The horizontal axis shows the respiratory hazard index. The vertical axis shows individual source sectors (i.e., major, area and other, onroad mobile, nonroad mobile, and background sources). The horizontal bar for each source sector indicates the range of typical hazard quotients for different census tracts across the US. For example, to read the hazard index for area and other sources on the chart, the right-hand end of the bar shows a hazard index of about 7 for the 99th percentile census tract. This means that only 616 census tracts (1% of 61,641 total census tracts) have a higher hazard index associated with inhalation of respiratory irritants. Other vertical lines across the area sources bar show different percentiles, as indicated by the key in the lower right of the chart. For example, the 50th percentile line shows that half the census tracts have hazard indexes higher than 0.7. The left-hand end of the bar shows the hazard index for the 5th percentile census tract, about 0.08. Ninety-five percent of all tracts have a higher hazard index for respiratory irritation associated with area and other sources.