the tempera-ture and humidity ranges that are comfort-able for most people engaged in largelysedentary activities. That information issummarized on page 57. The ASHRAEstandard assumes “normal” indoorclothing. Added layers of clothing reducethe rate of heat loss.

Uniformity of temperature is importantto comfort. When the heating and coolingneeds of rooms within a single zonechange at different rates, rooms that areserved by a single thermostat may be atdifferent temperatures. Temperaturestratification is a common problem causedby convection, the tendency of light, warmair to rise and heavier, cooler air to sink. Ifair is not properly mixed by the ventilationsystem, the temperature near the ceilingcan be several degrees warmer than atfloor level. Even if air is properly mixed,uninsulated floors over unheated spacescan create discomfort in some climatezones. Large fluctuations of indoortemperature can also occur when controlshave a wide “dead band” (a temperaturerange within which neither heating norcooling takes place).

A number ofvariables, includingpersonal activitylevels, uniformity oftemperature, radiantheat gain or loss, andhumidity, interact todetermine whetherpeople arecomfortable with thetemperature of theindoor air.

Radiant heat transfer may cause peoplelocated near very hot or very cold surfacesto be uncomfortable even though thethermostat setting and the measured airtemperature are within the comfort range.Buildings with large window areas some-times have acute problems of discomfortdue to radiant heat gains and losses, withthe locations of complaints shifting duringthe day as the sun angle changes. Largevertical surfaces can also produce asignificant flow of naturally-convecting air,producing complaints of draftiness.Adding insulation to walls helps tomoderate the temperature of interior wallsurfaces. Closing curtains reduces heatingfrom direct sunlight and isolates buildingoccupants from exposure to windowsurfaces (which, lacking insulation, arelikely to be much hotter or colder than thewalls).

Humidity is a factor in thermal comfort.Raising relative humidity reduces theability to lose heat through perspiration andevaporation, so that the effect is similar toraising the temperature. Humidity ex-tremes can also create other IAQ problems.Excessively high or low relative humiditiescan produce discomfort, while high relativehumidities can promote the growth of moldand mildew (see Appendix C).

Ventilation to Meet OccupantNeeds

Most air handling units distribute a blendof outdoor air and recirculated indoor air.HVAC designs may also include units thatintroduce 100% outdoor air or that simplytransfer air within the building. Uncon-trolled quantities of outdoor air enterbuildings by infiltration through windows,doors, and gaps in the exterior construc-tion. Thermal comfort and ventilationneeds are met by supplying “conditioned”air (a blend of outdoor and recirculated airthat has been filtered, heated or cooled, andsometimes humidified or dehumidified).

8 Section 2

Large buildings often have interior(“core”) spaces in which constant coolingis required to compensate for heat gener-ated by occupants, equipment, andlighting, while perimeter rooms mayrequire heating or cooling depending onoutdoor conditions.

Two of the most common HVACdesigns used in modern public andcommercial buildings are constant volumeand variable air volume systems. Con-stant volume systems are designed toprovide a constant airflow and to vary theair temperature to meet heating andcooling needs. The percentage of outdoorair may be held constant, but is oftencontrolled either manually or automaticallyto vary with outdoor temperature andhumidity. Controls may include a mini-mum setting that should allow the systemto meet ventilation guidelines for outdoorair quantities under design conditions.

Variable air volume (VAV) systemscondition supply air to a constant tempera-ture and ensure thermal comfort by varyingthe airflow to occupied spaces. Most earlyVAV systems did not allow control of theoutdoor air quantity, so that a decreasingamount of outdoor air was provided as theflow of supply air was reduced. Somemore recent designs ensure a minimumsupply of outdoor air with static pressuredevices in the outdoor air stream. Addi-tional energy-conserving features such aseconomizer control or heat recovery arealso found in some buildings.

