EPA's focus on IAQ

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Between work, sleep and recreation, most Americans spend more time indoors than outdoors. So why do we have ambient - outdoor - air quality standards and no indoor air quality standards? Since 1970, the federal Clean Air Act has empowered the U.S. Environmental Protection Agency (EPA) to establish and enforce national ambient air quality standards (NAAQS) to protect the public health and welfare - yet no similar mandate exists regarding indoor air quality.

Perhaps part of an answer lies in how we perceive the threat of a problem. To many people, the term "air pollution" conjures visions of huge smokestacks belching voluminous quantities of black smoke, or of a great brown haze shrouding the spires of a modern city. But what of indoor air pollution? Typically, there are no visible signs pointing to deteriorated indoor air quality. You may associate this concept more closely with the short-term, sickening smell that lingers briefly after the exterminator has passed through your office laying down a thin spray of pesticide, or with the faintly nauseating odor emanating from that new carpet that was just put down in the hall outside your office. While the NAAQS strive to produce a healthful environment that meets a common standard throughout the country, each indoor environment is unique, and consequently less amenable to the concept of a common standard.

In recent years, indoor air quality issues have been gaining increased attention, especially within EPA. Comparative risk studies at EPA have ranked indoor air quality among the top five environmental risks to public health. EPA has been conducting studies to understand indoor air pollution in order to devise solutions. One ongoing study is the Building Assessment and Survey Evaluation (BASE) study. A goal of the BASE study is to quantify many of the parameters associated with indoor air quality in a minimum of 100 office buildings across the country. EPA has released some of the preliminary findings of the BASE study.

Factors affecting indoor air quality
Indoor air quality is affected by a number of variables, most notably various sources of emissions and the dynamics of air movements within a building. Air pollutants found within a building can include both natural and anthropogenic materials. Examples of natural air pollutants include pollen, spores, microorganisms, dust and radon. Sources of these materials inside the building might include a small pool of stagnant water in the ventilation ductwork, or old stacks of books and papers that have been collecting dust for years. Sources of these materials outside the building might include the local landscaping or materials borne on the wind from across town.

Examples of anthropogenic air pollutants include volatile organic compounds (VOCs), semi-volatile organic compounds (SVOCs), carbonyl compounds, carbon monoxide, oxides of nitrogen and particulate matter. Sources of these materials inside the building might include maintenance activities, furnishings and certain commercial activities. Sources of these materials outside the building might include automobiles, industrial processes, waste handling and fuel storage.

Once these materials are inside a building, the movement of air within the building largely dictates the exposure of an individual to any of them. In commercial office buildings, air handling within the building is regulated mainly by way of the heating, ventilating and air conditioning (HVAC) system. A well maintained and operated HVAC system can improve the quality of indoor air. Conversely, a problematic HVAC system can contribute to worsening indoor air quality.

The BASE study
EPA set out to develop several sets of data on indoor air quality in office buildings nationwide when the BASE study was conceived in the early 1990s. Among the multiple objectives of the study were to provide a distribution of indoor air quality, building and HVAC characteristics; to examine relationships of symptoms to building characteristics; and to assist in developing guidance on building design, construction, operation and maintenance.

Buildings were selected randomly from cities with a population of more than 100,000, from 10 climate zones in the United States. A building could not be included in the study if it had been involved in any highly publicized indoor air quality problem. A study area within a chosen building had to have a minimum of 50 people and be served by no more than two air handling systems.

Researchers spent one week in each building selected, collecting data following an agreed-upon protocol. The types of data collected for the indoor study areas included: (1) air concentrations of VOCs, carbonyls, particulate matter, carbon monoxide, carbon dioxide, radon and microorganisms; (2) values of temperature and relative humidity; (3) building characteristics; (4) HVAC characteristics; and (5) occupant perceptions of indoor air quality.

