Risk of Mercury in Fish
- Mercury is an element which exists naturally in the environment in abundance.
- It can be released into the air and water both naturally and from industrial sources and entered into the food chain.
Occurrence of Mercury
- Mercury exists in three forms, namely metallic, inorganic and organic.
- In streams, lakes and oceans, mercury can be transformed by bacteria into methyl mercury, which is the most prevalent form of organic mercury and is the most hazardous form. Methyl mercury 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 binds tightly to the protein in fish tissues. Hence, fish is the major dietary source of mercury exposure in humans.
- As methyl mercury tends to bioaccumulate in the food chain, large predatory fish such as shark, swordfish, marlin, alfonsino and tuna (especially bigeye and bluefin species) would have a greater tendency to accumulate higher amount of the chemical than non-predatory fish species at lower levels in the food chain.
- Cooking could not appreciably reduce the concentration of mercury in fish.
Adverse Effects of Mercury
- Exposure to high level of mercury can cause adverse effect to the nervous system, especially the developing brain. Hence, unborn foetuses, infants and young children are more sensitive to such toxic effects.
- The Provisional Tolerable Weekly Intake (PTWI) of methylmercury as recommended by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) in 2003 is 1.6 μg/kg body weight.. The JECFA also established a PTWI of 4 μg/kg body weight for inorganic mercury in 2010, based on the assumption that the predominant form of mercury in foods other than fish and shellfish is inorganic mercury. In the absence of evidence to the contrary, this PTWI for inorganic mercury was considered applicable to dietary exposure to total mercury from foods other than fish and shellfish. The PTWI of 5 μg/kg body weigh for total mercury established by JECFA in 1972 was withdrawn by JECFA in 2010.
- PTWI is an estimate of the amount of a contaminant that can be ingested over a lifetime without appreciable risk. An intake above the PTWI does not automatically mean that health is at risk. Transient excursion above the PTWI would have no health consequences provided that the average intake over long period is not exceeded as the emphasis of PTWI is a lifetime exposure.
Situation in Hong Kong
- Level of mercury in food in Hong Kong is regulated under the Food Adulteration (Metallic Contamination) Regulations made under the Public Health and Municipal Services Ordinance ((Cap.132), which stipulates that the maximum permitted concentration of mercury in food is 0.5 ppm (parts per million). Offenders shall be liable to a maximum fine of $50,000 and imprisonment for six months.
- The Centre for Food Safety (CFS) has a food surveillance programme in place for mercury in food. In 2009, 2,503 food samples including 320 samples of fish and fish sashimi had been taken for testing of mercury. Predatory fish such as swordfish had the highest content of mercury. The mercury levels of 7 out of 2,503 food samples taken were above legal limit and they had been announced in Food Safety Reports. The results of the remaining samples were all below the legal limit.
- In 2008, CFS has conducted a study to estimate the dietary exposure to methylmercury of secondary school students. The results showed that the estimated dietary exposure to methylmercury for average and high consumers of secondary school students were 0.58 μg/kg bw/week and 1.61 μg/kg bw/week respectively. The results for average consumers were within the PTWI of methylmercury (1.6 μg/kg body weight) as recommended by JECFA. This implies that the average consumers among secondary school students in Hong Kong would be unlikely to experience major toxicological effects of methylmercury. For the high consumers, the estimated dietary exposures to methylmercury may exceed the PTWI (94 – 106 % of PTWI). Hence, the risk to adverse effects of methylmercury for high consumers cannot be excluded. However, transient excursion above the PTWI would have no health consequences provided that the average intake over long period is not exceeded as the emphasis of PTWI is a lifetime exposure. (For reference of full report of the study, please click here).
Advice to the Public
- Pregnant women, women planning pregnancy and young children when selecting fish species in their diet should avoid eating large predatory fish and the types of fish which may contain high levels of mercury such as shark, swordfish, marlin, alfonsino and tuna (especially bigeye and bluefin species)
- Maintain a balanced diet to avoid excessive exposure to contaminants from a small range of food items.
- 3. Moderate consumption of a variety of fish is recommended as fish is an excellent source of many essential nutrients, such as omega-3 fatty acids and high quality proteins.
Risk Assessment Section
Centre for Food Safety