The Mickey Leland National Urban Air Toxics Research Center (Leland Center) was established by Congress under Title III, Section 301(p) of the 1990 Clean Air Act Amendments. Congress created the Leland Center as a non-profit, public/private research organization to sponsor research on the potential human health impacts of the 188 listed air toxics. The Leland Center is governed by a nine-member Board of Directors, appointed by Congress and the President. A thirteen-member Scientific Advisory Panel, composed of nationally-recognized scientists and physicians, establishes the Leland Center's peer-reviewed research program. The Leland Center's mission is to contribute meaningful and relevant data to the scientific literature on the potential human health effects of air toxics. We view this contribution as a fundamental component in the national effort to develop cost-effective and balanced regulations to protect the public health from the potential risks of air toxics.
After exploring the most critical public health aspects of air toxics risks, the Leland Center's Board of Directors identified two fundamental research data gaps: (1) the determination of the actual human exposures to air toxics in urban environments, and (2) the non-cancer health effects of such exposures. The Leland Center chose to pursue personal exposure research. We were the first research institution to develop a research program on personal exposures to air toxics in urban populations.
Exposure
Traditionally, ambient air concentrations of air toxics have been equated with adverse health effects. Under this approach, the larger the airborne concentration, the larger the potential human health risk. However, it is actual exposure, and not air concentrations, that is the critical component needed to determine potential adverse health effects from a pollutant emission into the environment. High airborne concentrations of air pollutants in an area without people means there is no exposure. Without exposure there is no human health risk.
In its March 1998 report Research Priorities for Airborne Particulate Matter-Volume I, the National Research Council states that the relationship among outdoor, indoor, and personal exposure is a fundamental factor in determining potential human health effects. Exposure is one of the two major elements (the other being the determination of the most biologically active constituents (of a pollutant or particle) on which other research, such as epidemiological studies, should be based. The National Research Council named exposure research as one of the 10 most critical research areas. Only with exposure information can the potential public health impacts be calculated.
Exposure Defined
Exposure is the contact of a chemical, biological or physical agent at the boundary of the body over a specified time period. People are exposed to chemicals through inhalation, ingested through food or absorbed through the skin. People are exposed to air pollutants primarily through inhalation. However, deposition onto soil, food, and water, can result in other exposure routes. Actual human exposure is a function of outdoor sources, indoor sources, and human activity patterns. (NRC, 1998). Thus, what people are actually exposed to is a result of where they spend their time and what air pollutants are present in those areas. People do not spend their time in just one location, but rather move through a series of locations (or microenvironments) such as the home, car, office, outside, throughout the day.
Exposure Assessment
Exposure assessment is the science of determining what people are exposed to and how they come into contact with various contaminants. Exposures can be estimated by a number of methods. See Attachment 1. The most accurate measurements are obtained by measuring people directly, such as through the use of personal monitors, breath, urine, and blood samples. Exposure assessments are used in epidemiology studies, risk assessments, analysis of trends, and risk management decisions.
Exposure Sources
Most individuals in the United States spend the majority of their time indoors. See Attachment 2. In some cities, such as Houston, people spend approximately 90 percent of their time indoors. Outdoor pollutants may be brought inside through open windows, ventilation systems, food, water, tracked-in soil, and consumer products. These pollutants may even undergo chemical reactions once inside a building or home producing yet other pollutants. While ambient air toxics can penetrate into homes, offices, and cars, many chemicals, classified as air toxics under the Clean Air Act, are also emitted directly into the indoor air from consumer and cleaning products and building materials. Carpet, paint, and air deodorizers may all release chemicals into the indoor environment. Even taking a hot shower, washing dishes or clothes in hot water, may release chemicals, such as chloroform. (Wallace et al., 1993).
In addition, some emissions near a person's face can contribute significant concentrations to personal exposure, while contributing a negligible amount to ambient concentrations. Wearing recently dry-cleaned clothing is an example. (Wallace et al., 1993). Smoking is another example. Smoking accounts for the largest percentage of personal exposure to benzene, yet the activity of smoking releases little benzene into the surrounding air.
Thus, when the contribution of outdoor sources is minimal compared to indoor sources or the air immediately surrounding an individual, ambient air emissions are not a good indicator of personal exposure. It is therefore important to determine indoor air concentrations, the sources of those concentrations and the relative contribution from outdoor, indoor and personal sources in determining what people are really exposed to in their daily lives.
Personal Exposure Studies
Several studies have been carried out to assess the relationship among outdoor, indoor, and personal air to determine the sources of the exposure to air toxics. The most comprehensive U.S. study to date has been the Total Exposure Assessment Methodology (TEAM) study. This study was conducted in phases from 1980-1987 (e.g., Wallace et al., 1987, 1988). In addition, Phase I of the National Human Exposure Assessment Survey (NHEXAS), conducted from 1995-1997 (Pellizzari et al., 1995, Sexton et al., 1995) also examined this relationship. The major purpose of the TEAM study was to measure the personal exposures to select chemicals in urban populations in several U.S. cities. One phase of the TEAM study examined personal exposures of 600 people to a number of toxic or carcinogenic chemicals in the air and drinking water. One central hypothesis of the TEAM study was that emissions from major industrial sources in urban areas would be the primary source of the personal exposures to volatile organic compounds of study participants who lived in these areas. In addition, it was further surmised that these industrial sources would be the major source of indoor air pollutant concentrations.
However, one of the primary findings of the TEAM study was that for air toxics, indoor sources were the primary contributor to indoor air concentrations and to personal exposures for the majority of air toxics measured. See Attachment 3. Researchers determined that in many instances, the contribution of outdoor air toxics concentrations to personal exposure was negligible. The TEAM study concluded that it was sources other than outdoor air, that were controlling indoor and personal air concentrations.
