AKRON, Ohio - FirstEnergy Corp. on Thursday said it will retire six coal-fired power plants, including four in Ohio, because of stricter federal anti-pollution rules.

The six older and dirtier plants will be closed by Sept. 1.

"It was a tough decision," said Charles D. Lasky, vice president of fossil fleet operations for FirstEnergy Generation Corp.

FirstEnergy will be among the first American utilities to close aging, polluting power plants after tighter federal clean-air rules were finalized last month.

FirstEnergy had been keeping a close eye on proposed federal rules on mercury, heavy metals and air toxics from coal-burning power plants for years, Lasky said.

The new rules provided FirstEnergy with "sufficient certainty" to proceed with the closings, he said.

The federal mandate that improvements be completed within three years was a factor in the decision to retire the six plants, which represent 12 percent of the utility's generation capacity, he said.

The average age of the six plants is 55 years, Lasky said.

The closings were triggered by the U.S. Environmental Protection Agency's new Mercury and Air Toxic Standards (MATS), which were finalized Dec. 21.

Reducing emissions of mercury, heavy metals and airborne toxics from coal-burning power plants will protect people's health, the EPA said.

Installing anti-pollution equipment on small, old power plants was not economically feasible, FirstEnergy concluded.

Lasky declined to say how much it would have cost FirstEnergy to equip the plants with bag houses, activated carbon filters and lime or sorbent injection systems to meet the new federal rules.

FirstEnergy saw no advantage to waiting to see whether legal challenges might overturn the new rules, said Ray Evans, executive director of environmental for FirstEnergy Services.

In some cases, there is not enough land around the old plants to install anti-pollution equipment, he said.