To estimate potential public health benefits from ozone (O
3) pollution reduction attributable to the use of methyl tertiary-butyl ether (MTBE) in gasoline, O
3 dose-response estimates from the biomedical literature were combined with model estimates of O
3 reduction. Modeling employed EPA MOBILE5a and Complex models to predict emission changes, industry AQIRP techniques to predict ambient O
3 changes, and the National Exposure Model to predict human exposures. Human health effects considered were lung function decrements and respiratory irritant symptoms (using dose-response functions measured in laboratory and field studies), and increased death rates (using concentration-response functions inferred statistically from public-health data). Other reported health effects, such as lung inflammation, increases in asthma attacks, and hospitalizations, were not addressed because of inadequate dose-response information. Even for the health responses considered, quantitation of improvements due to MTBE use is problematical, because MTBE affects only a small percentage of existing O
3 pollution, and because exposure-response relationships are not well understood for population subgroups most likely to be affected. Nevertheless, it is reasonable to conclude that even small MTBE-associated reductions in peak ambient O
3 levels (1–5 ppb, according to model estimates) should yield considerable public health benefits. Tens of millions of Americans are potentially exposed to O
3 in the concentration range associated with health effects. Even if only a small percentage of them are susceptible, any incremental reduction in O
3 (as with MTBE use) must mitigate or prevent effects for a meaningful number of people. Better quantitative estimates of benefit must await a more detailed understanding of each link in the chain of causation.
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