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EPA Strongly Cautions that These Modeling Results Should Not be Use

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

EPA strongly cautions that these modeling results should not be used to draw conclusions about local exposure concentrations or risk. The results are most meaningful when viewed at the state or national level; for smaller areas, the modeling becomes less certain. In addition, these results represent conditions in 1996 rather than current conditions and only include exposures from outdoor sources of air toxics.

The exposure estimates presented above represent midrange estimates of population exposures. Due to a number of factors, some individuals may have substantially higher or lower exposures. It is important to note that the model, as applied on the national scale, is not designed to quantify these extreme values of individual exposures.

Note that for certain chemicals, exposure pathways other than inhalation as well as indoor sources of air toxics may contribute substantially to total exposures of concern. This assessment does not address these other routes of exposure (i.e., ingestion or dermal) or inhalation exposure resulting from indoor sources.

The emissions used in this assessment do not reflect potentially significant emission reductions that have taken effect since 1996, including those from: 1) mobile source regulations which are being phased in over time; 2) many of the air toxics regulations EPA has issued for major industrial sources; 3) State or industry initiatives; and 4) any facility closures.

Simplified modeling assumptions may introduce significant uncertainties into each component of the assessment. See the full discussion of these limitations.

Because of these uncertainties, EPA will not use the results of this assessment to determine source-specific contributions or to set regulatory requirements. However, EPA expects to use these results to inform decisions about the priorities of the air toxics program as well as to guide the collection of additional data that could lead to regulatory decisions.
  • EPA provides cancer risk estimates for carcinogens at various exposure levels (e.g., 100 in a million) to assist readers in placing the exposure levels in context. The interpretation of a risk level of 100 in a million is that an individual continually exposed to a pollutant at this level over the course of a lifetime sustains a 100 in a million chance of contracting cancer as a result of that exposure. All risk estimates are considered conservative, but not worst-case. True risk would probably be less, but could be greater.

  • EPA also provides hazard quotient (HQ) values for non-carcinogens at various exposure levels (e.g., HQ=1.0) to assist readers in placing the exposure levels in context. The HQ is the ratio of a given exposure level to the reference concentration (RfC), which in turn is an estimate of the continuous lifetime inhalation exposure which the EPA believes is likely to be without appreciable risk of deleterious non-cancer effect. Thus, if HQ=1.0, the predicted exposure concentration is equivalent to the RfC level for that pollutant. While exposures below the RfC are believed safe, exposures above the RfC are not necessarily harmful. Nevertheless,as exposure increases above the RfC, the risk for adverse effects also increases.

  • It is important to realize that these risk and HQ levels are not regulatory levels - they are provided to help in judging the relative importance of these different pollutants with regard to their potential to create adverse health effects . The determination of what risk or HQ levels are acceptable or unacceptable depends on additional factors and more refined information, and is not addressed in this assessment.

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