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ST. LOUIS - A national environmental group is threaten to sue Ameren Missouri over what it claims are several thousand violations of the Clean Air Act at three of its St. Louis-area coal-fired power plants.

The group attributes thousands of asthma attacks, hundreds of heart attacks and premature deaths every year to the Labadie coal plant in Franklin County, Meramec coal plant in St. Louis County, and Rush Island coal plant in Jefferson County.

On Thursday, the group submitted a "notice of intent letter" to Ameren Corporation, saying the utility company needs to address these violations within two months' time or face legal action. The "citizen suit" provision of the Clean Air Act requires the Sierra Club to give 60 days' notice of its intent to file a suit in federal court.

For its part, Ameren Corporation released a statement saying the EPA has not cited any of its Missouri coal-fired power plants for permit or pollution violations.

"Ameren Missouri has at all times operated our energy centers in accordance with EPA regulations and permitting requirements. Contrary to the assertions by the Sierra Club, unauthorized opacity exceedances occur infrequently and existing pollution control equipment removes approximately 99 percent of particulates from the flue gas stream," said Michael Menne, Vice President of Environmental Services for Ameren Corporation "We report all of our emissions as required to EPA, and the agency has not cited our Missouri coal-fired energy centers for opacity or permit violations. The filing of such litigation is a common tactic used by opponents who are striving to remove coal from energy choices available to customers."

Ameren Missouri says sulfur dioxide output at its power plants have dropped by more than 80 percent between 1990 and 2012.