The potential health effects of exposure to environmental tobacco smoke (ETS) is a controversial subject, especially in the United States. Most studies on this subject have relied on reports of exposure from interviews or from questionnaires administered to the subjects. Other studies have derived data from actual measurements of markers of ETS in areas where tobacco products, especially cigarettes, are "being smoked" and the exposure of the subjects have been extrapolated from the values obtained. In this study, measurements of airborne ETS markers were made in approximately 1000 non-smokers in the US, who wore personal air sampling systems "at work" and "away from work". Participants in this study stated that they had not used tobacco products, nicotine patches, or nicotine gum for at least 6 months. It was found that the largest concentrations of ETS markers were encountered "away from work" and that, in general, the amount of ETS to which the subjects were exposed was markedly lower than those estimated in previous studies. Additionally, it appears that there is an important difference between the exposure to ETS perceived by participants and the actual exposures measured in this study.