According to EPA's 2011 National Emissions Inventory Technical Support Document (June 2014), coal-fired electric power plants are the largest source of human-caused mercury air emissions in the U.S. These power plants account for 25.4 tons per year of emissions -- about 49% of total U.S. man-made mercury emissions. Other large sources are:
- Electric arc furnaces used in steelmaking (about 10% of U.S. mercury emissions),
- Industrial boilers (about 7%), and
- Burning waste from the manufacture of Portland cement (about 5.5%)
Burning municipal waste and medical waste was once a larger source of emissions. Today, in response to EPA and state regulations and reductions in mercury use, emissions from these sources have declined over 95 percent.