For example, mercury vapor from volcanoes, coal-fired power plants, crematoria, ore smelters, forest fires, and other sources may disperse into the atmosphere and fall into distant surface waters. Batteries, thermometers and fluorescent light bulbs in old landfills may also release small, but environmentally significant, amounts of mercury into waters, soils and sediments. Once absorbed, animals do not effectively excrete methylmercury and it accumulates in the brain and other parts of the nervous system. Carnivorous fish would essentially accumulate all of the methylmercury from the fish that they eat. The methylmercury is then passed up the food chain to humans that like to catch and eat large fish.
Large fish tend to be older and enriched in methylmercury.
Over their relatively long life spans, the larger fish may accumulate harmful or even deadly concentrations of methylmercury. Some mercury-sensitive people have even died from eating tuna fish dinners. Normally, the mercury vapor goes out of the crematorium stacks. Busy crematoriums may release a lot of mercury into the atmosphere that may pollute nearby lakes, which may lead to methylmercury poisoning of wildlife and humans. Elemental selenium reacts with mercury vapor during cremations and reduces mercury pollution to the atmosphere.