Some facts about mercury

ALTHOUGH best known as the silver liquid in thermometers, mercury has over 3,000 industrial uses. It occurs in nature mostly as mercuric sulphide, known as cinnabar, which has been refined for its mercury content since the 16th century. Mercury is present in numerous compounds and is toxic even in its elementary form.

Properties such as the ability to alloy with most metals, liquefy at room temperature, vapourise and freeze with ease and conduct electricity make mercury an important industrial metal. Mercury in elemental or inorganic form is transformed into organic forms (especially the methylated form) by biological systems when it comes into contact with micro-organisms in the soil and water.

Humans come into contact with mercury through environmental, occupational or accidental exposure. Methylated mercury compounds are toxic and can accumulate in the environment and in the food chain.

Mercury magnifies up to 100,000 times in predatory fish. In the 1950s, the consumption of fish contaminated by methylated mercury led to the poisoning of thousands of fishing families in Minimata, Japan. The pollution, caused by an acetaldehyde manufacturing unit owned by the Chisso industrial group, affected over 10,000 people over a period of three decades.

Occupational exposure to mercury occurs through the inhalation of contaminated air, skin contact with liquid mercury, or oral exposure through contaminated hands or food. Mercury can cause acute or chronic health problems. It accumulates in the kidney, the brain and the blood. The amount of mercury absorbed by the body - and thus the degree of toxicity - depends upon its chemical form. Elemental mercury, the form workers are exposed to, is the most hazardous when inhaled. Only about 25 per cent of the inhaled dose is exhaled.

Very high exposure to mercury vapour can cause acute poisoning. This may lead to pneumonia and can be fatal. People with chronic mercury poisoning experience wide mood swings and become irritable, depressed or excited for no apparent reason. The earliest symptom is tremor of the hand, tongue and eyelids, which can progress to problems with balancing and walking.

Over 40 States in the United States and the U.S. Food and Drug Administration have issued at least 2,000 public health warnings about mercury contamination. Several States and municipal administrations have begun to check the amount of mercury in the environment by banning mercury thermometers, as "even if one thermometer contaminates a drinking water supply source, it can be permanently unusable".