There are limited data on exposures to ambient air toxics experienced by inhabitants of urban areas in developing countries that have high levels of outdoor air pollution. In particular, little is known about exposures experienced by individuals working outdoors - typically as part of the informal sector of the economy - as compared to workers in office-type environments that approach the indoor air quality conditions of the more developed countries. The objective of this study is to explore these differences in personal exposures using a convenience sample of 68 outdoor and indoor workers living in Mexico City (higher outdoor air pollution) and Puebla (lower outdoor air pollution), Mexico. Occupational and non-occupational exposures to airborne volatile organic compounds (VOCs) were monitored during a 2 day period, monitoring 2 consecutives occupational and non-occupational periods, using organic vapor monitors (OVMs). Socio-demographic and personal time-location-activity information were collected by means of questionnaires and activity logs. Outdoor workers experienced significantly higher exposures to most VOCs compared to indoor workers in each of these cities. The outdoor workers in Mexico City had the highest exposures both during- and off-work, with maximum occupational exposures for toluene, MTBE, n-pentane, and d-limonene exceeding 1 mg/m(3). The inter-city pattern of exposures between the outdoor workers is consistent with the higher outdoor air pollution levels in Mexico City, and is above exposures reported for urban areas of the more developed countries. Results from this study suggest that elevated outdoor air pollution concentrations have a larger impact on outdoor workers' personal exposures compared to the contribution from indoor pollution sources. This contrasts with the more dominant role of indoor air VOC contributions to personal exposures typically reported for urban populations of the more developed countries.