Projection of health benefits from ambient ozone reduction related to the use of methyl tertiary butyl ether (MTBE) in the reformulated gasoline program

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.