4.3 Mercury

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).

The effects of mercury compounds on wildlife and on people

Mercury compounds have no effect on plants, but adverse effects have been demonstrated in a wide range of animals, including fish and amphibians (Boening, 2000). Very high levels of mercury have been found in the livers of American alligators in the severely polluted Everglades of Florida; these can be as much as 400 times greater than levels in alligators born and reared in alligator farms (Linder and Grillitsch, 2000).

SAQ 13

By what process would such high levels of mercury arise in alligators?

Answer

Bioaccumulation. Alligators are large, predatory animals that feed on fish.

Methyl mercury pollution is implicated in the near extinction of populations of stream-living salamanders in Acadia National Park, Maine (Bank et al., 2006).

The most important effect that mercury compounds have on people is on children born to women exposed to high levels during pregnancy (WHO, 2004). In extreme cases they have seizures and cerebral palsy; they may also be born blind or deaf. In less extreme cases, they have reduced intelligence, poor memory and attention deficit disorder. Mercury compounds have no detectable effect on the mother, but can be detected in her hair, and mercury levels in maternal hair are strongly related to the severity of post-birth effects in children (Cohen et al., 2005).

Infants can also be exposed to mercury compounds via breast-milk. In some fishing communities the concentration of mercury in children's hair is correlated with the duration of breast-feeding. Reports of high mercury levels in mothers and children mostly come from regions where people eat a lot of fish; for example, high levels of blood mercury have been detected in people in the USA who identify themselves as Asians, Pacific Islanders or Native Americans (Hightower et al., 2006). The unsaturated fats that occur in fish have beneficial consequences for human health and people are encouraged to eat fish in many countries. Currently, the US government encourages the eating of fish in the general population, but discourages it in women of childbearing age because of the risk posed to unborn children by mercury compounds (Cohen et al., 2005). Around the Faeroe Islands especially high levels of mercury have been found in pilot whales and, as a consequence, pregnant women are encouraged to avoid eating whale meat (Booth and Zeller, 2005).

Mercury compounds represent a major threat to human health in the future. Mercury emissions from power stations and other sources are not controlled in most countries. For example, at the time of writing in 2006 they were not covered by US Clean Air legislation (Clean Air Network, 1999). The rate of emissions has been increasing; there was a 10% increase in the USA from 2001 to 2002, and, in countries such as China and India, whose rapidly expanding economies are heavily dependent on fossil fuels, emissions are predicted to increase even faster (Booth and Zeller, 2005). The effects of mercury pollution will be global; because mercury can be dispersed as a vapour it can be deposited anywhere in the world.

Current inaction over mercury contrasts strongly with what has happened in relation to another toxic metal, lead. Lead is recognised as a very important toxin for children (WHO, 2004). Like mercury, it has serious effects on the developing nervous system, causing impaired brain function, leading, for example, to attention deficit disorder. In many high-income countries, the most obvious sources of lead have been eliminated; lead is no longer used for water pipes and has been removed from petrol and paint. Lead has long been subject to a surveillance programme in the USA, but no such programme yet exists for mercury (Schweiger, 2005).

Original Copyright © 2007 The Open University. Now made available within the Creative Commons framework under the CC Attribution – Non-commercial licence (see http://creativecommons.org/by-nc-sa/2.0/uk/).