Coal-fired utilities

Besides nature's own emissions, humans release mercury as well, mostly via the air and eventually into water where fish absorb particles.

Power plants that use coal to produce electricity are the largest single source of U.S. mercury emissions at 40 percent of the total.

The EPA estimates up to 15 percent of mercury emissions from these utilities fall within 30 miles of a plant, and up to 50 percent falls within 600 miles.

Technology to significantly reduce emissions, the EPA estimates, would cost $1.8 billion a year for the industry, and eventually consumers. That's 0.5% of the industry's annual revenue of $400 billion.

Other commerical sources
After coal-fired utilities, the biggest mercury sources are commercial heating boilers powered by coal (10 percent), burning hazardous waste (5 percent) and chlorine production (5 percent).

Humans

Scientists still debate the amount of exposure required to harm a child's development, but most agree kidney and/or brain damage can be done to unborn babies, infants and young children who consume fish poisoned by mercury.

The EPA notes that some Americans, often families from lower income groups, are eating fish with mercury levels 10 times higher than what's advised for humans.

Because mercury is also a natural element often found in soil and water, scientists have been unable to be specific about how significant manmade emissions are.

Wildlife

Mercury's main path into the food chain is via fish, since they pick it up as it falls into lakes, streams and coastal areas. As a result, the vast majority of states have issued fishing advisories because of mercury levels.

Even if mercury emissions ceased, it would still take decades for levels in fish and other wildlife to disappear.

A 1997 EPA report to Congress identified the mink, river otter, kingfisher, loon, osprey and bald eagle as examples of species with increased risk of mercury poisoning because they feed on fish.