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