Coal Plants Emit Highest Level of Pollutants
While coal is the cheapest fossil fuel for generating electricity, it is also the dirtiest, releasing the highest levels of pollutants into the air. Coal-fired power plants are responsible for 60% of U.S. sulfur dioxide emissions, 33% of U.S. mercury emissions, 25% of nitrogen oxide emissions and more than 33% of the nation’s carbon dioxide emissions.

Most existing power plants between 30-50 years old and are up to 10 times dirtier than new power plants. When the Clean Air Act of the 1970s was passed, Congress included a "grandfathering" loophole that allowed older power plants to avoid meeting the modern pollution control standards that new facilities had to adapt. At the time, Congress allowed the loophole because it expected that these "grandfathered" plants would soon retire and be replaced by cleaner, new plants. However, many of these older coal-fired power plants have sidestepped the new source review provision and have illegally avoided installing modern pollution controls. As a result, we are now faced with a disproportionate amount of pollution coming from these old, dirty, under-controlled plants.  Source: Sierra Club