The NHEXAS study was designed to determine population-base exposures to select air toxics, PM2.5 and pesticides in urban, suburban and rural settings. NHEXAS pilot study results were similar to that of the TEAM Study. (The full NHEXAS study has not yet been initiated.)
There are instances where outdoor sources may be primary source. If compounds have minimal or no indoor sources, then penetration from outside source contaminants can be the dominant source. For example, carbon tetrachloride has been banned from use in consumer products, but has a long residence time in the ambient air. Thus, outdoor air is the source for the indoor air levels and personal exposure to this compound. (Baek, 1997).
Outdoor sources may be a significant contributor to indoor and personal exposures in homes immediately adjacent to ambient sources, such as factories, parking garages, heavily trafficked streets or dry cleaners. One current study, The Relationship Among Indoor, Outdoor, and Personal Air (RIOPA) Study, being conducted by the Environmental and Occupational Health Sciences Institute, is evaluating this hypothesis. The RIOPA study is examining this relationship in 100 homes in three urban areas (Houston, Texas; Los Angeles, California; and Elizabeth, New Jersey). All of these homes are near major outdoor sources of air pollutants or in heavily trafficked areas. In these homes, researchers are placing monitors outside and inside the home to measure concentrations of select compounds. In addition, participants will wear a personal monitor for 48 hours and keep a diary of their activities during this time period. The RIOPA study is measuring select VOC, aldehydes, and PM 2.5 in a three-year study. Some of the pilot study results are attached as Attachment 4. This study will provide specific information on the impact of outdoor sources of air toxics to personal exposures for residents living close to ambient source concentrations.
Future Research Directions
Only by understanding the relationship among outdoor, indoor and personal exposures can public health impacts be assessed. Additional research on indoor air, including indoor air chemistry (what happens to air pollutants in a home or building) and the sources of such concentrations needs to be examined. Additional research on the relationship among outdoor, indoor and personal exposure needs to be conducted. While an exposure research program exists for particulate matter, there is no such overall integrated program for the 188 air toxics listed in Section 112 of the Act.
Exposure is the link between ambient air concentrations and public health impacts. A dialogue is needed about the role of ambient air toxics monitoring in exposure research along with the role of monitoring microenvironments and "hot spots" and their relationship to personal exposure.
Conclusion
It is personal exposure to air pollutants, and not air concentration that is the critical component in assessing the public health impacts from air pollutants. Science has now established that indoor air pollutants can be a major contributor to a person's overall exposure to air pollutants. In addition, the findings from these studies indicate that the same air pollutants subject to regulation under the Clean Air Act are often found at much higher levels indoors. The Clean Air Act Amendments of 1990 rely solely on the assumption that outdoor levels are determinative of an individual's exposure and hence risk. The Act does not address the contribution of indoor sources of air pollution or the differences between indoor and outdoor quality.
Absent information about personal exposures, the real public health risk of air toxics cannot be accurately assessed. Merely reducing the ambient emission levels may not result in improved public health. By focusing on exposure, we can determine where the greatest risk to public health lie and tailor the solution to correct the problem. The Leland Center will continue to focus our research on addressing the critical area of exposure, thus allowing for a more cost-effective approach to protecting public health under the Clean Air Act.
Attachments
REFERENCES
Baek, S-O, Kim, Y-S, Perry, R (1997). Indoor air quality in homes, offices and restaurants in Korean urban areas-indoor/outdoor relationships. Atmospheric Environment 31:529-544.
National Research Council (1998). Research priorities for airborne particulate matter volume I-immediate priorities and a long-range research portfolio. National Academy Press. March 1998
Ott, W.R., Roberts J.W. (1998). Everyday exposure to toxic pollutants. Scientific American. February 1998
Pellizzari, E, Lioy, P, Quackenboss, J, Whitmore, R, Clayton, A, Freeman, N, Waldman, J, Thomas, C, Rodes, C, Wilcosky, T (1995). Population-based exposure measurements in EPA region 5: A Phase I field study in support of the National Human Exposure Assessment Survey. Journal of Exposure Analysis and Environmental Epidemiology 5:327-358.
Sexton, K, Kleffman, DE, Callahan, MA (1995). An introduction to the National Human Exposure Assessment Survey (NHEXAS) and related Phase I field studies. Journal of Exposure Analysis and Environmental Epidemiology 5:229-232.
Wallace, L (1991). Comparison of risks from outdoor and indoor exposure to toxic chemicals. Environmental Health Perspectives 95:7-13.
Wallace, L (1996). Indoor particles: A view. Journal of Air & Waste Management Association 46:98-126.
Wallace, LA, Pellizarri, ED, Hartwell, TD, R, W, Zelon, H, Peritt, R, Sheldon, L (1988). The California TEAM study: breath concentrations and personal exposure to 26 volatile compounds in air and drinking water of 188 residents of Los Angeles, Antioch, and Pittsburg, CA. Atmospheric Environment 22:2141-2163.
Wallace, LA, Pellizzari, ED, Hartwell, TD, Sparacino, C, Whitmore, R, Sheldon, L, Zelo, H, Perritt, R (1987). The TEAM study: Personal exposures to toxic substances in air, drinking water and breath of 400 residents of New Jersey, North Carolina, and North Dakota. Environmental Research 43:290-307.
Weisel, C. The influence of ambient air sources on exposure to air toxics: results from the RIOPA pilot. Prepared for the Air Toxics Session of the 1999 American Bar Association Meeting.