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U.S. air only fair. (Air Pollution).

In the first federal study to assess ambient concentrations, human exposures, and estimated risks of a wide range of air pollutants, the U.S. Environmental Protection Agency (EPA) has found that millions of people live in areas where air toxics may pose potentially significant health concerns. Although this study of 1996 data only partially addresses the question of how much risk air toxics actually currently pose, all indications point to the risk being much higher than goals set by the EPA.

The National-Scale Air Toxics Assessment (NATA) found that the risk of developing any kind of cancer over a lifetime due to exposure to certain air toxics exceeded 10 in 1 million for the contiguous 48 states, Puerto Rico, and the U.S. Virgin Islands, substantially higher than the EPA's goal of 1 in 1 million. More than 20 million people lived in counties with much higher risk, exceeding 100 in 1 million, and localized "hot spots" posed an even higher risk within some counties.

To assess noncancer risks, the agency adopted a hazard index based on an evaluation of the respiratory hazard posed by eight of the air toxics. Many of the data for noncancer toxic mechanisms are not yet available, so the index is considered the best method to reflect a range of noncancer effects. A rating above 1.0 indicates the potential for adverse health effects, while a rating below 1.0 suggests that exposures pose little lifetime risk. The vast majority of the population lived in areas with an index above 1.0, and more than 20 million people lived in counties with an index above 10.0.

NATA uses data updated every three years as part of the National Toxics Inventory. The first findings, released 31 May 2002, are based on 1996 data, the latest available for all the evaluated substances. The study estimated the presence of and health risks posed by 32 air toxics, including benzene, chromium, formaldehyde, arsenic, acetaldehyde, acrylonitrile, acrolein, and cadmium. In addition, the EPA calculated the presence of diesel particulate matter in each county, but did not include it in the risk assessment because it doesn't yet have an agency-assigned numerical risk factor (and isn't expected to for several years).

The 33 air toxics--selected based in part on the availability of emissions and risk assessment data--are among the 188 air toxics for which the EPA must still develop emissions standards. The selected air toxics come from a variety of sources. On- and off-road vehicles account for about half the emissions. Other sources include industry (sources such as chemical plants and oil refineries generate more than 40% of 20 air toxics) landfills, fertilizers, fireplaces, wildfires, and globally transported pollutants.

This first generation of NATA was not able to fully answer the question of how much risk air toxics pose. For instance, when diesel risks are eventually included, the cumulative risk ratings could rise substantially [see "Fuel for the Long Haul? Diesel in America," p. A454 this issue]. And when the EPA compared modeled emissions to seven that it had actually monitored, it found that the modeled emissions were sometimes as low as 15% of the monitored emissions. In addition, risks posed by noninhalation pathways, such as ingestion or dermal absorption, are not included in the risk assessment, nor are many risks from indoor exposures or from sources such as agriculture operations. The 155 other air toxics not studied but considered high priority by the EPA also contribute some risk, as do tens of thousands of other chemicals in regular use.

On the other hand, actual risks from the substances assessed may be lower, because there are built-in margins of safety in the risk assessment for each substance. And risks borne by any one individual may be lower (or higher) than those assumed for a "typical" person in this study.

EPA officials plan to use NATA information to help identify the air toxics of greatest concern, improve understanding about sources and exposures, and set priorities for future work. However, the results are not intended for direct use in regulatory actions.

One potentially affected industry, represented by the American Chemistry Council of Arlington, Virginia, says it sanctions the EPA's fledgling efforts. "We support the EPA release of NATA as a first step in making broad assessments about where there are remaining risks and the relative contributions from cars, small business, and larger industry," council spokesman Chris VandenHeuvel says.

EPA and ACC officials say that emissions of the 33 air toxics are lower now, or should be soon, following implementation of numerous pollution control actions since 1996. Some of those reductions may be reflected in the 1999 data to be used for the next iteration of NATA, scheduled for release at the end of 2003. The current results are available on the Internet at
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Author:Weinhold, Bob
Publication:Environmental Health Perspectives
Date:Aug 1, 2002
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