Good quality design, installation, andtesting and balancing are critically impor-tant to the proper operation of all typesof HVAC systems, especially VAVsystems, as are regular inspections andmaintenance. (See Appendix B for furtherdiscussion of HVAC system types.)

The amount of outdoor air consideredadequate for proper ventilation has variedsubstantially over time. The currentguideline issued by ASHRAE is ASHRAEStandard 62-1989. The building code thatwas in force when your building HVAC

system was designed may well haveestablished a lower amount of ventilation(in cubic feet of outdoor air per minute perperson) than is currently recommended.(A table of outdoor air quantities recom-mended by ASHRAE is reproduced onpage 136 in Appendix B. Note that otherimportant aspects of the standard are notincluded in this table.)

Control of Odors andContaminants

One technique for controlling odors andcontaminants is to dilute them withoutdoor air. Dilution can work only ifthere is a consistent and appropriate flowof supply air that mixes effectively withroom air. The term “ventilation effi-ciency” is used to describe the ability ofthe ventilation system to distribute supplyair and remove internally generatedpollutants. Researchers are currentlystudying ways to measure ventilationefficiency and interpret the results of thosemeasurements.

Another technique for isolating odorsand contaminants is to design and operatethe HVAC system so that pressurerelationships between rooms are con-trolled. This control is accomplished byadjusting the air quantities that aresupplied to and removed from each room.If more air is supplied to a room than isexhausted, the excess air leaks out of thespace and the room is said to be underpositive pressure. If less air is suppliedthan is exhausted, air is pulled into thespace and the room is said to be undernegative pressure.

Control of pressure relationships iscritically important in mixed use buildingsor buildings with special use areas.Lobbies and buildings in general are oftendesigned to operate under positive pressureto prevent or minimize the infiltration ofunconditioned air, with its potential tocause drafts and introduce dust, dirt, andthermal discomfort. Without properoperation and maintenance, these pressure

The amount ofoutdoor airconsidered adequatefor proper ventilationhas variedsubstantially overtime. The currentguideline issued byASHRAE is Standard62-1989.

Factors Affecting Indoor Air Quality 9

Chases, crawlspaces, andother hidden spaces can beboth sources and pathwaysfor pollutants.

differences are not likely to remain asoriginally designed.

A third technique is to use local exhaustsystems (sometimes known as dedicatedexhaust ventilation systems) to isolate andremove contaminants by maintainingnegative pressure in the area around thecontaminant source. Local exhaust can belinked to the operation of a particular pieceof equipment (such as a kitchen range) orused to treat an entire room (such as asmoking lounge or custodial closet). Airshould be exhausted to the outdoors, notrecirculated, from locations which producesignificant odors and high concentrationsof contaminants (such as copy rooms,bathrooms, kitchens, and beauty salons).

Spaces where local exhaust is used mustbe provided with make-up air and the localexhaust must function in coordination withthe rest of the ventilation system. Undersome circumstances, it may be acceptableto transfer conditioned air from relativelyclean parts of a building to comparativelydirty areas and use it as make-up air for alocal exhaust system. Such a transfer canachieve significant energy savings.

Air cleaning and filtration devicesdesigned to control contaminants are foundas components of HVAC systems (forexample, filter boxes in ductwork) and canalso be installed as independent units. Theeffectiveness of air cleaning depends uponproper equipment selection, installation,operation, and maintenance. Cautionshould be used in evaluating the many newtechnological developments in the field ofair cleaning and filtration.

POLLUTANT PATHWAYS ANDDRIVING FORCES

Airflow patterns in buildings result fromthe combined action of mechanicalventilation systems, human activity, andnatural forces. Pressure differentialscreated by these forces move airbornecontaminants from areas of relativelyhigher pressure to areas of relatively lowerpressure through any available openings.

The HVAC system is generally thepredominant pathway and driving force forair movement in buildings. However, allof a building’s components (walls, ceilings,floors, penetrations, HVAC equipment, andoccupants) interact to affect the distributionof contaminants.

