In 2011, EPA released the results of its 2005 national-scale assessment (NATA) of air toxic emissions. The purpose of NATA is to identify and prioritize air toxics, emission source type, and locations that are of greatest potential concern in terms of contributing to population risk. EPA uses the results of these assessments in many ways, including:

• To work with communities in designing their own local-scale assessments,
• To set priorities for improving data in emissions inventories, and
• To help direct priorities for expanding and improving the network of air toxics monitoring.

About the Assessment

The 2005 NATA provides information on 177 of the 187 Clean Air Act air toxics plus diesel particulate matter (diesel PM was assessed for noncancer only). The assessment includes four steps that focus on the 2005 emissions year:

1. Compiling a national emissions inventory of air toxics emissions from outdoor sources,
2. Estimating ambient and exposure concentrations of air toxics across the United States,
3. Estimating population exposures across the United States,
4. Characterizing potential public health risk due to inhalation of air toxics including both cancer and noncancer effects.

For information summarizing the 2005 assessment, see:

• Fact Sheet
• Summary of Results

NATA results are useful in identifying potential patterns in emissions, concentrations and risk from air toxics nationwide and is intended as a tool to prioritize specific air toxics and sources for further study or regulation.

NATA assessment methods continue to be improved. One important improvement in this 2005 assessment is the use of the Community Multiscale Air Quality (CMAQ) model to estimate the atmospheric transformation (formation and decay) of certain air toxics for which such transformation is important. The 2005 NATA predicted the resulting ambient concentrations due to the atmospheric transformation of four air toxics (includes acetaldehyde, formaldehyde, and acrolein) and the atmospheric decay for one air toxic (1,3-butadiene decays to acrolein). The most notable effect of this improvement is the increased estimated, ambient levels of formaldehyde. Nearly 90% of the estimated formaldehyde is a result of this atmospheric formation process. This change coupled with the use of the IRIS unit risk estimate (URE) for formaldehyde resulted in significantly higher formaldehyde cancer risks in many locations than were indicated in previous versions of NATA. Further it is important to note that the inventory that serves as input to CMAQ was the base 2005 NEI which included emissions from all sources including those from forest and wildfires.

This assessment also includes a Technical Methods Document (TMD). This document presents the approaches EPA used to conduct this NATA, including descriptions of how:

• emissions data are compiled and prepared for use as model inputs,
• ambient concentrations of air toxics are estimated,
• exposures to air toxics for populations are estimated,
• toxicity values are selected and assigned to chemicals,
• human health risks and hazards are characterized, and
• variability and uncertainty are addressed.