Mercury, the only common liquid metal, is a nerve toxin that may impair sight, hearing, walking and talking. The main hazard arises when mercury is deposited in lakes or waterways. Bacteria in the water convert it into "organic "alkyl forms, notably methylmercury, which are easily ingested by aquatic life of all types and stored in their fatty tissue. Methylmercury contaminates the food chain and builds up in the fatty tissue of fish and of wildlife and humans who eat the fish. Because of high mercury concentrations in the fish in many places authorities issue advisories each year cautioning people to limit how much fish they eat.
About two-thirds of the mercury in the atmosphere comes from human-made sources like coal-burning power plants and incinerators. The remaining mercury comes from natural sources, such as volcanoes, forest fires and natural ore bodies. However, industrial sources are generally more concentrated and are released in populated areas. Mercury is released directly into the air during mining and smelting, from weathering of paint and from losses during industrial processing. It also enters soils directly on industrial sites, through application of pesticides and through inadequate waste disposal of old batteries and other discarded products containing mercury, and from where it leaches into groundwater and surface waters. Soil erosion and direct discharge of effluents and sewage also carries a significant amount of mercury into waterways.
Acute poisoning by organomercurial compounds has been only infrequently reported and the culprits are methyl and other alkyl compounds. However, there have been many cases of chronic poisoning involving organomercurials, mostly associated with repeated exposure to alkyl compounds during their manufacture or their use for treating seed against pest attack. Such use of alkylmercury-treated seed as food has produced epidemics of poisoning of humans, and more widespread, the poisoning of domestic animals. The exact amount of any alkylmercury compound necessary to produce poisoning in humans is not known, but is obviously small. The average intake of persons who have been poisoned by dressed seed has been estimated at 2.7 mg per person per day and the highest intake at 8.2 mg per person per day.
In the USA, inappropriate disposal of mercury in medical waste incinerators is responsible for emitting 10% of all mercury air emissions each year.