1996 National Air Toxics Assessment Exposure and Risk Data

Q: Why don't the nationwide cumulative cancer risk estimates in the table of county risk levels match the maps on this website? The table on this page shows a median nationwide upper bound cancer risk levels of about 50 in a million, but the map indicates the 50th percentile cumulative risk level is only about 30 in a million.

A: In the table, the percentiles shown for each county reflect the risks of all the census tracts in the county, and the nationwide percentiles at the top of the table reflect all the census tracts in the US -- not the county medians. In contrast, the map legend reflects the percentiles for the county medians, not the individual tracts. The reason the nationwide median of county risks is different from the nationwide median for tract risks is that different counties contain vastly different numbers of census tracts. Furthermore, counties with larger numbers of tracts tend to be urban (and therefore higher-risk). All these high-risk tracts get represented on the map only by a single county median. In the other extreme, some rural counties have only a one census tract, and that single tract's risk therefore represents an entire county. For this reason, the median of county risks is much more influenced by rural census tracts -- and is correspondingly lower -- than the median of census tract risks.