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Indoor Air Quality

Indoor Air Quality

Factors influencing indoor air quality

Aspects of indoor air quality

Sick building syndrome


Indoor air quality (IAQ) is the condition and content of interior air, especially with regards to how it affects health and safety of humans. The chemical, physical, and biological characteristics of the atmosphere inside of dwellings and in commercial and institutional buildings are influenced in numerous ways.

Sometimes, effects on indoor air quality can be sufficient to cause people to experience significant discomfort, and even to become physically ill. Recent studies have proven that indoor air quality is generally worse than outside air quality.

People vary greatly in their sensitivity to air pollution, both inside and outside of buildings. People also differ in the sorts of symptoms that they develop in response to deterioration of air quality. Consequently, it has proven difficult for scientists to characterize the dimensions of indoor air quality, and to precisely define the nature of the subsequent environmental illnesses that some people appear to develop. This has led to a great deal of environmental and medical controversy, concerning the extent and intensity of a syndrome of air-quality related illnesses, known as the sick building syndrome.

Factors influencing indoor air quality

Air quality inside of buildings is related to a diverse range of chemical, physical, and biological factors. In any situation, the importance of these many influences can vary greatly, depending on the emission rates of various chemicals, the frequency with which inside air is exchanged with ambient air, the efficiency of atmospheric circulation within the building, and numerous other factors.

In response to the need to conserve energy (and money), modern buildings are well insulated to retain their heat in winter and their coolness in summer. Such buildings receive almost all of their inputs of relatively clean, outside air through their carefully designed, ventilation system. Such systems have only a few, discrete intakes of ambient air, and outputs of used air back to the outside, as well as particular, internal-circulation characteristics. It is not possible, for example, to open any windows in many modern office buildings, because this ability would interfere with pressure gradients and upset the designed balance of the ventilation system. Of course, the ventilation characteristics of many recently constructed modern buildings have a substantial influence on the quality of the internal atmosphere of the structure.

When ventilation systems are operated with a view to saving energy, there are relatively few exchanges of indoor air with relatively clean, ambient air. Sometimes, too much attention to the efficiency of energy use in air-tight buildings can lead to the buildup of excessive concentrations of indoor air pollutants, because of on-going emissions of chemicals within the building.

In addition, in some cases the intake pipes for ambient air to buildings are located too close to ducts that exhaust contaminated air from the same or a nearby building. This faulty design can lead to the intake of poor-quality outside air, impairing atmospheric quality within the building. Similarly, sinks and other water drains installed without proper systems to prevent the back-up of sewer gases can lead to incursions of noxious smells and chemicals into buildings. In other cases, the poor faulty design or operation of internal ventilation systems can lead to the development of local zones of restricted air circulation, which can develop into areas of degraded air quality within the building.

Clearly, the appropriate design and operation of air-handling systems in modern, air-tight buildings is a critical factor affecting indoor air quality.

Emission rates of chemicals and dusts within buildings are affected by many factors. The sorts of materials of which the building or its furnishings are constructed may be important in this regard. For example, minerals contained in cement or in stone may emit gaseous radon, or may slowly degenerate to release fine, inhal-able dusts. The oxidation of materials in humidification systems and ventilation duct works can also generate large quantities of fine, metallic dusts, as can the wear of painted surfaces. Many composite wood products, such as plywood and particle boards, emit gaseous formaldehyde, as do many types of synthetic fabrics.

Chemicals may also be emitted to the internal air from laboratories that do not have adequate fume hoods to vent noxious vapors and gases to the atmosphere. Similarly, industrial processes involving chemicals may be an important source of emissions in some buildings. The use of some kinds of solvents, detergents, and other substances during cleaning and sanitation of the building may also be important.

Even the human occupants of buildings emit large quantities of gases and vapors that affect air quality, for example, carbon dioxide. In addition, although the practice is increasingly being restricted, many people smoke tobacco (such as through cigarettes) inside of buildings, releasing diverse chemicals to the atmosphere. More than 2,000 chemicals have been identified in tobacco fumes, including various carcinogens such as benzo(a)pyrene and nickel carbonyl, as well as many other toxic chemicals.

