Background

On December 17, 2015, the U.S. Environmental Protection Agency (EPA) released the fifth version of the National Air Toxics Assessment (NATA) – a state-of-the-science screening tool that provides information on the potential risks from breathing air toxics. This tool can inform efforts to identify and prioritize air toxic pollutants, source types, and locations of potential concern.

NATA uses emissions data compiled for a single year as inputs for modeling ambient air concentrations and estimating health risks. Results include estimates of ambient concentrations and exposure concentrations (ECs) of air toxics and estimates of cancer risks and potential noncancer health effects associated with chronic inhalation exposure to air toxics. The estimates are generated within each state, at both county and census-tract levels.

NATA provides a “snapshot” of outdoor air quality and the risks to human health that might result if air toxic emission levels were to remain at the same levels as those estimated for the assessment year. The estimates reflect only risks associated with chronic (relatively long-term) exposures to the inhalation of air toxics at the population level.

•EPA developed NATA because there is not a large, nationwide monitoring network in place for the 187 pollutants identified as air toxics. To understand potential health risks from breathing air toxics, the agency developed the model-based NATA.

•NATA is useful in informing national and localized efforts to collect air toxics information, characterize emissions and prioritize pollutants and geographic areas for more refined data collection and analyses, and other activities designed to reduce pollution and risks to public health. However, it should not be used to single out or rank areas of the country as having the highest risk.

•EPA collaborates extensively with industry, state, local and Tribal air agencies to develop and quality assure the information contained in the assessment.

•For more information on NATA, visit the website at: www.epa.gov/national-air-toxics-assessment

•For additional questions and answers regarding NATA, go to: www.epa.gov/national-air-toxics-assessment/nata-frequent-questions

Highlights

The 2011 NATA uses improved emissions and modeling methods that help better characterize risks across the country.

•Monitoring data and emissions inventories show overall reductions in air toxics across the country as a result of Clean Air Act programs. Results from the 2011 NATA support this downward trend.

•Emissions from fires and biogenic sources are included in this version of NATA and more detailed location information was available for some sources, such as oil and gas wells and airports.

•Two air quality models - a long range transport and chemistry model and a near-field model - were combined to estimate ambient air concentrations of air toxics across the country.

•By combining the two models, EPA is able to provide an improved air quality prediction that accounts for air pollution transport across regions, secondary pollution formation in the atmosphere, and local conditions near specific sources.

Sources

•Secondary formation is the largest contributor to cancer risks nationwide, accounting for 47 percent of the risk. On-road mobile sources contribute the most risk from directly emitted pollutants (about 18 percent).

•Nationally, on-road mobile sources contribute the largest amount to non-cancer risks (34 percent). Nonroad mobile sources and nonpoint sources also contributed to the non-cancer risks in nearly equal amounts (15 percent).

•Contributing sources in some areas may be local industry sources.

Geographic Areas

•According to the 2010 U.S. census, there are approximately 78,000 census tracts in the country. Based on the 2011 NATA, EPA estimates there are approximately 130 tracts (or less than one percent) with cancer risks greater than 100-in-1 million.

•For census tracts with risk greater than 100-in-1 million, the risks are due to large and small industry, as well as mobile sources and secondary formation.

•Urban areas tend to have higher overall estimates of cancer and non-cancer risks than rural areas because there are more air toxics emissions from varied sources, in addition to higher population densities. Secondary formation also tends to occur more in urban areas because of the complex mixture of emitted pollutants.

Overview

The following tables present the EPA's 2011 NATA ambient concentration, exposure concentration, and risk estimates across the United States plus Puerto Rico, the Virgin Islands, and the District of Columbia. The 2011 NEI, after review by State, Local, and Tribal agencies, was used as input to the AERMOD and CMAQ models to generate ambient concentrations. The ambient concentrations were used as input to the inhalation exposure model (HAPEM7) to generate exposure concentrations. Exposure modeling is an important step in this assessment because it takes into account that people move from one location to another (e.g., from outside environments to inside environments). Exposure concentrations were then used with health-benchmark information to estimate risks or hazards. More information on the 2011 NATA methods can be found on the Assessment Methods page or in the Technical Support Document.

These data are presented in different ways. The Access and Excel files contain risks at nationwide, state, county, and census tract levels for each risk or hazard quotient endpoint. Additionally, the data are presented by 41 source sectors.

While the file, 2011 NATA National Respiratory Risk by Tract Source spreadsheet, presents the non-cancer results specifically for the respiratory endpoint, the individual state and pollutant files provide the non-cancer information for other endpoints (e.g., neurological, reproductive system), when a dose-response value was available for that air toxic. Information on these additional endpoints, (e.g., the endpoint and critical concentration used for each), can be found in Appendix H of the 2011 NATA Technical Support Document.

In the 2011 NATA results, results for stationary sources are broken into two groups: “point” and “nonpoint” sources. These designations reflect the way each emission source was modeled. Point sources are those that have the location of their emissions identified with latitude and longitude coordinates. These can include larger sources such as large industrial complexes as well as some smaller sources, such as dry cleaners. Nonpoint sources are those where the specific location for the emission source is not known. These are generally smaller sources or sources related to residential activity such as residential wood combustion or consumer and commercial solvent usage. Emissions from these nonpoint sources, which are generally inventoried on a county-wide basis, are allocated to a census tract for modeling using appropriate surrogates.

This nonpoint source approach was used for mobile source emissions as well.

For the airports modeled in NATA, the locations of these emissions were actually known and thus, were modeled at their actual locations. However, the risk results for airports are summarized under the mobile source category, in the nonroad source group.

The 2011 NATA is based on emissions for the 2011 calendar year – the most complete and up-to-date emissions data available at the time of the assessment.

The 2011 NATA assessed 180 air toxics such as benzene, formaldehyde and acrolein, plus diesel particulate matter (PM) (181 total), from the following types of emissions sources:

Stationary sources, e.g., industrial facilities such as coke ovens for the steel industry, refineries and smaller sources like gas stations

Mobile sources, e.g., cars, trucks and off-road vehicles like construction equipment and trains

Events, e.g., wildfires, prescribed burning

Biogenics, e.g., naturally-occurring emissions

In addition, NATA includes:

Secondary formation, e.g., pollutants that form from chemical reactions from other pollutants emitted into the air such as formaldehyde

Background, e.g., long-range transport from distant sources

The emissions data were then modeled to make broad estimates of health risks (both cancer and non-cancer risks) over geographic areas of the country to provide a snapshot of air quality in 2011. Estimated health risks are provided for about 140 air toxics for which we have health benchmarks.

Disclaimer

EPA suggests that the results of this assessment be used cautiously, as the overall quality and uncertainties of the assessment will vary from location to location as well as from pollutant to pollutant. In many cases more localized assessments, including monitoring and modeling, may be needed to better characterize local-level risk. It is important to consider when using these results that NATA is a screening-level assessment and only suitable to answer certain questions. The variability and uncertainty in the assessment should be kept in mind. For example, results are more uncertain at finer geographic scales. And, results should not be used to compare different areas of the country or to rank different areas. This is because emissions, air concentrations, exposures, and risks are not the same throughout the U.S., and are not the same for everyone. It is important to have an idea of how these factors vary throughout the U.S. In addition, the EPA seeks to protect health with reasonable confidence. But scientific estimates of air concentrations, exposures, and risks always involve assumptions that simplify the real situation and introduce uncertainties in order to make the assessment possible. A more complete discussion of variability and uncertainty is found in Section 7, Variability and Uncertainty Associated with NATA, of the TSD.