Mercury in Food and Its Impact on Human Health

Mercury is a naturally-occurring heavy metal that may enter the environment through natural phenomena (e.g. volcanic activity and erosion of mineral deposits) as well as from a wide range of anthropogenic activities such as mining, metal smelting, coal production, coal-fired power stations, paper/pulp production, residential heating systems, waste disposal/incinerators and chemical synthesis/use.

Mercury is present in the environment in a number of forms including elemental mercury (Hg0), inorganic mercurous (Hg+) and mercuric (Hg2+) salts and as organic compounds (e.g. methyl-, ethyl- and phenyl- mercury) ; each form possesses different physicochemical properties and toxicity profiles ([Clarkson, 197] and [Goldman and Shannon, 201]). However, once released into the environment, the various forms of mercury are subject to complex inter-conversions, principally through oxidation-reduction and methylation-demethylation reactions involving bacteria, and to transport on a global scale; processes that together are termed the ' mercury cycle' (Clarkson, 17). The only long-term sink for removal of mercury from the biosphere is thought to be deep-sea sediments ([Clarkson, 19] and [Goldman and Shannon, 201]). Among the organic forms, the principal human source is the consumption of fish species - particularly predatory species - since all forms of organomercury have high bioaccumulation potential (up to 5000-times that of surrounding waters; Sharma, 93) and concentrations may reach very high levels in predatory fish. Methylmercury (MeHg) is the most stable of the organomercury compounds and, hence, is the primary concern regarding human exposure to this type of mercury (EC, 26).

There is a considerable body of evidence from experimental animal studies of the toxic effects that may arise from short-term (acute) and prolonged (chronic) exposures to the various types of mercury compounds. The dangers associated with high-level (occupational) exposure to the various forms of mercury have also been recognised for centuries (Bull, 7). However, the extent of risk to the general population from long-term but low-level (environmental) exposure to mercury remains an area of continuing debate.

At a recent European Commission conference (EC, 2006), a call was made to adopt a highly precautionary approach, based upon suggestions that there may be observable effects in humans exposed to even very low levels of MeHg, especially with regard to impacts on early functional programming of the foetus and infant (Grandjean, 8). The WHO has also recommended the reduction or, wherever possible, elimination of the use of mercury (WHO, 7) and within Europe recent measures have restricted the use of many forms of mercury (EU, European Union, 25) and further measures have been proposed (EC, 29). It is therefore timely in this paper to seek to summarise current knowledge on mercury, focusing on assessing the strength of the epidemiological evidence base on the potential adverse effects of long-term low-level exposure to mercury in its various forms, and seek to identify those areas of uncertainty where further research might be of particular value. This assessment was based on literature identified through a structured search focusing mainly on authoritative reviews published during 1998 to 2008, using a comprehensive set of ...