For example, as air moves from supplyregisters or diffusers to return air grilles, itis diverted or obstructed by partitions,walls, and furnishings, and redirected byopenings that provide pathways for airmovement. On a localized basis, themovement of people has a major impact onthe movement of pollutants. Some of thepathways change as doors and windowsopen and close. It is useful to think of theentire building — the rooms and theconnections (e.g., chases, corridors,stairways, elevator shafts) between them —as part of the air distribution system.

Natural forces exert an importantinfluence on air movement between zonesand between the building’s interior andexterior. Both the stack effect and windcan overpower a building’s mechanicalsystem and disrupt air circulation andventilation, especially if the buildingenvelope is leaky.

Stack effect is the pressure driven flowproduced by convection (the tendency of

10 Section 2

warm air to rise). The stack effect existswhenever there is an indoor-outdoortemperature difference and becomesstronger as the temperature differenceincreases. As heated air escapes fromupper levels of the building, indoor airmoves from lower to upper floors, andreplacement outdoor air is drawn intoopenings at the lower levels of buildings.Stack effect airflow can transport contami-nants between floors by way of stairwells,elevator shafts, utility chases, or otheropenings.

Wind effects are transient, creating localareas of high pressure (on the windwardside) and low pressure (on the leewardside) of buildings. Depending on theleakage openings in the building exterior,wind can affect the pressure relationshipswithin and between rooms.

The basic principle of air movement fromareas of relatively higher pressure to areasof relatively lower pressure can producemany patterns of contaminant distribution,including:

■ local circulation in the room containingthe pollutant source

■ air movement into adjacent spaces thatare under lower pressure (Note: Even iftwo rooms are both under positivepressure compared to the outdoors, oneroom is usually at a lower pressure thanthe other.)

■ recirculation of air within the zonecontaining the pollutant source or inadjacent zones where return systemsoverlap

■ movement from lower to upper levels ofthe building

■ air movement into the building througheither infiltration of outdoor air orreentry of exhaust air

Air moves from areas of higher pressureto areas of lower pressure through anyavailable openings. A small crack or holecan admit significant amounts of air if thepressure differentials are high enough(which may be very difficult to assess.)

Even when the building as a whole ismaintained under positive pressure, there isalways some location (for example, theoutdoor air intake) that is under negativepressure relative to the outdoors. Entry ofcontaminants may be intermittent, occur-ring only when the wind blows from thedirection of the pollutant source. Theinteraction between pollutant pathways andintermittent or variable driving forces canlead to a single source causing IAQcomplaints in areas of the building that aredistant from each other and from thesource.

BUILDING OCCUPANTS

The term “building occupants” is generallyused in this document to describe peoplewho spend extended time periods (e.g., afull workday) in the building. Clients andvisitors are also occupants; they may havedifferent tolerances and expectations fromthose who spend their entire workdays inthe building, and are likely to be moresensitive to odors.

Groups that may be particularly suscep-tible to effects of indoor air contaminantsinclude, but are not limited to:

■ allergic or asthmatic individuals■ people with respiratory disease■ people whose immune systems are

suppressed due to chemotherapy,radiation therapy, disease, or othercauses

■ contact lens wearers

Some other groups are particularlyvulnerable to exposures of certainpollutants or pollutant mixtures. Forexample, people with heart disease may bemore affected by exposure at lower levelsof carbon monoxide than healthyindividuals. Children exposed to environ-mental tobacco smoke have been shown tobe at higher risk of respiratory illnessesand those exposed to nitrogen dioxide havebeen shown to be at higher risk fromrespiratory infections.

The basic principle ofair movement fromareas of relativelyhigher pressure toareas of relativelylower pressure canproduce manypatterns ofcontaminantdistribution.

