Dr. Koutrakis and his colleagues assessed the correlations between personal exposure to PM2.5 and gaseous copollutants and compare these measurements with those taken at central-site monitors. Three groups of possibly susceptible individuals (children, seniors, and individuals with chronic obstructive pulmonary disease) were recruited in two cities (Boston and Baltimore) in two seasons (summer and winter). Integrated 24-hour personal exposures were measured over 8 or 12 consecutive days by using pump-driven multipollutant personal exposure monitors equipped with filters for collecting PM and by using passive Ogawa samplers for collecting gaseous pollutants. Concentrations of ambient pollutants were obtained at central monitoring sites in Baltimore and Boston by using a variety of monitors. Participants in each city, season, and group completed questionnaires that provided information about housing characteristics and about activities they were engaged in at different locations throughout the day.