Not-so-clear alternative: in its air-quality effects, ethanol fuel is similar to gasoline.

Switching the nation's vehicles from gasoline to mostly ethanol will not reduce air pollution, a new study finds. The work joins other evidence questioning the benefits of ethanol fuel.

Mark Z. Jacobson, an atmospheric scientist at Stanford University, created a model that takes into account how the chemicals emitted in car exhaust transform through reactions in the atmosphere. He combined the resulting chemical profile with population and health-effects data to determine the risks associated with each of the fuels.

Jacobson looked at emissions from E85, the 85 percent ethanol, 15 percent gasoline blend considered a potentially large-scale replacement for gasoline. He examined a scenario based on expected emissions in 2020, the first year that he says that E85-ready cars might dominate the roads.

Jacobson's calculations predicted health effects in response to ozone and carcinogens attributable to an all-gasoline fleet or an all-E85 fleet. He found that E85 use may increase asthma, hospitalizations, and death caused by ozone exposure by about 9 percent in Los Angeles and by 4 percent, on average, across the nation. The rise in ozone-related problems partially stemmed from larger releases of two ozone precursors--acetaldehyde and formaldehyde--from E85 as compared with gasoline.

Acetaldehyde and formaldehyde are also two of the four major human carcinogens in E85 and gasoline exhaust. E85 use lowered atmospheric concentrations of the other two major carcinogens--1,3-butadiene and benzene--as compared with gasoline use. The results regarding cancer "somewhat cancel each other out," notes Jacobson, "so there's not much difference between E85 and gasoline.

"The bottom line is, you aren't getting an improvement in air quality," Jacobson says. Although the ozone effects suggest that E85 could pose a larger health risk to the public than gasoline, he hesitates to emphasize that result because of the uncertainty inherent in some of his projections. His study appears online and in an upcoming Environmental Science & Technology.

Jacobson says that other renewable energies offer a better solution. "To solve global warming and air pollution health problems, we need to focus on technologies we know are addressing these problems," he says.

Timothy E. Lipman, a research scientist working on energy at the University of California, Berkeley, agrees. "There are other ways to substitute for petroleum that are likely to yield better greenhouse-gas and air-quality benefits," he says.

The study "should remind policy makers and others to be really skeptical about claims that E85 will improve air quality," comments Jana B. Milford, an environmental engineer at the University of Colorado at Boulder. The atmospheric-chemistry model that Jacobson developed is well regarded, she says, although he has examined only one scenario.

"It's a solid study," Milford says, "but it won't be the last word."