Factors Affecting Indoor Air Quality 11

Because of varying sensitivity amongpeople, one individual may react to aparticular IAQ problem while surroundingoccupants have no ill effects. (Symptomsthat are limited to a single person can alsooccur when only one work station receivesthe bulk of the pollutant dose.) In othercases, complaints may be widespread.

A single indoor air pollutant or problemcan trigger different reactions in differentpeople. Some may not be affected at all.Information about the types of symptomscan sometimes lead directly to solutions.However, symptom information is morelikely to be useful for identifying the timingand conditions under which problemsoccur.

Types of Symptoms andComplaints

The effects of IAQ problems are often non-specific symptoms rather than clearlydefined illnesses. Symptoms commonlyattributed to IAQ problems include:

■ headache■ fatigue■ shortness of breath■ sinus congestion■ cough■ sneezing■ eye, nose, and throat irritation■ skin irritation■ dizziness■ nausea

All of these symptoms, however, may alsobe caused by other factors, and are notnecessarily due to air quality deficiencies.

“Health” and “comfort” are used todescribe a spectrum of physical sensations.For example, when the air in a room isslightly too warm for a person’s activitylevel, that person may experience milddiscomfort. If the temperature continues torise, discomfort increases and symptomssuch as fatigue, stuffiness, and headachescan appear.

Some complaints by building occupantsare clearly related to the discomfort end ofthe spectrum. One of the most commonIAQ complaints is that “there’s a funnysmell in here.” Odors are often associatedwith a perception of poor air quality,whether or not they cause symptoms.Environmental stressors such as improperlighting, noise, vibration, overcrowding,ergonomic stressors, and job-relatedpsychosocial problems (such as job stress)can produce symptoms that are similar tothose associated with poor air quality.

The term sick building syndrome (SBS)is sometimes used to describe cases inwhich building occupants experience acutehealth and comfort effects that are appar-ently linked to the time they spend in thebuilding, but in which no specific illness orcause can be identified. The complaintsmay be localized in a particular room orzone or may be widespread throughout thebuilding. Many different symptoms havebeen associated with SBS, includingrespiratory complaints, irritation, andfatigue. Analysis of air samples often failsto detect high concentrations of specificcontaminants. The problem may be causedby any or all of the following:

■ the combined effects of multiplepollutants at low concentrations

■ other environmental stressors(e.g., overheating, poor lighting, noise)

■ ergonomic stressors■ job-related psychosocial stressors

(e.g., overcrowding, labor-managementproblems)

■ unknown factors

Building-related illness (BRI) is a termreferring to illness brought on by exposureto the building air, where symptoms ofdiagnosable illness are identified (e.g.,certain allergies or infections) and can bedirectly attributed to environmental agentsin the air. Legionnaire’s disease andhypersensitivity pneumonitis are examplesof BRI that can have serious, even life-threatening consequences.

Environmentalstressors such asimproper lighting,noise, vibration,overcrowding,ergonomic stressors,and job-relatedpsychosocialproblems (such as jobstress) can producesymptoms that aresimilar to thoseassociated with poorair quality.

12 Section 2

A small percentage of the populationmay be sensitive to a number of chemicalsin indoor air, each of which may occur atvery low concentrations. The existence ofthis condition, which is known as multiplechemical sensitivity (MCS), is a matter ofconsiderable controversy. MCS is notcurrently recognized by the major medicalorganizations, but medical opinion isdivided, and further research is needed.The applicability of access for the disabledand worker’s compensation regulations topeople who believe they are chemicallysensitive may become concerns for facilitymanagers.

Sometimes several building occupantsexperience rare or serious health problems(e.g., cancer, miscarriages, Lou Gehrig’sdisease) over a relatively short time period.These clusters of health problems areoccasionally blamed on indoor air quality,and can produce tremendous anxietyamong building occupants. State or localHealth Departments can provide adviceand assistance if clusters are suspected.They may be able to help answer keyquestions such as whether the apparentcluster is actually unusual and whether theunderlying cause could be related to IAQ.