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Estimated Risk: Current draft of bar chart caveats for noncarcinogens

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

Limitations of the Risk Characterization for Noncarcinogens

EPA strongly cautions that these 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. Because of these limitations, and others described below, EPA recommends that the results be used only for their intended purposes.
  • The information presented on this chart represents hazard estimates for adverse effects other than cancer. These hazard estimates are surrounded by substantial uncertainties from a variety of sources. They should not be confused with measured risks, such as analyses of the frequency of automobile crashes, which are much more certain.

  • The hazard estimates are based on 1996 emissions of air toxics. Significant emission reductions have occurred since 1996 and more are expected in the future. EPA did not account for these reductions in developing the hazard estimates shown here.

  • This assessment includes only 32 pollutants from the full list of 188 air toxics included in the Clean Air Act plus diesel PM. Although EPA believes that these 33 are among the most likely to present important health hazards, it is not possible to be certain that every important pollutant has been included.

  • The methods used to estimate emissions and the assumptions used in modeling dispersion and exposure may introduce significant uncertainties into the hazard estimates. For more details, please see Limitations in the 1996 National-Scale Air Toxics Assessment.

  • The hazard estimates are limited to inhalation. EPA did not consider hazards from oral exposures. In some cases, people may receive substantial additional inhalation exposures to common indoor air pollutants such as formaldehyde and perchloroethylene.

  • The hazard estimates do not include indoor emission sources because appropriate data are not yet available. In some cases, people may receive substantial additional inhalation exposures to common indoor air pollutants such as formaldehyde and perchloroethylene.

  • The chart displays hazard based on the median of the "typical" individual exposures from each census tract in the county. Some individuals may have substantially higher or lower exposures based on where they spend their time. The model is not designed to quantify these individual extremes.

  • EPA has assigned an overall confidence level for each pollutant based on consideration of the combined uncertainties from emissions estimation, ambient concentration modeling, and exposure modeling.

  • The level of noncancer hazard associated with each pollutant is expressed in terms of the Hazard Quotient. While a Hazard Quotient of 1 is considered safe, higher Hazard Quotients are not necessarily harmful. Nevertheless, as the Hazard Quotient increases above 1, the potential for adverse effects also increases. Please see Table 2. of Health Effects Information Used In Cancer and Noncancer Risk Assessment(36K PDF) for more information.

  • EPA has expressed noncancer hazards associated with multiple pollutants in terms of the respiratory Hazard Index, based on an assumption that Hazard Quotients of different respiratory irritants are additive, in the absence of evidence showing otherwise. Furthermore, EPA protectively applied each Reference Concentration to respiratory irritation even when it was based on a different critical effect (e.g., effects to the nervous system). The true respiratory Hazard Index may be either greater or less than the sum of the Hazard Quotients, but it is more likely to be less.

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