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Limitations

EPA developed this assessment tool to inform both national and more localized efforts to collect information and characterize/reduce air toxics emissions (e.g., prioritize pollutants/geographic areas of interest for monitoring and community assessments). 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. The points below highlight limitations to consider when looking at the results:

  • gaps in data
  • limitations in computer models used
  • default assumptions used routinely in any risk assessment
  • limitations in the overall design of the assessment (intended to address some questions but not others).
  • variations in detail and completeness of inventories from different geographical regions

The following are important specific limitations to recognize:

  • The results apply to geographic areas, not specific locations.
  • The results do not include impacts from sources in neighboring countries (i.e., Canada or Mexico).
  • The results apply to groups, not to specific individuals.
  • The results are restricted to 2005 since the assessment used emissions data from 2005.
  • The results do not reflect exposures and risk from all compounds.
  • The results do not reflect all pathways of exposure.
  • The results reflect only compounds released into the outdoor air.
  • The results do not fully reflect variation in background ambient air concentrations.
  • The results might systematically underestimate ambient air concentration for some compounds (see summary of monitor to model comparison).
  • The results used default, or simplifying, assumptions where data were missing or of poor quality.
  • The results may not accurately capture sources that have episodic emissions (e.g., prescribed burning or facilities with short-term deviations such as startups, shutdowns, malfunctions, and upsets).
  • Estimates of risk are uncertain.

A more detailed explanation of each of these limitations can be found in Chapter 7 of the TMD.

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