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The ASPEN model produces exposure estimates for census tracts, with boundaries defined by the 1990 U.S. Census. A new universe of census tracts (and tract-specific) boundaries was created during the 2000 U.S. Census. In order to make use of population and demographic data from the 2000 U.S. Census, it was necessary to spatially reallocate ASPEN results from the 1990 universe of tracts to the 2000 universe of tracts.
Using GIS, tract boundaries from the 2000 census were overlaid on 1990 census tract boundaries. The percentage of each 1990 tract incorporated into a 2000 census tract was calculated, based on overlapping areas. These overlap percentages were used to reapportion cancer risks and noncancer hazards for all modeled chemicals. For each 2000 census tract, the products of each contributing 1990 tract's overlap percentage, the 1990 average individual cancer risk or noncancer hazard, and the 1990 population were summed. Each sum was then divided by the 2000 population of the tract to obtain a reapportioned cancer risk or noncancer hazard. Tracts with 2000 populations of zero were assumed to have a population of one in order to be included in the reapportionment.
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