These are just a few of the diverse sources of emissions of gases, vapors, and particulates inside of modern buildings. All of these sources of emissions contribute to the degradation of the quality of the indoor atmosphere.

Some buildings can develop indoor-air problems associated with fungi and other microbes that grow in damp places, and whose spores or other so-called bioaerosols become spread within the building through the ventilation system. This microbial problem can develop in systems designed to humidify the indoor air, in places where stagnant water accumulates within the air-circulation system, or in other damp places. Some people may be allergic to these spores, or in rare cases the microorganisms may be pathogens. The latter is the case of Legionnaire disease, a rare condition involving pathogenic bacteria spread through the ventilation system of buildings.

Aspects of indoor air quality

Indoor air quality has many components, some of which are physical, others chemical, and a few biological. The most significant of these are briefly described below.

The most important physical aspects of indoor air quality are air temperature and humidity. Air temperatures that are too warm or cool for human comfort can be caused by improper placement or adjustment of thermostats, and by an inability of the heating or air-conditioning system to compensate for extremes of outdoor weather, or to adequately deal with heat generated by machinery or large numbers of people. Excessive or insufficient humidity can be caused by similar problems, including poorly operating or nonexistent humidity-control mechanisms within the ventilation system.

Carbon dioxide (CO2) is a normal constituent of the ambient atmosphere, occurring in a concentration of about 350 parts per million (ppm, on a volumetric basis). However, there are many sources of emission of carbon dioxide inside of buildings, including potted plants and their soil, respiration by humans, and stoves or space heaters fueled by kerosene, propane, or methane. Consequently, the concentrations of carbon dioxide are typically relatively large inside of buildings, especially in inadequately ventilated rooms that are crowded with people. Commonly measured concentrations of this gas are about from 600 to 800 ppm, but in some situations concentrations of thousands of ppm can be achieved. Longer-term exposure to concentrations of carbon dioxide greater than about 5,000 ppm is not recommended. Symptoms of excessive exposure to carbon dioxide include drowsiness, dizziness, headaches, and shortage of breath.

Carbon monoxide (CO) is a product of the incomplete oxidation of organic fuels. Indoor emissions are mostly associated with stoves or space heaters fueled by kerosene or natural gas, with cigarette smoke, or with poorly vented emissions from automobiles in garages or loading docks. Longer-term exposures to carbon monoxide concentrations greater than nine ppm should be avoided, as should shorter-term (about one-hour) exposures greater than 35 ppm. Carbon monoxide is a relatively toxic gas because it combines strongly with the hemoglobin of blood, thereby restricting the ability of the circulation system to transport an adequate supply of oxygen to the various parts of the body. Excessive exposures to carbon monoxide under poorly ventilated conditions can cause headaches, drowsiness, nausea, fatigue, impaired judgment, and other symptoms of insufficient oxygen supply. Anoxia and death can be the ultimate result.

Formaldehyde is a pungent, organic vapor that can be detected by smell at a concentration greater than about 0.2 ppm. There are diverse sources of emission of formaldehyde, including poorly sealed plywood and particle boards, urea-formaldehyde foam insulation, and many fabrics, carpets, glues, and copy papers. Some people are quite sensitive to formaldehyde, developing symptoms that can include a dry or sore throat, headaches, fatigue, nausea, and stinging sensations in the eyes. Most people can tolerate formaldehyde concentrations of less than 0.5 ppm without developing these sorts of symptoms, but other, hypersensitive people may be adversely affected at concentrations as small as 0.01 ppm. In general, exposures to formaldehyde exposure in work areas should be less than 0.1 ppm.

Volatile organic compounds (VOCs) are a wide range of molecular species that vaporize at normally encountered temperatures. Common examples of volatile organic compounds found in buildings include (in alphabetical order): acetone, butyl acetate, dichloro-benzene, dichloromethane, hexane, octane, toluene, trichloroethane, and xylene. These organic chemicals have diverse sources, including synthetic materials used to manufacture carpets and fabrics, paints, solvents, adhesives, cleaning solutions, perfumes, hair sprays, and cigarette smoke. All of the common VOCs and many others have recommended indoor-exposure limits, which vary depending on the toxicity of the particular chemical, and on the length of the exposure. Human responses to large concentrations of volatile organic compounds include dizziness, fatigue, drowsiness, tightness of the chest, numbness or tingling of the extremities, and skin and eye irritation. Some people are hypersensitive to specific compounds or groups of VOCs.

