A federal program allows power plants that reduce pollution levels to sell "credits" for the pollution they would have produced to other power companies. The program has been credited with reducing acid rain.

Officials at the Solid Waste Authority of Central Ohio said they have been selling pollution credits since 2000.

American Electric Power recently bought the four years' worth of pollution credits for $3.8 million. It can now legally release 9,552 tons of sulfur dioxide into the air from its coal-fired power plants, many of which are in Ohio.

"It's something that's rather routine for us," AEP spokesman Pat Hemlepp said of the purchase.

Power companies find that it is more economical to buy the credits than to clean up their operations or switch to more expensive, cleaner-burning fuels such as natural gas, said Peter Rosenthal, an analyst with Argus Media, which publishes an energy market trade journal.

Demand for sulfur dioxide credits has risen dramatically nationwide, Rosenthal said.

Because more power companies are burning more coal, they have fewer credits to sell. Credits that used to sell for $120 per ton of sulfur dioxide now sell for more than $500 per ton, Rosenthal said.

For example, in 2000, the Solid Waste Authority of Central Ohio made $300,088 by selling one year's worth of credits.

In Ohio, the state EPA allocates the credits to companies.

Large power plants that reduced pollution were first eligible to sell credits in 1995. At the time, the Columbus plant did not qualify because of its size. But in 2000, when smaller plants were made eligible, the trash-burning power plant was given its credits.

It didn't have to reduce its pollution because it wasn't operating.

An environmental advocate who fought to shut down the plant wondered how it could end up helping other power companies pollute the air.

"Those credits should have been taken away from them," said Teresa Mills, director of the Buckeye Environmental Network.

John Remy, a spokesman for the waste authority, said it can sell the credits until at least 2030.

"It's a nice windfall for us, and the city of Columbus," Remy said. "This is money we weren't expecting."

U.S. EPA spokesman John Millet said power plants that were awarded the credits don't lose them, even if they shut down.

"By closing down and not operating, they are able to turn their allotment of pollution into credits," Millet said. "Those credits turn into a commodity."

The $3.8 million the authority just made will go to the city of Columbus to help pay off a massive debt that the waste authority still owes under a lease agreement it signed to operate the plant.

Remy said the authority still owes Columbus $40 million.