Switching from gasoline to ethanol, promoted as a green alternative at the pump, may create dirtier air and cause 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's 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 populous Northeast and Los Angeles, California.

``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 George W. 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 an increase in smog of about 1 percent.

Jacobson's study troubles some environmentalists, even those who work with him. Roland Hwang of the Natural Resources Defense Council, said 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.''

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.''

Jacobson found that depends on where you live, with ethanol worsening the ozone problem in most urban areas.

Based on computer models of pollution and air flow, Jacobson predicted that the increase in ozone, and in the diseases it causes, would be worst in areas where smog is already a serious problem: Los Angeles and the Northeast.

Most of those projected 200 deaths would be in Los Angeles, he says, and the only place where ozone would fall is the Southeast because of the unique blend of chemicals in the air and heavy vegetation in the region.

The science behind why ethanol might increase smog is complicated, but according to Jacobson, part of the explanation is that ethanol produces more hydrocarbons than gasoline. Ozone is the product of hydrocarbons and nitrogen oxide cooking in the sun.

Also, ethanol produces longer-lasting chemicals that eventually turn into hydrocarbons that can travel farther. ``You are really spreading out pollution over a larger area,'' he said.

Finally, while ethanol produces less nitrogen oxide, that can be a negative in some very smoggy places. When an area like Los Angeles reaches a certain high level of nitrogen oxide, that excess chemical begins eating up spare ozone, Jacobson said.

Hwang agreed that that is a ``well-known effect.''

While praising Jacobson as one of the top atmospheric chemists in the nation, Hwang said he had problems with some of Jacobson's assumptions, such as an entire switch to ethanol by 2020. Also, he said that the ozone difference that Jacobson finds is so small that it may be in the margin of error of calculations.

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