The gases nitric oxide (NO), nitrogen dioxide (NO2), and sulfur dioxide (SO2) may also be important pollutants of the indoor atmosphere, especially where there are fuel-burning appliances or stoves used for cooking or space heating. These gases can be irritating to the eyes and upper respiratory system of people exposed to large concentrations.

Radon is a radioactive gas emitted by a wide range of geological sources, including mineral-containing building materials and ground water. Many poorly vented homes and some commercial buildings become significantly contaminated by radon, a gas that carries a risk of causing human toxicity through the development of cancers, especially lung cancer.

Particulates are various sorts of solid or liquid materials that are small enough to be suspended in the atmosphere as fine dusts or aerosols. Particulate emissions inside of buildings are associated with smoke, physical-chemical deterioration of ducts, insulating materials, walls, ceiling tiles, and paints, fibers from clothing and other fabrics, and many other sources. Particulates may also be drawn into buildings along with unfiltered, ambient air. Particulates are aggravating to many people, who may develop irritations of the upper respiratory tract, such as asthma. Some chemicals contained in particulates, especially certain metals and polycyclic aromatic hydrocarbons, are widely regarded as toxic substances, and unnecessary exposures should generally be avoided. The particulate size range of 0.004 to 0.4 in (0.1 to 10 mm) is of particular importance in terms of human exposures, because this size range is efficiently retained in the deepest parts of the lungs. Particulates smaller than 0.004 in (0.1 mm) are generally re-exhaled, while particles larger than 0.4 in (10 mm) are trapped in the upper respiratory system and have little toxic effect.

Sometimes, microbial matter (or bioaerosols) can be an indoor-air problem. Usually, this involves spore-producing fungi that occur in damp places in the ventilation system, carpets, or other places. Many people have allergies to fungal spores, and can be made ill by excessive exposures to these bioaerosols in indoor air. Bioaerosols of other microbes such as yeast, bacteria, viruses, and protozoan may also be important problems in the atmosphere of buildings. On a rare occasion, pathogenic bacteria such as the Legionella associated with pneumonialike Legionnaire disease, can be spread through the ventilation system of buildings. Other potential pathogens in the inside air of buildings include the fungi Aspergillus fumigatus and Histoplasma capsulatum.

Sick building syndrome

The sick building syndrome exists. However, it has proven very difficult for scientists to characterize


Bioaerosols Spores or actual microorganisms that occur suspended in the atmosphere.

Hypersensitivity The occurrence of extreme sensitivity to chemicals or pathogens in a small fraction of a larger human population. Hypersensitivity may be related to an extreme allergic response, or to a deficiency of the immune system.

Sick building syndrome A condition in which people frequently complain about a number of ailments while they are in a particular building, but feel relief when they go outside.

Ventilation rate This refers to the amount of outside or ambient air that is combined with re-circulating inside or return air, and is then supplied to the interior space of a building. This may also apply to some part of a building, such as a particular room.

the causes, treatment, or human responses to the sick building syndrome. This is because of the extremely variable natures of both the exposures to environmental stressors in buildings, and the responses of individual people, a small fraction of whom appear to be hypersensitive to particular aspects of the indoor atmosphere.

The effects of the sick building syndrome on people range from drowsiness and vague feelings of discomfort, with subsequent decreases in productivity, to the development of actual illnesses. In many cases it may be necessary for the afflicted people to leave the building for some length of time. Sometimes, sensitive people must give up their jobs, because they find the indoor air quality to be intolerable.

As a result of the difficult-to-define nature of the sick building syndrome, important medical and environmental controversies have developed. Some scientists suggest that people who display building-related illnesses are imagining their problems. It is suggested that these people may have developed so-called psychosomatic responses, in which clinical illnesses are caused by non-existent factors that the victim believes are important. Increasingly, however, scientists are convinced that the relatively sensitive physiologies of severely afflicted people are direct responses to physical, chemical, or biological stressors in the poorly ventilated, enclosed spaces where they live or work. Increasingly, indoor air quality issues are being taken seriously by private individuals, commercial property owners, health organizations, and federal, state, and local governments.

