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1996 National-scale Air Toxics Assessment

As a part of the National-scale Air Toxics Assessment, U.S. EPA created maps that illustrate the county-by-county modeled risk posed by the inhalation of the toxic air pollutants considered in the 1996 study. Lifetime cancer risk and noncancer hazard are depicted by map colors.

Maps depicting the lifetime cancer risk are expressed as N-in-a-million. According to the U.S. EPA, a risk level of "N" in a million implies a likelihood that up to "N" people, out of one million equally exposed people would contract cancer if exposed continuously (24 hours per day) to the specific concentration over 70 years (an assumed lifetime). This would be in addition to those cancer cases that would normally occur in an unexposed population of one million people. Note that this assessment looks at lifetime cancer risks, which should not be confused with or compared to annual cancer risk estimates. If you would like to compare an annual cancer risk estimate with the results in this assessment, you would need to multiply that annual estimate by a factor of 70 or alternatively divide the lifetime risk by a factor of 70.

A national map representing the median risk posed by all carcinogens (map) indicates, as one might expect, that the greatest cancer risk occurs in urban, highly populated areas where sources of air toxics are concentrated. The risk results in Vermont reflect this as well. Chittenden County, Vermont's most populous county, has the greatest cancer risk in the range of 32 to 43 in a million additional cases of cancer due to toxic air pollution. Compared to the counties across the rest of the nation, Chittenden County falls into the 75th to 90th percentile.

The U.S. EPA also created maps illustrating the noncancer hazard posed by air toxics that do not cause cancer. As defined by the U.S. EPA, the hazard quotient is the ratio of the potential exposure to the substance and the level at which no adverse effects are expected. If the Hazard Quotient is calculated to be less than 1, then no adverse health effects are expected as a result of exposure. If the Hazard Quotient is greater than 1, then adverse health effects are possible. The Hazard Quotient cannot be translated to a probability that adverse health effects will occur, and is unlikely to be proportional to risk. It is especially important to note that a Hazard Quotient exceeding 1 does not necessarily mean that adverse effects will occur.

Again, both on a national and statewide basis, the greatest hazard occurs in urban, highly populated areas. As the statewide map depicting the total noncancer hazard in Vermont indicates, Chittenden County has the highest hazard index level in the state falling in the range of 3.9 to 4.9. This is equal to the 90th to 95th percentile. The rest of the state has a lower hazard index.


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Last Updated: 1/22/03


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