An aide to South Dakota Sen. John Thune says a bill to exempt livestock from the Clean Air Act is has been filed with the clerk of the U.S. Senate.

Thune, a Republican, says such legislation would, in effect, eliminate the chance that cattle and other livestock will be taxed as a way to cut down on their alleged contribution to air pollution.

Thune says the Clean Air Act was designed to target smokestacks in industrial states and not farm and ranch livestock.

A spokeswoman for Democratic Sen. Tim Johnson says there's no effort to impose a tax on livestock but added that Johnson would not support higher taxes on family farmers.

The EPA has denied it is targeting livestock as a potential tax source.