Further research and monitoring will be required before a better understanding of the sick building syndrome can be achieved. This knowledge is required in order to design sensible systems of avoiding or treating the problems of poor-quality indoor air of buildings, and to better protect people who are exposed to this type of pollution. Federal and state agencies are working with homeowners, developers and building maintenance professionals to develop plans and programs for dealing with indoor air quality programs. Particular attention is being paid to schools, because the relatively more-sensitive physiologies of children make them particularly susceptible to health threats from poor indoor air quality.



Burroughs, H.E. Managing Indoor Air Quality. Lilburn, GA: Fairmont Press, 2004.

Hocking, Martin B., and Diana Hocking, eds. The Handbook of Environmental Chemistry. Berlin, Germany, and London, UK: Springer, 2005.

Morawska, Lidia, and Tunga Salthammer, eds. Indoor Environment: Airborne Particles and Settled Dust. Weinheim, UK: Wiley-VCH, 2003.

Zhang, Yuanhui. Indoor Air Quality Engineering. Boca Raton, FL: CRC Press, 2005.

Bill Freedman

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Indoor Air Quality

Indoor air quality

An assessment of air quality in buildings and homes based on physical and chemical monitoring of contaminants, physiological measurements, and/or psychosocial perceptions. Factors contributing to the quality of indoor air include lighting, ergonomics, thermal comfort, tobacco smoke , noise, ventilation, and psychosocial or work-organizational factors such as employee stress and satisfaction. "Sick building syndrome" (SBS) and "building-related illness" (BRI) are responses to indoor air pollution commonly described by office workers. Most symptoms are nonspecific; they progressively worsen during the week, occur more frequently in the afternoon, and disappear on the weekend.

Poor indoor air quality (IAQ) in industrial settings such as factories, coal mines, and foundries has long been recognized as a health risk to workers and has been regulated by the U.S. Occupational Safety and Health Administration (OSHA). The contaminant levels in industrial settings can be hundreds or thousands of times higher than the levels found in homes and offices. Nonetheless, indoor air quality in homes and offices has become an environmental priority in many countries, and federal IAQ legislation has been introduced in the U.S. Congress for the past several years. However, none has yet passed, and currently the U.S. Environmental Protection Agency (EPA) has no enforcement authority in this area.

Importance of IAQ

The prominence of IAQ issues has risen in part due to well-publicized incidents involving outbreaks of Legionnaires' disease, Pontiac fever, sick building syndrome , multiple chemical sensitivity , and asbestos mitigation in public buildings such as schools. Legionnaire's disease, for example, caused twenty-nine deaths in 1976 in a Philadelphia hotel due to infestation of the building's air conditioning system by a bacterium called Legionella pneumophila. This microbe affects the gastrointestinal tract, kidneys, and central nervous system. It also causes the non-fatal Pontiac fever.

IAQ is important to the general public for several reasons. First, individuals typically spend the vast majority of their time8090%indoors. Second, an emphasis on energy conservation measures, such as reducing air exchange rates in ventilation systems and using more energy efficient but synthetic materials, has increased levels of air contaminants in offices and homes. New "tight" buildings have few cracks and openings so minimal fresh air enters such buildings. Low ventilation and exchange rates can increase indoor levels of carbon monoxide , nitrogen oxides , ozone , volatile organic compounds, bioaerosols , and pesticides and maintain high levels of second-hand tobacco smoke generated inside the building. Thus, many contaminants are found indoors at levels that greatly exceed outdoor levels. Third, an increasing number of synthetic chemicalsfound in building materials, furnishing, cleaning and hygiene productsare used indoors. Fourth, studies show that exposure to indoor contaminants such as radon , asbestos, and tobacco smoke pose significant health risks. Fifth, poor IAQ is thought to adversely affect children's development and lower productivity in the adult population. Demands for indoor air quality investigations of "sick" and problem buildings have increased rapidly in recent years, and a large fraction of buildings are known or suspected to have IAQ problems.

