My mouth was open, numb and full of instruments by the time I realised what was happening. Only my eyebrows could register alarm that yet another mercury amalgam filling was being inserted in to my tooth. A question mark, I knew, hung over the use of mercury in the mouth, but the dentist dismissed my fears. I can't tell you exactly how much mercury is present in amalgam," he announced, glancing fretfully at the clock, "but as part of a compound, it is inert." Afterwards, I soon found out that mercury comprises more than 50 per cent of amalgam. Mercury is one of the most poisonous substances known to man. If amalgam were proposed now as a dental filling, it would not be approved, but it has been around for 150 years. There is also now abundant evidence that the metal leaks out. It vapourises at body temperature, particularly during tooth-brushing, chewing and the consumption of hot drinks - and the vapour is more dangerous than the mercury that occurs naturally in food. Some people are known to be unable to tolerate even small amounts. But that doesn't mean that we should all worry about it; according to BDA spokesman Stephen Challacombe, Professor of Oral Medicine at Guy's, King's and St Thomas's hospitals: "Approximately five per cent of the population would react to contact with mercury, as they do to all heavy metals." White fillings are now, their advocates claim ... just as durable and tight-fitting as amalgam. Finding dentists able to insert them in heavily damaged rear teeth is difficult.