Toxicity of mercury

A ruling by the European Union heralds the demise of those useful clinical instruments, the mercury thermometer and the mercury sphygmomanometer. The new laws have been passed because of worries about mercury poisoning. Yet y ou can drink metallic mercury and come to no harm. What does it all mean? T here are three forms of mercury from a toxicological point of view: inorgan ic mercury salts; organic mercury compounds; and metallic mercury. Inorgani c mercury salts are water soluble, irritate the gut, and cause severe kidne y damage. Organic mercury compounds, which are fat soluble, can cross the b lood brain barrier and cause neurological damage. Mercury metal poses two d angers. It can be vaporised: the vapour pressure at room temperature is abo ut 100 times the safe amount, so poisoning can occur if mercury metal is sp illed into crevices or cracks in the floorboards. Dentists are occasionally poisoned this way. Mercury easily crosses into the brain, and causes tremo r, depression, and behavioural disturbances. A second danger from metallic mercury is that it is biotransformed into organic mercury, by bacteria at t he bottom of lakes. This can be passed along the food chain and eventually to man. It was this process that led to the Japanese tragedy at Minimata Ba y in the late 1950s when over 800 people were poisoned. It is the need to r educe mercury contamination of the environment which should encourage us to cut the usage of metallic mercury. However, much more metallic mercury is spilled as waste by the chemical industry than is dropped on the floor in t he clinic.