A Case Study of MTBE Exposures and Risk in California. P. R. D. Williams and D. J. Paustenbach, Exponent
The oxygenate methyl tertiary butyl ether (MTBE) has been added to gasoline in significant quantities (11-15% by volume) to meet the requirements of the 1990 Clean Air Act Amendments. MTBE reformulated gasoline (RFG) has notably reduced criteria and toxic air pollutants, such as carbon monoxide and benzene, and has been estimated to decrease cancer risks in California by 40%. Despite significant air quality benefits, reported health effects from acute MTBE ambient exposures (i.e., during refueling) and detections of MTBE in some drinking water sources, have raised concerns about potential environmental contamination and human health risks. In response, various states have attempted to reduce or eliminate MTBE in RFG by waiving out of the federal oxygenate mandate and the state of California has declared a phase out of MTBE effective December 31, 2002. To evaluate whether the use of MTBE in California is likely to pose a public health hazard, we conducted a probabilistic analysis of MTBE exposures from air and drinking water based on available state monitoring and microenvironment exposure data. Under the assumption that MTBE is carcinogenic to humans, we estimate lifetime cancer risks from MTBE exposures in California from 1995 to 1998 to be 2.6x10-7 and 4.8x10-7 at the 95th percentile for adults and children, respectively. Longer-term MTBE exposures-i.e., those occurring from 1995 to 2002-were also estimated to pose a negligible health risk. Service station workers were estimated to be at greatest risk from MTBE exposures. Study findings suggest that although MTBE may pose an aesthetic concern for some individuals, actual MTBE exposures are unlikely to pose a health hazard for the general population in California.
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