Gasoline is a complex mixture of many constituents in varying proportions. Not only does the composition of whole gasoline vary from company to company and season to season, but it changes over time. The composition of gasoline vapors is dominated by volatile compounds, while "gasoline" in groundwater consists mainly of water-soluble constituents. Hydrocarbons, including alkanes, alkenes, and aromatics, make up the large majority of gasoline, but other substances, such as alcohols, ethers, and additives, may also be present. Given this inability to define "gasoline,h' exposures to individual chemicals or groups of chemicals must be defined in a meaningful exposure assessment. An estimated 111 million people are currently exposed to gasoline constituents in the course of refueling at self-service gasoline stations. Refueling requires only a few minutes per week, accruing to about 100 min per year. During that time, concentrations in air of total hydrocarbons typically fall in the range 20-200 parts per million by volume (ppmV). Concentrations of the aromatic compounds benzene, toluene, and xylene rarely exceed 1 ppmV. Some liquid gasoline is also released, generally as drops less than 0.1 g each, but with enough larger spills to raise the average loss per gallon dispensed to 0.23 g for stations with conventional nozzles and 0.14 g per refueling for stations with vapor recovery nozzles (Stage II controls). Some skin exposure may occur from these spills but the exposure has not been quantified. Two major types of vehicular emissions have been studied. Evaporative emissions include emissions while the vehicle is driven (running losses), emissions after the engine has been shut off but is still warm (hot soak), and emissions during other standing periods (diurnal) emissions. These evaporative emissions are dominated by the more volatile gasoline components. Tailpipe emissions include some unreacted gasoline constituents as well as products of combustion (including chemicals identical to some of the original constituents of the gasoline) and a variety of hydrocarbons and related compounds. Running losses are reported to fall in the range of 0.2 to 2.8 g of total hydrocarbons per mile driven, while benzene evaporative emissions range from 0.002 to 0.007 g/mile. Benzene levels inside travelling vehicles have been reported to average about 13 ppbV in Los Angeles. Tailpipe emissions amount to 0.3 to 1.0 g/mile of total hydrocarbons; emissions of benzene, polycylic aromatic hydrocarbons, and 1,3-butadiene have been reported to range from 0.015 to 0.04 g/mile, 0.00025 to 0.00046 g/mile, and 0.001 to 0.005 g/mile, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)