Mercury poisoning

Mercury, familiar as the silvery liquid inside thermometers, looks harmless enough, but the vapour it gives off is an insidious poison when it's inhaled. One early symptom of mercury poisoning is bleeding gums and slobbering at the mouth. The victim of mercury poisoning typically undergoes a personality change, becoming nervous, irritable, depressed, insomniac or erratic in behaviour. Tremors develop in the body, and in severe cases the shaking movements may make it impossible to walk unaided.

The dire effects of mercury poisoning have been known for hundreds of years. It affected goldsmiths, gilders, mirror makers and even doctors treating syphilis patients with mercury.

People working in the felt hat industry were exposed to mercury vapour and were notorious for their bizarre behaviour - this is the origin of the phrase 'as mad as a hatter'.

Today, you're only likely to encounter mercury in a few occupations such as dentistry (where it's used in amalgam fillings) or if you repair arc valves in the electricity generation industry. Strict health and safety procedures are followed, and cases of mercury poisoning are rarely if ever seen today.