Mercury is a naturally occurring metal which, in its pure form, is not particularly toxic. Under normal conditions of temperature and pressure, it is a silvery-white liquid which readily transforms into a vapour. When vaporised, it enters the atmosphere, remains there for a long time, and is circulated globally (WHO, 2005b). Through chemical reaction and precipitation it enters freshwater lakes and rivers, where it accumulates in the sediments at the bottom. Here it is transformed by bacteria into a variety of mercury compounds, particularly methyl mercury (chemical formula: CH3Hg+) which is highly toxic. From freshwater sediments methyl mercury is taken up by small organisms and enters aquatic food chains, accumulating in the fat of animals and, by bioaccumulation, reaching high levels in animals towards the top of the food chain, such as larger fish and fish-eating birds (Zahir et al., 2005).
The realisation that mercury compounds pose a serious threat to human health began with an unfolding tragedy in Minamata Bay, Japan, beginning in the mid-1950s (Connell et al., 1999). As is often the case, the first evidence that something was amiss came from observations of animals. Birds flew erratically and sometimes fell into the sea; children were able to catch usually evasive octopuses with their bare hands; cats had convulsions and died. It was not until the 1960s that many local people became overtly ill. They had convulsions, began to stagger about and salivated excessively; deaths began to occur, including newly born children. The source of the problem was a chemical factory that was discharging its waste into Minamata Bay. This waste included large amounts of methyl mercury, estimated at 600 tons between 1932 and 1970. The animals and the people were suffering from mercury poisoning, now sometimes called ‘Minamata disease’.
The principal sources of atmospheric mercury are the burning of fossil fuels in power stations and of domestic and industrial wastes in incinerators. Mercury compounds are also released directly to the land in many fungicides (chemicals used to protect crops from fungal diseases) (Clean Air Network, 1999). Mercury compounds have been used as an ingredient of some cosmetics, and even some vaccines. A compound of mercury called thiomersal in the UK (or thimerosal in the USA) has been used as a preservative in vaccines since 1931. In the late 1990s, some safety concerns about thiomersal led to its gradual withdrawal from some of the vaccines in which it had been an ingredient (note: it was never used in the measles, mumps and rubella MMR vaccine in the UK), but a WHO expert committee concluded that there is no evidence of any toxicity and it remains in use (GACVS, 2006).