This study was aimed to describe the personal exposure of permanent residents in Mexico City's Metropolitan Area (MCMA) to particulate matter of less than 2.5 μm diameter (PM2.5) during their daily activities. A total of 40 healthy volunteers (30 women and 10 men) with sedentary activities were included. All of them carried a PM2.5 personal monitor during 13 h and registered their activities in a written diary that classified them in indoor and outdoor microenvironments in each 30 min period. All sample collections started at 0900 hours, and even though measurements were obtained during the rainy season (April-August 2002), the relative humidity was less than 70%. The data were categorized and evaluated under the following criteria: morning and afternoon exposure, indoor and outdoor activities, and geographical location. The descriptive analysis showed that the overall outdoor median concentration of PM2.5 (89.50 μg/m3) was higher than the indoor one (67.55 μg/m3). PM2.5 concentrations in the morning to early afternoon were more elevated than in the late afternoon, suggesting a circadian-like behavior. In the indoor microenvironment, the highest concentration occurred in the subway (106.2 μg/m3) followed by school (93.27 μg/m3), and the lowest at home (53.1 μg/m3). The outdoor microenvironment with the highest concentrations was the public transportation (bus) (99.95 μg/m3), while the automobile had the lowest (64.9 μg/m3). The geographical zone with the highest concentration was the Center city area (87.87 μg/m3), and the one with the lowest concentration was the northeast area of the city (50 μg/m3). All the differences were statistically significant (P<0.05). Multivariate analysis corroborated that PM2.5 concentrations are mainly determined by geographical locations and hour of the day, but not by the type of microenvironment. The inclusion of covariables in the multivariable analysis ensures a more accurate estimation and prediction of the real PM2.5 concentrations. In conclusion, PM2.5 personal exposure of healthy adult permanent residents of MCMA is usually higher than recommended by the international standards in outdoor and even in indoor microenvironments. Particulate matter personal exposure varies in relation to hour of the day, daily activities and microenvironments.