The horizontal axis shows the number of people who live in census tracts where typical individuals have an inhalation hazard quotient above certain levels. The vertical axis shows each individual pollutant having a potential to cause adverse non-cancer effects. To the right of each pollutant name is a symbol that indicates the relative level of overall confidence in the emissions information in the National Toxics Inventory, the ASPEN dispersion modeling, and the HAPEM4 exposure modeling for that pollutant. The three horizontal bars for each chemical show numbers of people in census tracts exceeding three different hazard quotient thresholds. For example, to read the bars for acrolein on the chart for hazard quotient from all sources, the longest bar indicates that about 250 million people live in census tracts where inhalation exposure to acrolein exceeds one-tenth the reference concentration. The shorter bar immediately above indicates that about 230 million people live in census tracts where typical inhalation exposures exceed the reference concentration, and the shortest bar shows that about 28 million people live in census tracts where such exposures exceed 10 times the reference concentration. The 28 million in high-exposure tracts and the 230 million in medium-exposure tracts are also included in the low-exposure population counts.