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Occurrence and Use
Benzene is a natural constituent of crude oil, coal tar, and other fossil fuels (Sandmeyer, 1981). Most of the millions of gallons of benzene that are used in the United States each year are produced by petroleum refining. A major use of benzene is as a component in gasoline, particularly in unleaded fuels, because of its antiknock properties (ATSDR, 1989). Its content in gasoline is estimated to range from 1% to 2% in the United States and up to 5% in European countries. Large quantities are also used to synthesize chemicals for the manufacture of various plastics, resins, elastomers, dyes, and pesticides (Marcus, 1987). Minimal amounts are now used as a solvent in paints, cements, adhesives, and paint removers. Sources of atmospheric contamination include fugitive emissions from gasoline handling, thermal degradation of plastics, solid waste gasification, and tobacco smoke (Sandmeyer, 1981; Marcus, 1987).
Benzene has been detected in approximately 10% of recent air samples in the space-shuttle cabin and in Spacelab at concentrations of 0.01-0.1 mg/m3 (James et al., 1994). Benzene has not been used as a payload or system chemical aboard the space shuttle; hence, the low concentrations observed are due to materials out-gassing. Benzene has been found as a pyrolysis product of electronic components identical to ones that failed in data-display units aboard STS-35 (J. Boyd, NASA, unpublished data).
Pharmacokinetics and Metabolism
In humans, benzene vapor is rapidly absorbed by the lungs in amounts equivalent to about 50% of the doses inhaled over several hours of exposure to concentrations of 50-100 ppm (Nomiyama and Nomiyama, 1974a,b; Sato and Nakajima, 1979; R. Snyder et al., 1981; IARC, 1982). In men and women exposed to 52-62 ppm for 4 h, respiratory uptake was 47%, with little difference between the sexes (Nomiyama and Nomiyama, 1974a,b; IARC, 1982). Absorption was greatest during the first 5 min of exposure and reached a constant level