Food Safety Focus (18th Issue, January 2008) – Food Safety Platform
Mercury and Food Safety
Reported by Ms. Joey KWOK, Scientific Officer,
Risk Communication Section, Centre for Food Safety
The following article, being the fifth and last in this series on “Metallic Contaminants in Food”, will focus on mercury. The various sources from which people are exposed to mercury, as well as the measures that people may take to reduce their exposure to this particular metal will be discussed.
What is Mercury? Where does it Come from?
Mercury is a metallic element found at low concentrations in the earth's crust. Pure mercury is shiny, silver-white and liquid at room temperature. Traditionally it has been used to make products like thermometers, electrical switches, and some light bulbs. Mercury exists in three forms, namely metallic (elemental), inorganic and organic. Methylmercury, which is the most prevalent form of organic mercury, is the most hazardous form.
Mercury can be released into the air and water as a result of volcanic eruptions or weathering of rocks, as well as human activities such as combustion of fossil fuels (especially coal), gold and mercury mining, electroplating, waste incineration, etc.
How are People Exposed to Mercury?
People can be exposed to mercury in their occupational environment. In addition, dental amalgam fillings, Traditional Chinese Medicines and cosmetics are also possible sources of exposure to mercury. Diet, however, is by far the most important source of exposure in most general public.
Mercury in fish
In streams, lakes and oceans, mercury can be transformed by bacteria into methylmercury, the most hazardous form. Methylmercury is readily taken up by living organisms and is passed along the microscopic plants and animals to larger organisms via the food chain. It can accumulate in fish and bind tightly to the protein in fish tissues, thus large predatory fish such as swordfish, shark and marlin would have a greater tendency to accumulate higher amount of the chemical (Table 1). Fish is the major dietary source of mercury exposure in humans. Cooking cannot reduce the concentration of mercury in fish effectively.
Table 1: Fish that can contain higher levels of mercury
- Certain types of tuna, e.g. bluefin, bigeye
Illustration: Swordfish fillet
Illustration: A kind of alfonsino: Beryx splendens
What are the Effects of Mercury on Health?
Mercury and its compounds have no known physiological functions in animals. Acute toxicity is often a result of occupational exposure, but that from dietary exposure is rare. Exposure to high level of mercury can cause adverse effect to the nervous system, especially the developing brain. Hence, developing foetuses, infants and young children are more sensitive to such toxic effects.
The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has established Provisional Tolerable Weekly Intakes (PTWIs) of 5 μg/ kg bw/week for total mercury and 1.6 μg/ kg bw/week for methylmercury, respectively. A local study conducted in 2004 revealed that dietary exposures to total mercury and methylmercury among secondary school students were below their respective PTWIs, meaning that their dietary exposures to these chemicals were unlikely to cause harmful effects. The food group "fish" was identified as the main contributor (59% for both total mercury and methylmercury), followed by the food group "seafood other than fish" (14% of total mercury and 18% methylmercury). Swordfish was found to have the highest concentrations of both total mercury and methylmercury.
Advice to Consumers
Susceptible groups such as young children, pregnant women and women planning pregnancy should be careful in their selection of food, in particular, they are advised to avoid eating large predatory fish.
Moderate consumption of fish is recommended as fish is a good source of high-quality protein and omega-3 fatty acids.
Maintain a balanced diet and avoid overindulgence of food items that may have high mercury contents (e.g. fish and shellfish).
Advice to the Trade
Obtain food supplies from reliable sources, and do not obtain fish and shellfish from contaminated areas.