Reductions in human benzene exposure in the California South Coast Air Basin

Abstract

Benzene typically contributes a significant fraction of the human cancer risk associated with exposure to urban air pollutants. In recent years, concentrations of benzene in ambient air have declined in many urban areas due to the use of reformulated gasolines, lower vehicle emissions, and other control measures. In the California South Coast Air Basin (SoCAB) ambient benzene concentrations have been reduced by more than 70% since 1989. To estimate the resulting effect on human exposures, the Regional Human Exposure (REHEX) model was used to calculate benzene exposures in the SoCAB for the years 1989 and 1997. Benzene concentration distributions in 14 microenvironments (e.g. outdoor, home, vehicle, work) were combined with California time-activity patterns and census data to calculate exposure distributions for 11 demographic groups in the SoCAB. For 1997, the calculated average benzene exposure for nonsmoking adults in the SoCAB was 2 ppb, compared to 6 ppb for 1989. For nonsmokers, about half of the 1997 exposure was due to ambient air concentrations (including their contributions to other microenvironments), but only 4% for smokers. Passive tobacco smoke contributed about one-fourth of all exposure for adult nonsmokers. In-transit microenvironments and attached garages contributed approximately 15 and 10%, respectively. From 1989 to 1997, decreases in passive smoke exposure accounted for about one-sixth of the decrease in exposure for nonsmoking adults, with the remainder due to decreases in ambient concentrations. The reductions in exposure during this time period indicate the effectiveness of reformulated fuels, more stringent emission standards, and smoking restrictions in significantly reducing exposure to benzene.