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