Summary of preliminary BASE VOC findings
Three indoor samples and one outdoor sample were collected and analyzed for each building for VOCs following EPA method TO-14. Based on available VOC data for the first 41 buildings studied, a total of 50 different VOCs were measured in quantifiable concentrations. Table 1 presents a listing of the 10 most frequently quantified VOCs among these initial samples and their range of concentrations. Table 1 also includes corresponding odor threshold concentrations, where available, from published data. It can be noted that even the maximum measured concentrations for each of these 10 VOCs are typically below their individual odor threshold values (in this example, toluene is an exception). However, cumulative concentrations of all VOCs present may still have perceptible odors to some individuals.

Table 1

Ten most frequently measured VOCs in office buildings


Range of concentrations

Odor threshold


3.8 - 390



12 - 240



1.6 - 130



1.3 - 52



1.3 - 22



1.7 - 61



1.5 - 300



3.7 - 570



5.1 - 2,000

None available

m- and p-Xylenes

4.0 - 69


Of the 50 VOCs detected indoors in the samples from the initial 41 buildings, 47 had median indoor-to-outdoor ratios greater than 1. Twenty-one of these 50 VOCs detected indoors had median indoor-to-outdoor ratios greater than 2. The highest median indoor-to-outdoor ratio for any VOC was 24.

While many of the VOCs measured in the preliminary stage of the BASE study are listed in the Clean Air Act as hazardous air pollutants (HAPs), it is important to remember that there are no NAAQS for HAPs. EPA's strategy for dealing with HAPs is through control technology, by establishing national emission standards. These compounds have a wide range of potential health effects, and long-term exposure to some of them, if occurring both indoors and outdoors, may dispose particularly sensitive individuals to certain deleterious effects. Table 2 summarizes some of these potential health effects for the 10 most frequently quantified VOCs among these initial BASE samples.

Table 2

Potential health effects of commonly measured VOCs in indoor air


Health effects


Narcotic; may cause anemia


Eye, skin and respiratory tract irritant


Structural change in nerves; eye and skin irritant


Hallucinations or distorted perceptions; narcotic; eye and skin irritant


Degenerates central nervous system; affects liver, kidneys, and heart


Carcinogen; respiratory tract irritant


Central nervous system depressant; eye and skin irritant


Respiratory system depressant; narcotic


Conjunctiva irritant; affects liver; narcotic

m- and p-Xylenes

Affects heart, liver, kidneys and nervous system; eye and skin irritant

Virtually any of the wide array of air pollutants we find in the ambient air can be found in indoor air - often in even higher concentrations than in outdoor air. Although there are no indoor air quality standards for the various compounds that exist inside public and private buildings alike, EPA is conducting the BASE study to describe the many parameters that characterize the indoor air quality of large office buildings. As of September 1998, EPA had completed sample collection at 100 buildings under the BASE study, and the agency plans to issue initial findings on the complete data set by the end of 1999. These findings should be instrumental in the ultimate development of guidelines to improve the quality of our indoor air.

Resources: (1) American Industrial Hygiene Association. Odor Thresholds for Chemicals with Established Occupational Health Standards, Fairfax, Virginia, 1989. (2) Hadwen, G.E., McCarthy, J.F., Womble, S.E., Girmin, J.R., Brightman, H.S. "Volatile Organic Compound Concentrations in 41 Office Buildings in the Continental United States," Proceedings of Healthy Buildings/IAQ 97, Washington, D.C, 1997. (3) Lewis, R.J. Sax's Dangerous Properties of Industrial Materials, 8th ed., New York , New York, 1992. (4) U.S. Environmental Protection Agency. Reference Guide to Odor Thresholds for Hazardous Air Pollutants Listed in the Clean Air Act Amendments of 1990 (EPA-600/R-92-047), Research Triangle Park, North Carolina, 1992.


EPA's indoor air quality page

EPA's BASE study

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This article originally appeared in the December, 1999 issue of Environmental Protection magazine, Vol. 10, Number 12, pp. 28-32.

This article originally appeared in the 12/01/1999 issue of Environmental Protection.

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