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 were to run on a fuel blend made mostly of ethanol by 2020, the research concludes. The author of the study 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 author of the study, die from respiratory problems related to ozone, the unseen component of smog, combined with small particles. Ethanol would raise ozone levels, particularly in certain regions of the country.

“It’s not green in terms of air pollution,” said Mark Jacobson, a Stanford University civil and environmental engineering professor and author of the study. “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, was published Wednesday in the online edition of the peer-reviewed journal Environmental Science and Technology. It added 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 of the gases linked to global warming.

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 such a change could mean approximately 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 the California 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 that “ethanol is a greener fuel than gasoline.”

But Jacobson found that it all depended on where you lived, 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 the diseases it causes – would be worst in areas where smog already is a serious problem: Los Angeles and the Northeast.

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

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. And ozone is the product of hydrocarbons and nitrogen oxide cooking in the sun.

While praising Jacobson as one of the top atmospheric chemists in the nation, Hwang said that he had problems with some of Jacobson’s assumptions, like that of an entire switch to ethanol by 2020.

Jacobson is also ignoring that ethanol reduces greenhouse gases, which cause global warming, and that global warming will increase smog and smog-related deaths, according to an international scientific panel, Hwang said.