This study examines the influence of the indoor environment on exposure to a suite of air pollutants, designated by the United States Environmental Protection Agency (EPA) as hazardous air pollutants (HAP's). The EPA found that several of these compounds were responsible for a large percentage of overall excess cancer risk in the population. However, their analysis only examined outdoor concentrations. To fully quantify human exposure, we need to include the indoor environment, where we spend about 80% of our total time. We assemble data from several monitoring and modeling studies to better understand the average levels of total personal exposure, and to determine the relative contribution of exposure resulting from indoor versus outdoor origin. For many HAP's, indoor concentrations are higher than outdoor concentrations, due to the presence of indoor sources combined with infiltration from outdoor air. We find that many of HAPs examined have a significant proportion of exposure coming from the indoor environment.