Liquid
gasoline is a
complex mixture of at least 150
hydrocarbons with about 60-70%
alkanes (paraffins), 25-30% aromatics, and 6-9%
alkenes. In order to evaluate the potential for health effects from inhaling
gasoline vapors, it is essential to understand the major differences in the composition of vapors versus liquid
gasoline. The small chain, low
carbon-numbered components are more volatile and thus in higher percentages in the vapor phase than the larger and heavier molecules. It is noteworthy that the concentrations of aromatics (the more toxic of the
gasoline components), are depleted to about 2% in the vapor phase, with the light paraffins (the less toxic) enriched to about 90%. Actual measurements of vapor exposure at service stations confirm that the vapor composition is primarily to low weight
alkanes although
benzene is also emitted and represents the chemical of greatest concern. A perceived health concern from inhaling
gasoline vapors is the potential for carcinogenicity based on the induction of kidney
tumors in male rats and liver
tumors in female mice exposed to wholly-vaporized
gasoline. However, the results of the animal studies are of questionable relevance for human risk assessment due to the unique mechanism operative only in the male rat and since the exposure was to wholly-vaporized
gasoline rather than the
gasoline vapor mixture to which humans are exposed. Recent research supports the hypothesis that branched-chain-
alkanes bind to a
globulin specific to make rats, alpha 2-u-globulin. The
protein complex can not be degraded in the usual manner so that
protein accumulation occurs in renal cells, leading to cytotoxicity, death, proliferation, and with prolonged exposure,
kidney cancer. The results of epidemiology studies fail to link an increase in
cancer to exposure to
gasoline vapors.