U.S. EPA Mercury Web Site Mercury is a naturally occurring element that is present throughout the environment.
Human activity can release some of that mercury into the air, water and soil.
In the U.S., coal-fired power plants are the biggest source of mercury emissions to the air. Mercury concentrations in air are usually low and of little direct concern. But when mercury enters water, biological processes transform it to a highly toxic form that builds up in fish and animals that eat fish. People are exposed to mercury primarily by eating fish. The Environmental Protection Agency is working to reduce the amount of mercury in the environment.