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      Characterization of the Chronic Risk and Hazard of Hazardous Air Pollutants in the United States Using Ambient Monitoring Data

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          Ambient measurements of hazardous air pollutants (air toxics) have been used to validate model-predicted concentrations of air toxics but have not been used to perform risk screening at the national level.


          We used ambient concentrations of routinely measured air toxics to determine the relative importance of individual air toxics for chronic cancer and noncancer exposures.


          We compiled 3-year averages for ambient measurement of air toxics collected at monitoring locations in the United States from 2003 through 2005. We then used national distributions of risk-weighted concentrations to identify the air toxics of most concern.


          Concentrations of benzene, carbon tetrachloride, arsenic, 1,3-butadiene, and acetaldehyde were above the 10 −6 cancer risk level at most sites nationally with a high degree of confidence. Concentrations of tetrachloroethylene, ethylene oxide, acrylonitrile, and 1,4-dichlorobenzene were also often greater than the 10 −6 cancer risk level, but we have less confidence in the estimated risk associated with these pollutants. Formaldehyde and chromium VI concentrations were either above or below the 10 −6 cancer risk level, depending on the choice of agency-recommended 10 −6 level. The method detection limits of eight additional pollutants were too high to rule out that concentrations were above the 10 −6 cancer risk level. Concentrations of 52 compounds compared with chronic noncancer benchmarks indicated that only acrolein concentrations were greater than the noncancer reference concentration at most monitoring sites.


          Most pollutants with national site-level averages greater than health benchmarks were also pollutants of concern identified in modeled national-scale risk assessments. Current monitoring networks need more sensitive ambient measurement techniques to better characterize the air toxics problem in the United States.

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          Most cited references 23

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          Spatial and monthly trends in speciated fine particle concentration in the United States

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            Risk Assessment in the Federal Government: Managing the Process

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              National estimates of outdoor air toxics concentrations.

              The Clean Air Act identifies 189 hazardous air pollutants (HAPs), or "air toxics," associated with a wide range of adverse human health effects. The U.S. Environmental Protection Agency has conducted a modeling study with the Assessment System for Population Exposure Nationwide (ASPEN) to gain a greater understanding of the spatial distribution of concentrations of these HAPs resulting from contributions of multiple emission sources. The study estimates year 1990 long-term outdoor concentrations of 148 air toxics for each census tract in the continental United States, utilizing a Gaussian air dispersion modeling approach. Ratios of median national modeled concentrations to estimated emissions indicate that emission totals without consideration of emission source type can be a misleading indicator of air quality. The results also indicate priorities for improvements in modeling methodology and emissions identification. Model performance evaluation suggests a tendency for underprediction of observed concentrations, which is likely due, at least in part, to a number of limitations of the Gaussian modeling formulation. Emissions estimates for HAPs have a high degree of uncertainty and contribute to discrepancies between modeled and monitored concentration estimates. The model's ranking of concentrations among monitoring sites is reasonably good for most of the gaseous HAPs evaluated, with ranking accuracy ranging from 66 to 100%.

                Author and article information

                Environ Health Perspect
                Environmental Health Perspectives
                National Institute of Environmental Health Sciences
                May 2009
                9 January 2009
                : 117
                : 5
                : 790-796
                Sonoma Technology, Inc., Petaluma, California, USA
                Author notes
                Address correspondence to M.C. McCarthy, Sonoma Technology, Inc., 1455 N. McDowell Blvd., Petaluma, CA 94954-6503 USA. Telephone: (707) 665-9900. Fax: (707) 665-9800. E-mail: mmccarthy@

                Current address: Department of Land, Air and Water Resources, University of California, Davis, Davis, California.

                M.C.M., T.E.O., and H.R.H. have all been employed at STI for > 5 years, and J.G.C. is now a student at UC Davis. STI consults for federal, state, and local agencies (e.g., Southeastern States Air Resources Managers, Arizona Department of Environmental Quality) on this topic.

                This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original DOI.

                Public health

                ambient air quality, air quality, air toxics, hazardous air pollutants, risk screening


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