Jump to main content.

Estimated Risk: Current Draft of "Page 2."

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 estimates should not be used to draw conclusions about local risks. 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 the results are based on inhalation exposure to outdoor sources of air toxics only.


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


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


  • This risk characterization assumes pollutant exposures remain at 1996 levels over a 70 year lifetime. Therefore, it does not take into account significant reductions that have taken effect since 1996, including those from: 1) mobile source regulations; 2) many of the air toxics regulations EPA has issued for major industrial sources; 3) State or industry initiatives; and 4) facility closures. In addition, it does not take into account future reductions. For example, by 2007, EPA expects mobile source emissions of benzene to be cut in half due to EPA regulations.


  • 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 will use these results to inform decisions about the priorities of the air toxics program and to guide the collection of additional data that could lead to regulatory decisions.


  • EPA provides cancer risk estimates for carcinogens in the form of a number such as "in a million". The meaning of a risk level of 1 in a million (for example) is that an individual continually exposed to a pollutant at a specified concentration over the course of a lifetime sustains a 1 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 provides hazard quotient (HQ) values for non-carcinogens in the form of a number such as 0.1 or 1.0. Hazard quotient is the ratio of a predicted exposure concentration to the reference concentration (RfC) for a pollutant. For example, a hazard quotient of 1.0 means that the predicted exposure concentration equals the reference concentration, and a hazard quotient of 0.1 means the predicted exposure concentration is one-tenth the reference concentration. Reference concentration is an estimate of the continuous lifetime inhalation exposure that the EPA believes is likely to have no appreciable risk of deleterious non-cancer effects. Although exposures below the RfC are believed to be safe, exposures above the RfC are not necessarily associated with adverse effects. Nevertheless, as exposure increases above the RfC, the risk for adverse effects also increases.


  • These risk and hazard quotient levels are not regulatory levels. They are provided to help gauge the relative potential of the air pollutants considered in this assessment to cause adverse health effects based on inhalation exposures. The determination of what is an acceptable or unacceptable risk or hazard quotient level depends on additional factors and more refined information, and is not addressed in this assessment.


  • A summary of the cancer risk potencies and the non- cancer reference concentrations used in this assessment is available at www.epa.gov/airtoxics/nata/nettables.pdf.

Local Navigation

Jump to main content.