A Microenvironment (ME) is a small space in which human contact with a Pollutant takes place which can be treated as a well -characterized, relatively homogenous location with respect to pollutant concentrations for a specified time period. In this national-scale assessment, HAPEM4 considers cohort activities in thirty-seven ME locations that include: (1) indoor locations (e.g., residence, office, store, school, restaurant, church, manufacturing facility, auditorium, health care facility, service station, other public building, garage); (2) outdoor locations (e.g., parking lot/garage, near road, motorcycle, service station, construction site, residential grounds, school, sports arena, park/golf course); and (3) in-vehicle locations (e.g., car, bus, truck, other, train/subway, airplane).