Mercury finds its way into the atmosphere from natural processes such as volcano eruptions, groundwater seepage and evaporation from the oceans as well as human activity. The latter accounts for between 50 and 75 percent of total releases to the air each year, according to the latest estimates.

An estimated 243 tons of mercury gets pumped into U.S. air each year from human activities. Of this amount, combustion sources contribute 85 percent. Medical waste incineration is the largest component (27 percent), following closely by municipal waste incinerators (23 percent), utility boilers (21 percent) and commercial/industrial boilers (12 percent).

By far the least mercury-polluting fossil fuel is natural gas, which contains almost no mercury at all. Oil is the next cleanest, followed by coal, the dirtiest.

Once released into the environment, mercury can be recirculated through the air, soils and water for extremely long periods, possibly even hundreds of years. Total global atmospheric mercury has at least doubled during the industrialized age and may have increased five-fold since 1890.

Thirty-seven states issue warnings about eating mercury-contaminated fish, and in 1994 six of these had health advisories posted on every freshwater lake, river and stream in the state. A 1992 national study by the U.S. EPA found mercury in fish at 92 percent of more than 314 water bodies tested across the nation.

Mercury use by industry in the U.S. has been in a steep decline for decades, with the peak year thought to be 1970. Dramatic reductions in the use of mercury in the manufacture of paint, pesticides and batteries are credited with the decline.

Humans can be exposed to methylmercury, one of mercury’s most toxic forms, by drinking contaminated water, breathing contaminated air, or eating contaminated beef, milk, chicken, eggs, lamb, pork, leafy vegetables, potatoes, cereal grains and fruits--but by far the most important exposure route is through eating freshwater fish.

In general, people who eat more than 3.5 ounces of fish each day may be eating enough methylmercury to cause health problems.

For the most part, marine seafood consumed in the U.S. is safe. Elevated levels of mercury are principally found in shark, swordfish and some tunas.

In humans, methylmercury has a half-life estimated to range from a month to nine months, with excretion occurring principally via the feces, breast milk and urine.

The population groups considered most at risk from methylmercury consumption are women of child-bearing age (between 15 and 44) and children below age 15.

The long-term health effects of low-level exposure to methylmercury aren’t fully known. However, the EPA reports that a person could consume a tenth of a microgram for every kilogram of body weight every day (about 47 micrograms for a 175-lb person) and never be adversely effected in his or her lifetime.