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But 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 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 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.

Also, the 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.

And finally, while ethanol produces less nitrogen oxide, that can actually 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.

Jacobson is also ignoring that ethanol - especially the kind made from cellulose, like switchgrass - reduces greenhouse gases, which cause global warming. And global warming will increase smog and smog-related deaths, an international scientific panel just found this month, Hwang said.

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