Indoor contaminants

Indoor air contains many contaminants at varying but generally low concentration levels. Common contaminants include radon and radon progeny from the entry of soil gas and groundwater and from concrete and other mineral-based building materials; tobacco smoke from cigarette and pipe smoking; formaldehyde from polyurethane foam insulation and building materials; volatile organic compounds (VOCs) emitted from binders and resins in carpets, furniture, or building materials, as well as VOCs used in dry cleaning processes and as propellants and constituents of personal use and cleaning products, like hair sprays and polishes; pesticides and insecticides; carbon monoxide, nitrogen oxides, and other combustion productions from gas stoves, appliances, and vehicles; asbestos from high temperature insulation; and biological contaminants including viruses, bacteria, molds, pollen, dust mites, and indoor and outdoor biota. Many or most of these contaminants are present at low levels in all indoor environments.

The quality of indoor air can change rapidly in time and from room to room. There are many diverse sources that emit various physical and chemical forms of contaminants. Some releases are slow and continuous, such as outgassing associated with building and furniture materials, while others are nearly instantaneous, like the use of cleaners and aerosols. Many building surfaces demonstrate significant interactions with contaminants in the form of sorption-desorption processes. Building-specific variation in air exchange rates, mixing, filtration , building and furniture surfaces, and other factors alter dispersion mechanisms and contaminant lifetimes. Most buildings employ filters that can remove particles and aerosols. Filtration systems do not effectively remove very small particles and have no effect on gases, vapors, and odors. Ventilation and air exchange units designed into the heating and cooling systems of buildings are designed to diminish levels of these contaminants by dilution. In most buildings, however, ventilation systems are turned off at night after working hours, leading to an increase in contaminants through the night. Though operation and maintenance issues are estimated to cause the bulk of indoor air quality problems, deficiencies in the design of the heating, ventilating and air conditioning (HVAC) system can cause problems as well. For example, locating a building's fresh air intake near a truck loading dock will bring diesel fumes and other noxious contaminants into the building.

Health impacts

Exposures to indoor contaminants can cause a variety of health problems. Depending on the pollutant and exposure, health problems related to indoor air quality may include non-malignant respiratory effects, including mucous membrane irritation, allergic reactions, and asthma ; cardiovascular effects; infectious diseases such as Legionnaires' disease; immunologic diseases such as hypersensitivity pneumonitis; skin irritations; malignancies; neuropsychiatric effects; and other non-specific systemic effects such as lethargy, headache, and nausea. In addition indoor air contaminants such as radon, formaldehyde, asbestos, and other chemicals are suspected or known carcinogens. There is also growing concern over the possible effects of low level exposures on suppressing reproductive and growth capabilities and impacting the immune, endocrine, and nervous systems.

Solving IAQ problems

Acute indoor air quality problems can be greatly eliminated by identifying, evaluating, and controlling the sources of contaminants. IAQ control strategies include the use of higher ventilation and air exchange rates, the use of lower emission and more benign constituents in building and consumer products (including product use restriction regulations), air cleaning and filtering, and improved building practices in new construction. Radon may be reduced by inexpensive subslab ventilation systems. New buildings could implement a day of "bake-out," which heats the building to temperatures over 90°F (32°C) to drive out volatile organic compounds. Filters to remove ozone, organic compounds, and sulfur gases may be used to condition incoming and recirculated air. Copy machines and other emission sources should have special ventilation systems. Building designers, operators, contractors, maintenance personnel, and occupants are recognizing that healthy buildings result from combined and continued efforts to control emission sources, provide adequate ventilation and air cleaning, and good maintenance of building systems. Efforts toward this direction will greatly enhance indoor air quality.

[Stuart Batterman ]



Godish, T. Indoor Air Pollution Control. Chelsea, MI: Lewis, 1989. Kay, J. G., et al. Indoor Air Pollution: Radon, Bioaerosols and VOCs. Chelsea, MI: Lewis, 1991.

Samet, J. M., and J. D. Spengler. Indoor Air Pollution: A Health Perspective. Baltimore: Johns Hopkins University Press, 1991.


Kreiss, K. "The Epidemiology of Building-Related Complaints and Illnesses." Occupational Medicine: State of the Art Reviews 4 (1989): 57592.

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