Washington ? Politicians hail ethanol, the corn-based gasoline additive, as a boon to the environment and a way to reduce America’s dependence on foreign oil.

But ethanol also comes with its own environmental problems, and scientists disagree about whether producing ethanol actually uses more fossil energy than it replaces.

The Senate this week will decide whether to double the amount of ethanol to be used in gasoline, to 5 billion gallons a year. Critics say the plan is just one more subsidy for corn growers. Supporters make the case that the proposal is essential to an energy policy that is less reliant on oil.

“It will reduce our dependence on foreign oil. It will protect the environment,” Senate Majority Leader Bill Frist, R-Tenn., said.

There is skepticism about those claims.

Ethanol’s benefits are “a mixed bag,” said Blake Early, a lobbyist for the American Lung Assn.

Ethanol’s clearest air quality benefit is that it significantly cuts carbon monoxide, he said. But ethanol also releases more nitrogen oxide, a key element of smog, and evaporates more easily than gasoline, causing still other air pollution problems.

On balance, ethanol “certainly isn’t worse than gasoline,” Early said, but “it’s not that helpful from a smog perspective.”

The government also has identified ethanol plants as significant air polluters, but it has reached deals to curtail plant emissions.

And some scientists now say that ethanol, while not as troublesome as a methanol-based additive known as MTBE, also may complicate cleaning up gasoline spills into waterways and groundwater.

“It certainly is not all that benign,” says Tom Curtis, an official of the American Water Works Assn., which represents professionals involved in the drinking water supply business.

Curtis cites research indicating that gasoline plumes containing ethanol degrade more slowly in groundwater than plumes of only gasoline. Toxic chemicals such as benzene in ethanol-blended gasoline disperse more widely and take longer to degrade, the studies found.

These studies “are far from conclusive” and should be pursued further, said Monte Shaw, a spokesman for the Renewable Fuels Assn., which represents the ethanol industry.

But he maintains that because ethanol replaces 10 percent of the gasoline, there is also less benzene and other toxic chemicals — normally found in gasoline — going into the water in the first place. And, he says, refiners can blend their gasoline in ways to counter the air pollution concerns caused by ethanol’s evaporation.