According to its letter, the group said it was planning legal action in the next 60 days stemming from the issues of “opacity violations” at the Meramec, Labadie and Rush Island facilities in St Louis, Jefferson and Franklin counties.

It determined the basis for the violations from data released by Ameren, officials said, of plants using electronic continuous opacity monitoring systems, on a quarterly schedule to the Missouri Department of Natural Resources.

“In an effort to investigate and protect Missouri’s air, the Sierra Club requested the data from Missouri DNR, which revealed egregious violations of the Clean Air Act by exceeding the opacity limits allowed in Ameren’s permit on thousands of occasions between 2008 and 2013,” the green group said.

Citing research information from the Clean Air Task Force, the Sierra Club said pollution from Ameren’s Labadie coal plant in Franklin County, Meramec and Rush Island coal plants contribute to 3870 asthma attacks, 360 heart attacks and 226 premature deaths annually.

It also said the utility drew about 75% of its energy from coal-fueled facilities, higher than the 42% national average.

“Our neighbors in the Midwest are doubling down on clean energy and locking in record-low wind energy prices, but Ameren keeps Missouri lagging behind and hooked on coal,” Sierra Club Beyond Coal campaign representative for Missouri Andy Knott said.

“Between the tons of pollution and the rising costs, Ameren is well aware that coal is a dangerous risk for customers and for health yet they continue to do nothing about it.

“It’s time to move beyond coal and spur clean energy development here in Missouri, too.”

Sierra Club officials have invited Ameren to discuss the violations during the 60-day period prior to the filing, which it said it in hopes of an amicable resolution.

However, should a resolution not be met, it will move forward, they said.

The 60-day notice is required under federal law to permit for potential discussions.