Credit Sarah Skiold-Hanlin, St. Louis Public Radio
The Sierra Club's Andy Knott speaks at a rally on Thursday in front of a 15-foot tall inflatable inhaler in Keiner Plaza.
Credit Véronique LaCapra, St. Louis Public Radio
Ameren's 2,400-megawatt plant near Labadie, Missouri, is the state's largest coal-fired power plant.
Credit Sarah Skiöld-Hanlin, St. Louis Public Radio
Andy Knott uses an "opacity" scale to illustrate the disparity between acceptable emissions levels and those considered harmful. The darker squares represent increasing particulate matter in smokestack emissions.
An environmental group says it plans to soon sue utility provider Ameren over what it calls repeated violations of federal air pollution standards.
The Sierra Club told Ameren Corp. it plans legal action in the next 60 days over what the group says are nearly 10,000 violations of the Clean Air Act since 2008 at coal-fired power plants in St. Louis, Jefferson and Franklin counties
The alleged violations involve the amount of soot released from smokestacks at the Meramec, Labadie and Rush Island plants.