Limitations in the 1996 National- Scale Air Toxics Assessment

The national-scale assessment variability analysis provides a picture for 1996 of how ambient air concentration, exposure and risk vary throughout the United States. The study does not focus on individuals, or on the variation in exposure and risk between individuals. It focuses on variation between well-defined geographic areas such as counties, states and the Nation, all based on calculations of ambient air concentration, exposure and risk in different census tracts.

The information contained in the resulting maps and charts displaying predictions of risk, for example, are interpreted as follows: X% of the census tracts are characterized by a typical lifetime excess cancer risk of less than R. For example, if X is 25% and R is 1 in a million, the result would be interpreted as: 25% of the census tracts are characterized by a typical risk of less than 1 in a million. It does not necessarily mean that 25% of individuals in the U.S. have a cancer risk of 1 in a million. Some people in these census tracts would be expected to have a risk above 1 in a million. While an individual might live in a census tract where the typical risk is below 1 in a million, that individual might live nearer the source than the average person in the census tract, or might have an activity pattern that leads to greater exposure, or might be more susceptible, or might be more sensitive. All of these factors would cause that individual to experience a risk above the typical value for that census tract. Of course, the individual could also have a lower risk by living further from the source, or having an activity pattern that produces lower exposures, or being of lower susceptibility, or being of lower sensitivity.

It is important, therefore, to interpret the maps and charts of the national-scale assessment study as showing variation between values of ambient air concentration, exposure or risk in census tracts or larger groupings such as counties. It allows the identification of geographic regions (counties or states) where these values are higher or lower than the national average for all census tracts. It does not allow the identification of individuals who have higher or lower values of ambient air concentration, exposure and/or risk, and it does not allow identification of specific census tracts that are higher or lower than average. Despite these limitations, it may still be said that individuals with a high risk are more likely to found in geographic regions characterized by a high risk than in those characterized by a low risk. The same may be said for exposure (i.e. individuals with a high exposure are more likely to found in geographic regions characterized by a high exposure than in those characterized by a low exposure). You should review the information in the section on Limitations for a better understanding of the meaning of these results.

What are the components of variability?
Which components of variability did the national-scale assessment include?
How was the variability analysis conducted?
What are the results of the variability analysis

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