The air pollution data presented in the Atlas are derived from exposure to air toxics data from the National-Scale Air Toxics Assessment (NATA) developed by the U.S. Environmental Protection Agency. The NATA uses general information about emissions sources to develop risk estimates and does not incorporate more refined information about emissions sources, which suggests that the impacts of risks may be overestimated. Note, however, that because the Atlas indicators we developed using this data are relative—relative to the U.S. overall in the case of exposure index, and relative to each Atlas geography in the case of the unequal burden indicator—the fact that the underlying risk estimates themselves may be overstated is far less problematic. The NATA data include estimates of cancer risk and respiratory hazards (non-cancer risk) at the census tract level based on exposure to outdoor sources. It is important to note that while diesel particulate matter (PM) exposure is included in the NATA non-cancer risk estimates, it is not included in the cancer risk estimates (even though PM is a known carcinogen). Emissions source types were grouped into four categories for use in the Atlas: on-road mobile (e.g., cars, trucks, buses), off-road mobile (e.g., airplanes, trains, lawn mowers, construction vehicles, farm machinery, boats), major stationary (e.g., larger commercial/industrial facilities such as power plants, refineries, and factories), and area and other. The last of these categories is a residual category and includes smaller commercial/industrial facilities (e.g., dry cleaners and small manufacturers) along with fires and other natural sources and secondary and background sources. See here for more information about the NATA data.
The index of exposure to air pollution presented in the Atlas is based on calculations of three exposure indices at the census tract level, using the 2011 NATA: cancer risk, respiratory (or non-cancer) risk, and the two risk types combined. While the 2011 NATA includes estimates for Puerto Rico and the U.S. Virgin Islands, our analysis for this indicator (and all indicators in the Atlas) excludes all U.S. territories. For the cancer and non-cancer risk indices, the two NATA risk measures were simply ranked at the census tract level across the entire United States, from 1 to 100 (in ascending order, so that “100” includes the top one percent of all U.S. tracts in terms of risk). For the combined risk index, we followed the approach used by the U.S. Department of Housing and Urban Development (HUD) in developing its Environmental Health Index. The HUD Environmental Health Index includes neurological risk in addition to cancer and respiratory risk. However, given that neurological risk was is not included in the nationwide results for all years of the NATA we use for the Atlas air pollution indicator, it is excluded from our analysis. The cancer and non-cancer estimates were combined by calculating tract-level z-scores for each and adding them together as indicated in the formula below:
Where c indicates cancer risk, r indicates respiratory risk, i indexes census tracts, and μ and σ represent the means and standard deviations, respectively, of the risk estimates across all census tracts in the United States. As with the individual risk indices, the combined tract level index, COMBINEDi was ranked in ascending order across the entire United States, from 1 to 100.
With the three tract-level rankings in place (for cancer, non-cancer, and combined risk), we then estimated the contribution to each ranked value of four source categories. As noted above, these include on-road mobile, off-road mobile, major stationary, and area and other. To estimate their relative contributions to each ranked risk index, we multiplied each tract-level ranking by the tract-level ratio of the risk estimate for each source category to the total risk estimate for the tract (such that the sum of contributions across source categories was equal to the ranked risk value in each tract). For the combined cancer and non-cancer ranking, the tract-level ratio for each source category was calculated by taking a weighted average of the ratios for the two risk types, using their aforementioned z-scores as weight.
Finally, the tract-level rankings (and their contributions by source category) were summarized for each Atlas geography and demographic group (i.e., by race/ethnicity and poverty status) by taking a population-weighted average using the group population as weight, with group population data drawn from the relevant ACS summary file (see indicator-specific data notes for the particular NATA and Census/ACS summary files used for each year of the air pollution indicator reported on the Atlas). The result of that population-weighted average is the index of exposure to air pollution presented in the Atlas. The index value reported reflects the national percentile ranking in terms of pollution burden for each group and geography for which it is reported. For example, an exposure index value of 25 for all people suggests that the average person, in the indicated Atlas geography and for the indicated risk type, lives in a neighborhood (census tract) that ranks at the 25th percentile nationally (that is, it has greater pollution exposure than 24 percent of all census tracts in the U.S. but less exposure than 75 percent of all tracts). An exposure index value of 56 for Blacks suggests that the average Black person in the indicated Atlas geography and for the indicated risk type lives in a neighborhood that ranks at the 56th percentile nationally in terms of pollution exposure.