The PTEAM Study was the first large-scale probability-based study of personal exposure to particles. Sponsored by the U.S. Environmental Protection Agency (EPA) and the Air Resources Board of California, it was carried out by the Research Triangle Institute (RTI) and the Harvard University School of Public Health (HSPH). HSPH designed and constructed a 4-lpm, battery-operated personal monitor for inhalable particles (PM10) that could be worn comfortably for up to 14 hours by persons from 10 to 70 years old. The monitor was worn for two consecutive 12-hour periods (day and night) during the fall of 1990 by 178 participants representing 139,000 nonsmoking residents of Riverside, California. Nearly identical monitors were employed to collect concurrent indoor and outdoor samples. The monitors were equipped with a different sampling nozzle to collect fine particles (PM2.5). Population-weighted daytime personal PM10 exposures averaged 150 +/- 9 (SE) micrograms/m3, compared to concurrent indoor and outdoor concentrations of 95 +/- 6 micrograms/m3. This suggested the existence of excess mass near the person, a "personal cloud" that appeared related to personal activities. Fourteen of 15 prevalent elements also were evaluated in the personal samples. The two major indoor sources of indoor particles were smoking and cooking; even in these homes, however, more than half of the indoor particles came from outdoors, and a substantial portion of the indoor particles were of undetermined indoor origin. Outdoor concentrations near the homes were well correlated with outdoor concentrations at the central site, supporting the idea of using the central site as an indicator of of ambient concentrations over a wider area. Indoor concentrations were only weakly correlated with outdoor concentrations, however, and personal exposures were even more poorly correlated with outdoor concentrations. Elemental profiles were obtained for environmental tobacco smoke (ETS) (major contributions from potassium and chlorine) and cooking emissions (aluminum, iron, calcium, and chlorine). These profiles can be used in future source apportionment studies.