The horizontal axis shows the number of people who live in census tracts where typical individuals have an upper-bound lifetime cancer risk 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 risk thresholds. For example, reading the bars for major sources, the longest bar indicates that about 110 million people live in census tracts where the upper-bound major source-associated inhalation risk to a typical individual exceeds 1 in 1 million. The shorter bar immediately above indicates that about 17 million people live in census tracts where such risks exceed 10 in 1 million. The shortest bar shows that about 260,000 people live in census tracts where such risks exceed 100 in 1 million. The 260,000 exceeding the highest-risk threshold and the 17 million exceeding the medium-risk threshold are also included in the counts for people exceeding the lowest risk threshold.