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2005 National-Scale Air Toxics Assessment

About the 2005 Assessment

EPA's 2005 national-scale assessment characterizes risks from air toxics at a particular point in time. The assessment looks at human health impacts from estimated, chronic inhalation exposures based on emissions data from the 2005 National Emissions Inventory for hazardous air pollutants, assuming these emissions remain constant throughout one's lifetime (not today's levels or projected levels).

NATA produces results that 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. The 2005 assessment includes emissions, ambient concentrations, and exposure estimates for 177 air toxics (PDF) (3pp, 15k) plus diesel PM. For 139 of these air toxics (those with health data based on chronic exposure), the assessment includes cancer or noncancer health effects, or both including noncancer health effects for diesel PM. For the 39 air toxics with no health effects information, only the air concentration estimates (ambient and exposure) are provided. Ten air toxics (see FAQ #5) were not included in this NATA assessment because either no emissions information was reported for them in 2005 or emission estimates useful for modeling could not be determined reliably from their reported emissions (e.g., radionuclides).

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. (see Question #11 on the Frequent Questions page for further information). 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.

Specifically, this document summarizes the data sources, methods, models, and assumptions used in NATA that have been published in various EPA reports and have been available on the various NATA assessment web sites. Presenting this information in one place provides those interested in NATA with a more convenient, detailed resource than has been available in the past.

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