Health & Medical Environmental

Determining Risk from Pollutants of Ambient Origin

´╗┐Determining Risk from Pollutants of Ambient Origin
Human exposure research has consistently shown that, for most volatile organic compounds (VOCs), personal exposures are vastly different from outdoor air concentrations. Therefore, risk estimates based on ambient measurements may over- or underestimate risk, leading to ineffective or inefficient management strategies. In the present study we examine the extent of exposure misclassification and its impact on risk for exposure estimated by the U.S. Environmental Protection Agency (U.S. EPA) Assessment System for Population Exposure Nationwide (ASPEN) model relative to monitoring results from a community-based exposure assessment conducted in Baltimore, Maryland (USA). This study is the first direct comparison of the ASPEN model (as used by the U.S. EPA for the Cumulative Exposure Project and subsequently the National-Scale Air Toxics Assessment) and human exposure data to estimate health risks. A random sampling strategy was used to recruit 33 nonsmoking adult community residents. Passive air sampling badges were used to assess 3-day time-weighted-average personal exposure as well as outdoor and indoor residential concentrations of VOCs for each study participant. In general, personal exposures were greater than indoor VOC concentrations, which were greater than outdoor VOC concentrations. Public health risks due to actual personal exposures were estimated. In comparing measured personal exposures and indoor and outdoor VOC concentrations with ASPEN model estimates for ambient concentrations, our data suggest that ASPEN was reasonably accurate as a surrogate for personal exposures (measured exposures of community residents) for VOCs emitted primarily from mobile sources or VOCs that occur as global "background" source pollutant with no indoor source contributions. Otherwise, the ASPEN model estimates were generally lower than measured personal exposures and the estimated health risks. ASPEN's lower exposures resulted in proportional underestimation of cumulative cancer risk when pollutant exposures were combined to estimate cumulative risk. Median cumulative lifetime cancer risk based on personal exposures was 3-fold greater than estimates based on ASPEN-modeled concentrations. These findings demonstrate the significance of indoor exposure sources and the importance of indoor and/or personal monitoring for accurate assessment of risk. Environmental health policies may not be sufficient in reducing exposures and risks if they are based solely on modeled ambient VOC concentrations. Results from our study underscore the need for a coordinated multimedia approach to exposure assessment for setting public health policy.

The absence of human exposure information constitutes a critical source of uncertainty for risk-based regulatory decision making. Risk assessments are used by the U.S. Environmental Protection Agency (U.S. EPA) to estimate the likelihood that exposure to a given pollutant will produce an adverse health effect and to determine what regulatory actions are necessary to protect public health. In the absence of human exposure data, policy makers, risk assessors, regulators, researchers, and public health officials often must rely on estimates or surrogates of human exposure levels, such as proximity to a hazardous waste site or regional ambient air quality data. Such estimates may be derived from models that predict levels of environmental contamination in the air. These approaches are limited in identifying health risks because they rely on assumptions about actual exposures experienced by people, thus introducing uncertainty in their risk estimates and ensuing policies. Although monitoring is generally recognized as providing a more reliable estimate of exposure, it carries its own limitations, such as cost for implementing on a large population scale over long periods of time to estimate long-term exposures.

In 1995, the U.S. EPA released the results of its Cumulative Exposure Project (CEP). Under the CEP, the U.S. EPA used an air dispersion model, the Assessment System for Population Exposure Nationwide (ASPEN) model, and 1990 emissions inventory data to characterize the magnitude, extent, and significance of airborne outdoor concentrations for 148 hazardous air pollutants (HAPs) listed under the 1990 Clean Air Act Amendments (CAAA 1990) for each of the 60,803 census tracts in the contiguous United States (Woodruff et al. 1998). Although the model estimated exposures to HAPs of ambient origin, by default they were assumed to represent total human exposure forming the basis for human health risk estimation. Therefore, not only the validity of the ASPEN estimate relative to ambient measurements of interest but also the magnitude of the difference relative to personal and indoor exposure and the significance of this difference in risk estimation are important to understand. Results from the CEP suggested that HAP exposures were prevalent nationwide and that, in some locations, concentrations were significantly higher than concentrations associated with the one-in-one million excess cancer risk, levels considered by U.S. EPA researchers as a benchmark for acceptable "de minimus" risk (Caldwell et al. 1998; Woodruff et al. 1998). U.S. EPA researchers also concluded that HAP concentrations estimated by the model may pose a significant public health problem, especially in urban census tracts and census tracts of predominantly low-income and minority populations (Morello-Frosch 1997; Morello-Frosch et al. 2000). The main sources of the HAPs were found to be mobile (e.g., automobiles and trucks) and area sources (e.g., dry cleaners and gas stations).

The CEP has provided critical information about possible population exposures to HAPs and their relationship with population demographics (race, ethnicity, and income) never before revealed on a national scale. In addition, the CEP has served as a prototype for the National-Scale Air Toxics Assessment (NATA). The U.S. EPA released the NATA modeling and risk assessment results in two phases: first, in September 2000, the ASPEN modeling data only; and then later in May 2002, results from a human exposure module [Hazardous Air Pollutant Exposure Model 4 (HAPEM4)] added to ASPEN and related risk estimates using 1996 air toxics emissions data as input for the ASPEN model. Given the paucity of comprehensive ambient air monitoring data for HAPs and even fewer human exposure data, national air toxics modeling as carried out by the U.S. EPA will play a significant role in identifying effective control strategies to reduce public health risks from exposure to HAPs as required by 1990 CAAA, and in shaping national policies to reduce air pollution emissions.

Many volatile organic compounds (VOCs) are listed by the U.S. EPA as HAPs (e.g., benzene, carbon tetrachloride, and chloroform) and were included in the CEP. Beginning in the 1980s with the U.S. EPA's Total Exposure Assessment Methodology (TEAM) studies of VOCs, it has been demonstrated repeatedly that personal exposures typically exceed outdoor air concentrations and that levels of human exposure to VOCs depend on people's locations, especially indoors, where people spend up to 90% of their time (Akland et al. 1997; Buckley et al. 1997; Clayton et al. 1999; Cohen et al. 1989; Kinney et al. 2002; Leung and Harrison 1998; Lioy 1990; Otson et al. 1994; Ott 1990; Pellizzari et al. 1999; Seifert et al. 1989; U.S. EPA 1987; Wallace 1993; Wallace et al. 1985; Weisel 2002). Therefore, assessment of potential public health impacts from HAPs is limited by the uncertainty in exposure estimates based on fixed-site ambient monitoring or models that use ambient concentrations to estimate exposure. Environmental policies that focus solely on reducing HAP emissions from stationary or point sources may not be effective in reducing human exposures and risks when the indoor environment is a significant contributor to exposures.

At the same time that the U.S. EPA released results from the CEP, a community-based VOC exposure study, conducted in partnership with South Baltimore, Maryland, community leaders, was in the planning phases. As a result, an opportunity arose to examine a) whether ambient concentrations of VOCs based on the U.S. EPA's ASPEN model were adequate estimates of ambient air toxic concentrations in South Baltimore and b) the magnitude of the differences between ambient estimates and the more health relevant indoor and personal exposures and the differences between their associated health risks. In this article we present the results of this investigation.

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