Introduction

There is a potential accumulation risk of methylmercury when large quantities of fish and seafood products are consumed on a regulator basis. For most populations fish is the main source of methylmercury in food. In general, concentrations are below 0.4μg/g, but fish at the highest trophic levels may contain methylmercury above 5μg/g. Older and larger predatory fish species and certain marine mammals contain the highest levels of methylmercury. The half-life of methylmercury in fish is approximately 2 years and therefore large older fish accumulate considerably more mercury than young smaller fish. Fresh tuna typically contains higher Hg concentrations than canned tuna because it is produced from smaller and younger fish which have accumulated less methylmercury.

European Commission Member States have issued specific advice to vulnerable groups. This includes limiting the frequency of consumption of particular predatory fish, such as swordfish, marlin, pike and tuna. Some non EC countries have issued specific advice mainly for pregnant women and young children. For example Health Canada advises the general population not to eat more than 150 grams per week of tuna, shark, swordfish or marlin. For women that are pregnant, may become pregnant or are breast feeding consumption of less than 150 gram per month is recommended. This limit is reduced to 75 grams per month for children aged 1 to 4 years of age.

The toxicity of mercury is dependent on whether it is inorganic, elemental or organic. Methylmercury affects the kidneys and also the central nervous system, particularly during development, as it crosses both the blood-brain barrier and the placenta. Following ingestion approximately 95% of methylmercury is absorbed through the gastrointestinal tract and distributed to all tissues within 30 hours with approximately 5% found in blood and 10% in the brain. The excretion process of methylmercury involves transfer of the glutathione-mercury complex into the bile, demethylation by gut microflora to the inorganic form and the subsequent the elimination in the feces. The half-life of mercury in the body is approximately 70 days in adults, with a steady state being reached in one year.

Table 1. Provisional tolerable weekly intakes by country and organization.

The provisional tolerable weekly intake (PTWI) of methylmercury for the general population is between 0.7μg/Kg body weight/week to 2.8μg/kg body weight/week depending on the reference level set by organization and country (see Table 1).

Most countries have recognized that pregnant women and young children are at greater risk and have thus adopted lower PTWI reference limits for this susceptible group of the population. The PTWI is the amount of a substance that can be consumed weekly over an entire lifetime without appreciable risk to health. This includes heavy metals such as mercury which are known to have cumulative properties.

The European Commission Regulation 221/2002 defines the maximum mercury concentration in general fish as around 0.5mg Hg/Kg and 1.0mg Hg/Kg for certain larger predatory species of fish such as shark, swordfish, marlin, tuna and orange.