The horizontal axis shows the number of people who live in census tracts where typical individuals have an inhalation hazard index for respiratory irritation above certain levels. The vertical axis shows individual source sectors (i.e., major, area and other, onroad mobile, nonroad mobile, and background sources). The three horizontal bars for each source sector show numbers of people in census tracts exceeding three different hazard index thresholds. For example, reading the bars for major sources, the longest bar indicates that about 20 million people live in census tracts where inhalation exposure to respiratory irritants exceeds a hazard index of 0.1. The shorter bar immediately above indicates that about 5 million people live in census tracts where such inhalation exposures exceed a hazard index of 1.0, and the shortest bar shows that about 700,000 people live in census tracts where such exposures exceed a hazard index of 10. The 700,000 people in high-exposure tracts and the 5 million in medium-exposure tracts are also included in the low-exposure population counts.