Mercury in the environment

Mercury can be released into the environment from natural sources, such as volcanic and geothermal activity, marine environments or forest fires, or it can be released from man-made sources like coal-fired power plants and other industrial activities.
Recent studies suggest that human activity contributes 50 to 70 percent of the mercury in the environment globally (Office of Air Quality and Standards Report to Congress, 1997). Once it enters the environment, it circulates in and out of the atmosphere until it ends up in the bottoms of lakes and oceans where it is transformed by bacteria into a form that can accumulate in fish tissue.
Mercury is among a group of pollutants called persistent bio-accumulative toxins, or PBTs. These pollutants "persist" in the environment, meaning that they don’t break down or go away. Mercury can’t be destroyed or combusted and it doesn’t degrade.