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2005 National-Scale Air Toxics Assessment

National-Scale Air Toxics Assessment for 2002 - Fact Sheet

June 24, 2009

  • The Environmental Protection Agency (EPA) has released the 2002 national-scale air toxics assessment (NATA). The assessment is a state-of-the-science national-scale tool which provides broad estimates of health risks from breathing air toxics.
  • NATA helps air agencies take an important step toward reducing risks from air toxics by identifying exposures that should be evaluated further to gain a better understanding of risks and to determine steps to reduce those exposures where necessary.
  • Air toxics are pollutants known or suspected of causing cancer or other serious health problems, such as birth defects. The 2002 NATA assessed 181 air toxics, such as benzene, methylene chloride, acrolein, and diesel particulate matter (diesel PM). NATA estimates risks from exposure to emissions from large industries, smaller industries and from mobile sources such as cars and trucks.
  • EPA collaborated with state, local and tribal agencies to develop the information that is contained in the NATA.
  • EPA also used results from the 2002 NATA assessment as one of the tools to determine which schools to include in the Agency’s initiative to monitor the air outside a subset of schools.
  • Much progress has been made in reducing air toxics emissions. Since the passage of the 1990 Clean Air Act Amendments, emissions of air toxics have declined 40 percent from all sources. The NATA 2002 can be used to help target further necessary reductions in air toxics emissions.


  • NATA 2002 estimated chronic cancer risks and non-cancer hazards for the 285 million people in the U.S. based on the 2000 census.
  • NATA provides broad estimates of risk over geographic areas of the country and not definitive risks to specific individuals. The results are best used to prioritize pollutants and areas for further study, not as the sole basis for regulation or risk reduction activities.
  • To determine cancer risk, EPA assumes that people would be exposed to the specific concentration over 70 years (an assumed lifetime). This would be an excess cancer risk that is in addition to those cancer cases that would normally occur in an unexposed population.

Cancer Risk

  • NATA estimates that nearly all of the 285 million people in the U.S. have an increased cancer risk of greater than 10 in one million. The average cancer risk for 2002 is 36 in a million. This means that, on average, approximately 1 in every 28,000 people could contract cancer as a result of breathing air toxics from outdoor sources, if they were exposed to 2002 emission levels over the course of their lifetime.
  • Two million (less than 1 percent of the total U.S. population based on the 2000 census) have an increased cancer risk of greater than 100 in one million.
  • There are several contributors to these cancer risks:
    • Pollutants that are emitted locally by industrial sources and therefore have local or “hotspot” effects.  Examples of these include tetrachloroethylene which is emitted from dry cleaning operations and methylene chloride which is used as an industrial solvent in many operations
      • NATA results show that local industry emissions account for about 25 percent of the average overall cancer risk.
    • Pollutants that are emitted from mobile sources and are widespread, for example benzene.
      • NATA results show mobile emissions account for about 30 percent of the overall average cancer risk.  The majority of this risk is from benzene.
    • Pollutants that are considered to be “background” pollutants. These are pollutants for which there are currently no known emission sources, but because these pollutants persist in the environment, they still may be detected in the ambient air by monitors.
      • NATA results show that pollutants in this group account for about 45 percent of overall cancer risk, with carbon tetrachloride contributing nearly half of this risk. 

Non-cancer Hazard

  • Previous NATAs showed that respiratory and neurological health effects are the key chronic non-cancer effects of concern from air toxic exposures. For this reason, the 2002 NATA is only reporting non-cancer results for those two health effects.
  • Of the 23 air toxics showing the potential for neurological effects, 5 are responsible for approximately 95% of the national average neurological hazard with manganese being the most significant contributor at 28%. 
  • Of the 43 air toxics showing the potential for respiratory effects, acrolein is the most significant, contributing about 90 percent of the nationwide average non-cancer hazard.

Diesel PM

  • In this assessment, the potential cancer risk from diesel PM exhaust emissions is not addressed. This is because data are not sufficient to develop a quantitative estimate of the carcinogenic potency for this pollutant. However, EPA has concluded that diesel exhaust is among the substances that may pose the greatest risk.
  • In the 2002 NATA, diesel PM is assessed for non-cancer hazard and is shown to contribute approximately 3 percent to the national average respiratory hazard.


  • This iteration of NATA is based on air toxics emissions information from the year 2002. Emissions information from that year were the most complete and up-to-date available at the time EPA conducted the analysis. Working with industries and states, EPA updates information about air toxics emissions every three years. Once an inventory is complete, EPA conducts the analysis which is then reviewed by the states. Once the review is complete and the results are evaluated for accuracy, EPA releases the NATA.
  • NATA is a screening-level assessment. A screening-level assessment is performed with a limited amount of technical information and with several health-protective assumptions to identify exposures that should be evaluated more carefully with more technical information to gain a better understanding of risks. Also, given that NATA is a screening-level assessment, it is not designed to be used as the sole basis for regulatory action.
  • NATA results are used to target those geographical areas where more refined local-scale assessments or monitoring are needed to identify hotspots.  
  • The risks estimated in the assessment are associated with breathing the pollutants -- it does not address other methods of exposure such as eating or drinking. For the majority of air toxics, most exposure comes from breathing. For some air toxics, a separate assessment of other exposure routes is important.
  • Because of improvements in EPA's NATA methodology, it is not meaningful to directly compare results across the national-scale assessments. Any change in emissions, ambient concentrations, or risks may be due to either improvements in the methodology or to real changes in emission levels.


  • This NATA assessment generally included a four step process, all of which focus on 2002 air toxics data: 
    • Compile a national air toxics emissions inventory of outdoor stationary and mobile sources. The compiled information is called the National Emissions Inventory (NEI)
    • Estimate ambient concentrations of air toxics based on an air dispersion model.
    • Estimate population exposures based on a screening-level inhalation exposure model.
    • Characterize potential cancer and noncancer public health risks due to inhalation of air toxics.
  • EPA plans to develop new national-scale assessments as inventory data from subsequent years become available. The next such analysis will focus on 2005 emissions inventory data. Work on this analysis has begun.
  • The Agency has published two previous national-scale air toxics assessments for the years 1996 (published in 2002) and 1999 (published in 2006).


  • For more information about the National-Scale Air Toxics Assessment, contact the following people at EPA's Office of Air Quality Planning & Standards: Ted Palma at palma.ted@epa.gov; and Dennis Pagano at pagano.dennis@epa.gov.


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