Risk-benefit analysis of ethanol fuel blends in the U.S.

摘要

This paper presents a summary of the available literature concerning the potential air and water quality risks and benefits associated with ethanol-fuel blends in the U.S. The available weight-of-evidence indicates that ethanol-fuel blends are likely to produce measurable air quality benefits compared to conventional gasoline. These benefits may not be as significant as those produced by MTBE-fuel blends, however, because of ethanol's evaporative properties which tend to increase certain air contaminant levels. Specifically, ethanol-fuel blends are associated with increased exhaust emissions and/or ambient air concentrations of acetaldehyde and peroxyacetyl nitrate (PAN), and increased evaporative emissions of benzene, total hydrocarbons, and chemicals with ozone-forming potential. These air quality impacts may therefore negate some of the positive reductions in air contaminant levels associated with ethanol-fuel blends, and result in some degree of "back-sliding" on air quality associated with alternative fuel blends, such as MTBE. In regards to water quality, ethanol itself is not expected to have a detrimental impact on public drinking water sources due to its rapid degradation in water. Its predicted effect on lengthening the plume of other gasoline constituents, however, could result in greater drinking water detections of certain chemicals such as benzene. Unfortunately, few "real-world" data are available to evaluate the actual impact this will have on public exposures and health risks. Other issues that have been raised, but not fully evaluated, include the effect of neat ethanol releases on aquatic ecosystems or potential aesthetic impacts from ethanol by-products. It is noteworthy that the potential water quality impacts of ethanol-fuel blends are quite different from those of MTBE, because the former relates more to health risk while the latter relates more to aesthetics (i.e., taste and odor). Although the data suggest that ethanol-fuel blends have their own unique sets of risks and benefits, the paucity of data on actual environmental exposure levels or associated health impacts hinders the completion of a more comprehensive quantitative risk-benefit comparison. In addition, besides examining general data on air and water quality risks and benefits, policy decisions about whether to use a fuel oxygenate (and which one to use) require consideration of many broader contextual issues.