Toxicity of mercury A new European Union directive1 will prohibit the use of mercury in sphygmomanometers and clinical thermometers. We review here the toxic effect of mercury and its compounds and discuss their relevance to the environment and to modern medicine. The Chinese used mercury (II) sulphide 1000 years before the birth of Christ as the red dye pigment vermilion. It was used similarly in the GraecoRoman world, with both Hippocrates and Galen recording its toxic effects. Since then its toxicity has become well known in metalworkers, miners, felthat manufacturers, dyers and paint manufacturers. Despite this, mercury has been incorporated into the treatment of man’s maladies from ancient times. Its main use has been to treat syphilis, from its first appearance in the West in the 15th century up to World War II.2 Mercury and its salts have at various times been used as antiseptics, skin ointments, laxatives, diuretics, bowel washouts for the treatment of colorectal cancer, and scabicides. It is still used today as a solvent for the silver-tin amalgams used in dental fillings. So how toxic is mercury?