Human exposure to air pollution is a public health concern, especially for individuals who are particularly sensitive to the health effects of air pollution. STI scientists frequently work with epidemiologists and biostatisticians to refine the exposure assessments used in acute and chronic air pollution health effects research. STI designs and performs exposure assessments and develops new exposure assessment tools for government, university, and industry research groups. Study design services provide clients with plans and technical rationale for the collection of data most important for achieving study objectives. Prospective studies frequently include the collection and analysis of time-activity and housing questionnaire data and personal, microenvironmental, and ambient exposure data. Retrospective studies utilize STI’s national air quality and time-activity databases compiled from routine and special studies, and scientists' knowledge of the exposure literature and exposure modeling. STI scientists developed the Regional Human Exposure model (REHEX) for assessments of the general population exposure and special cohort exposure to ambient and indoor air pollutants.