A study by the National Academy of Sciences panel said the additive, which used in about a third of the gasoline sold in the United States, is believed to reduce overall peak ozone levels "by only a few percent."
The study said, however, the so-called reformulated gasoline with the additive has characteristics other than higher oxygen levels that lower pollution from automobiles and light trucks. The group made no recommendation whether the program should be continued.
The EPA has required reformulated gasoline since 1995 in areas with severe smog. All or parts of 17 states require the fuel, including most of the Northeast.
The most widely used oxygenate, MTBE, has been the focus of controversy with critics arguing that it presents its own environmental and health risks. Recently, California reversed course and said it would ban the additive beginning in 2002 because MTBE has recently been found in lakes and streams. Officials in Maine stopped requiring the additive this month and have asked to be taken out of the federal program.
Peak ozone levels in urban areas have decreased more than 10 percent since 1986 and continue to decline. But the 11-member panel of scientists said "it is not certain that any part of these trends can be significantly attributed to the use of reformulated gasoline."
Ozone results from sunlight's interaction with pollutants including hydrocarbons and nitrogen oxides, which forms what's commonly known as smog. At high levels, especially in summer, it is a severe irritant that can damage lung tissue and aggravate respiratory problems. About one-fourth of all ozone is attributed to automobiles and trucks, almost half to all forms of transportation.
The addition of MTBE or ethanol to gasoline, in itself, "is likely to have little air-quality impact in terms of ozone reduction," the study by the academy's National Research Council said. It said reductions in volatile organic compounds and carbon monoxide from cars appear to be quite small, and the additive may lead to more nitrogen oxide being released.
The study was requested by the EPA, which had asked scientists to evaluate the comparative benefits of the two oxygen additives MTBE, which is derived from methanol, and ethanol, which comes from corn. The study said ethanol is less effective, but the overall impact of MTBE in curbing ozone is not much better.
The EPA said in a statement the agency "supports the technical findings" of the report, but the findings should not dissuade the continued use of reformulated gasoline blends. "Cleaner gasoline is an important tool for achieving cleaner air and protecting public health, and we expect that some form of cleaner gasoline will always be in demand," said EPA spokesman David Cohen.
William Chameides, te science panel's chairman and a professor at the Georgia Institute of Technology, said the report should not be interpreted to suggest abandonment of reformulated gasoline. "Not at all," Chameides said in a telephone interview. "We're simply looking at the impact." He said reformulated gasoline may be environmentally beneficial overall, but "it is not possible to attribute a significant portion of past reductions in smog to the use of these gasoline additives."
Currently the cleaner gasoline is used in all or part of 16 states including: California, Connecticut, Delaware, Illinois, Indiana, Kentucky, Massachusetts, Maryland, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Texas, Virginia and Wisconsin.