Mercury sources and toxicity

ROCHESTER, NEW YORK. Mercury is a highly toxic metal associated with damage to the kidneys and central nervous system. Mercury vapour is emitted from volcanoes, coal-burning power stations, and municipal incinerators and returns to the earth through rain contaminated with metallic mercury. Metallic mercury is methylated to methyl mercury in oceans and lakes and enters the food chain via fish and other seafood. Long-lived predator fish such as shark, swordfish, tilefish, king mackerel, and pike and bass in fresh water are the main sources of methyl mercury. Dental amalgams are an important source of mercury vapour and the vaccine preservative thimerosal is a significant source of ethyl mercury.

Researchers at the University of Rochester School of Medicine recently published a review of what is currently known about mercury toxicity. Among the highlights:

Mercury vapour, methyl mercury and ethyl mercury all target the central nervous system and mercury vapour and ethyl mercury also target the kidneys. Inorganic (metallic) mercury primarily targets the kidneys and stomach.
Chelators such as DMSA are effective in removing all forms of mercury from the body, but cannot reverse central nervous system damage.
The allowable or safe intake of mercury has recently been reduced to 0.1 microgram/day per kilogram of body weight.
The concentration of mercury in the brain, blood and urine correlates with the number of amalgam fillings in one's mouth. The concentration increases markedly with increased chewing. Long-term use of nicotine gum by people with amalgam (silver) fillings may increase levels by a factor of 10, thus approaching occupational safety limits.
There is concern, but no clear evidence, that mercury emitted from amalgam fillings may cause or worsen degenerative diseases such as ALS, Alzheimer's disease, multiple sclerosis, and Parkinson's disease.
Ethyl mercury (thimerosal) is used as a preservative in vaccines. Recent concerns about its toxicity have caused US authorities to take steps to remove it by switching from multi-dose vials to single-dose vials that do not require a preservative.
A recent move by power companies to replace mercury containing pressure-control devices for domestic gas supplies has led to numerous spills of mercury in homes. Some 200,000 homes were affected in one recent incident. The liquid mercury is difficult to remove and gives off highly toxic vapours, which are particularly harmful to infants and children.
Several studies have found an association between mercury exposure and cardiovascular disease, but other studies have failed to confirm the connection.

Clarkson, Thomas W., et al. The toxicology of mercury – current exposures and clinical manifestations. New England Journal of Medicine, Vol. 349, October 30, 2003, pp. 1731-37

Editor's comment: The review makes it clear that exposure to mercury is detrimental, but hard to avoid. Nevertheless, avoiding the placement of new amalgam dental fillings and gradually replacing old ones with composite fillings, avoiding gum chewing if amalgam fillings are present, and limiting the intake of fish with high mercury levels are all steps that can be taken by everyone. It is important to realize that consuming just one 7 oz (198 grams) can of tuna per week translates into a mercury intake of 0.1 microgram/day of mercury per kilogram of body weight – equivalent to the currently recommended maximum daily intake.