WASHINGTON - Switching from gasoline to ethanol - touted as a green alternative at the pump - may create dirtier air, causing slightly more smog-related deaths, a new study says.

Nearly 200 more people would die yearly from respiratory problems if all vehicles in the United States ran on a mostly ethanol fuel blend by 2020, the research concludes. Of course, the study author acknowledges that such a quick and monumental shift to plant-based fuels is next to impossible.

Each year, about 4,700 people, according to the study's author, die from respiratory problems from ozone, the unseen component of smog along with small particles. Ethanol would raise ozone levels, particularly in certain regions of the country, including the Northeast and Los Angeles.

"It's not green in terms of air pollution," said study author Mark Jacobson, a Stanford University civil and environmental engineering professor. "If you want to use ethanol, fine, but don't do it based on health grounds. It's no better than gasoline, apparently slightly worse."

His study, based on a computer model, is published in Wednesday's online edition of the peer-reviewed journal Environmental Science and Technology and adds to the messy debate over ethanol.

Farmers, politicians, industry leaders and environmentalists have clashed over just how much ethanol can be produced, how much land it would take to grow the crops to make it, and how much it would cost. They also disagree on the benefits of ethanol in cutting back fuel consumption and in fighting pollution, especially global warming gases.

In January, President Bush announced a push to reduce gas consumption by 20 percent over 10 years by substituting alternative fuels, mainly ethanol. Scientists with the Environmental Protection Agency estimated that could mean about a 1 percent increase in smog.

Jacobson's study troubles some environmentalists, even those who work with him. Roland Hwang of the Natural Resources Defense Council, said that ethanol, which cuts one of the key ingredients of smog and produces fewer greenhouse gases, is an important part of reducing all kinds of air pollution.

Jacobson's conclusion "is a provocative concept that is not workable," said Hwang, an engineer who used to work for California's state pollution control agency. "There's nothing in here that means we should throw away ethanol."

And Matt Hartwig, spokesman for the Renewable Fuels Association, the largest Washington ethanol lobby group, said other research and real-life data show "ethanol is a greener fuel than gasoline."

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