Mercury is a naturally occurring element found in water, air, and soil. It is a liquid metal at room temperature that evaporates quickly. Inhaling, touching, or consuming mercury is extremely harmful to the brain, heart, kidneys, lungs, and immune system. The most common cause of mercury poisoning is by ingestion of methylmercury, a highly toxic form that builds up in the fish and shellfish that we consume. Species that live longer or are higher on the food chain, such as albacore tuna and swordfish, are more mercury-dense than others. Pregnant women and children must be especially careful, because mercury has been known to cause birth defects and developmental problems.

Dangerous amounts of mercury are released during coal burning, most notably by power plants and cement kilns. The EPA has set laws that regulate mercury levels in the air and water, and to make sure that mercury waste is disposed of properly, which is important for human health as well as the health of water ecosystems. Though mercury is being phased out of newer products, it can still be found in thermometers, compact fluorescent light bulbs (CFLs), old batteries, and the switches in certain electrical devices.