LEWISTON – For the fifth year in a row, students in the environmental economics course at Bates College have bought and will retire a government permit for the atmospheric release of sulfur dioxide, a pollutant that causes acid rain and contributes to respiratory illness.

Since 2001, students have bid successfully for permits, each licensing the release of a ton of sulfur dioxide, in an annual auction that’s part of the U.S. Environmental Protection Agency’s “cap-and-trade” approach to pollution control.

This year’s purchase brings the Bates total to 13 tons. By retiring the permits, the Bates students keep the sulfur dioxide – a product of fossil fuel combustion, particularly coal – from being discharged into the air.

According to Wayne Shirley, of the Regulatory Assistance Project office in Gardiner, every ton of sulfur dioxide released into the atmosphere causes health care costs amounting to $14,633. The Bates contingent paid $724 for this year’s permit.

“If that doesn’t pass a benefit-cost test, I don’t know what does,” said associate professor of economics Lynne Lewis, who teaches the course.

Under the 1990 Clean Air Act amendments, coal-burning utilities are required to have emissions permits. The permits are tradable, and the annual emissions-allowance auction held by the Chicago Board of Trade disposes of some 125,000 allowances.

Because of its location downwind of the nation’s dirtiest utilities, Maine is particularly vulnerable to the effects of pollutants such as sulfur dioxide.

“I feel great about purchasing the SO2 permit,” said Jamie Nissen, a student from Cumberland Foreside. “I know this project has planted a seed of awareness in every person that was involved.”

The environmental economics class was joined in its bid by students from Bates and Colby College environmental studies courses. Also taking part was Christopher Gwozdz of Boston, a recent graduate who took Lewis’ class in 2004.