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.