Mercury is among a group of pollutants called persistent bioaccumulative 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. Mercury also "bioacccumulates" in the environment, meaning it builds up in the food chain over time.
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 (EPA 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.
Bioaccumulation of Mercury
When mercury falls in rain or snow, or when it falls out of the air as dry deposition, it may eventually be washed into waterbodies by rain. Bacteria in soils and sediments convert mercury to methylmercury. In this form, it is taken up by tiny aquatic plants and animals. Fish that eat these organisms build up methylmercury in their bodies. As ever-bigger fish eat smaller ones, the methylmercury is concentrated further up the food chain and bioaccmulates in the larger species.
Methylmercury concentrations in fish depend on many factors, including mercury, the concentration in water, water pH and temperature, the amount of dissolved solids and organic matter in the water, and what organisms live in the water. Methylmercury concentrations in fish may also be affected by the presence of sulfur and other chemicals in the water. Because of these variables, and because food webs are very complex, bioaccumulation is hard to predict and can vary from one water body to another.
However, in a given water body, the highest concentrations of methylmercury are generally found in large fish that eat other fish. The concentrations of methylmercury in large fish can be over a million-fold larger than in the surrounding water. EPA discussions of estimates bioaccumulation can be found in Chapter 6 and Appendix A of the Water Quality Criterion for the Protection of Human Health: Methylmercury.
Mercury is tightly bound to proteins in all fish tissue, including muscle. There is no method of cooking or cleaning fish that will reduce the amount of mercury in a meal.
Methylmercury accumulates as you move up the food chain:
Fish are caught and eaten by humans and animals, causing methylmercury to accumulate in their tissues.