Overview
Mercury is a highly toxic element that is found both naturally and as a contaminant that is introduced in the environment. The potential of mercury as a contaminant is well known and well documented. However what is alarming is the fact that people and wildlife even in environments that are not obviously polluted are at a risk of mercury contamination. The level of risk is determined by the likelihood of exposure, the form of mercury present (some forms are more toxic than others) and the geochemical and ecological factors that influence how mercury moves and changes form in the environment.
Sources of Mercury
Alkali and metal processing, incineration of coal, medical and other waste and mining of gold and mercury lead to the high concentration of mercury in some areas. However the factor that is most responsible for the rise in mercury concentration over the landscape is atmospheric deposition. Natural sources of atmospheric mercury include volcanoes, geologic deposits of mercury and volatilization from the ocean. Scientists have also found some local mineral occurrences and thermal springs that are naturally high in mercury. Mercury is also contained in small amounts in some rocks, sediments, soils and certain water bodies.
Toxic Effects
The toxic effects of mercury depend on its chemical form and the route of exposure. Methyl mercury is the most toxic form of mercury and is particularly damaging to developing embryos, which are 5 to 10 times more sensitive than adults. This chemical form of the contaminant affects the immune system, alters the genetic and enzyme systems and damages the nervous system including coordination and the senses of touch, taste and sight. Methyl mercury enters the human system through ingestion, and is absorbed more readily and excreted more slowly than other forms of mercury thereby increasing its toxic potential. Elemental mercury is released from broken thermometers. This form is less toxic than methyl mercury but if inhaled for a long period of time causes tremors, gingivitis and excitability. However if elemental mercury is ingested it is absorbed slowly and may pass through the digestive system without causing much damage. Mercury chloride is another form of mercury that causes damage to the gastrointestinal tract and kidney failure. However contamination from this salt is unlikely from environmental sources. Some other forms of mercury also cause premature birth i.e. birth before the 37th week of pregnancy.
People are exposed to methyl mercury almost entirely through eating contaminated fish and wildlife that are at the top of the aquatic food chains. Generally fishes which live longer and eat other fishes have greater concentration of mercury. These include sharks, tuna, mackerel and sword fish and their consumption should be avoided during pregnancy. Fishes like all forms of salmon, rainbow trout, sardines, herring, pilchard and canned salmon are perfectly safe to eat during pregnancy and are also beneficial as they contain the necessary omega -3 fatty acids.
