Study on mercury in fish and food safety released

The Centre for Food Safety (CFS) today (April 16) released the findings of its study on mercury in fish and food safety.

"Mercury is a metallic element present widely in the environment from natural sources and human activities. It accumulates mainly in the organic form of methylmercury (MeHg) in the food chain, particularly in fish. Concerns about MeHg in food are related to its possible effects on the nervous system, particularly in developing foetuses," the Centre's Consultant (Community Medicine) (Risk Assessment and Communication), Dr Ho Yuk-yin, said.

According to the Food Adulteration (Metallic Contamination) Regulations, the legal level for mercury, including MeHg, in food is 0.5ppm, or 500 microgrammes per kilogramme (500 µg/kg).

For the study, the CFS had collected 280 fish samples, including 266 whole fish and 14 canned fish of 89 species found in the local market, to analyse their levels of total mercury and MeHg. The findings indicated that, except for three samples of alfonsino, the levels of total mercury in all the samples did not exceed 500 µg/kg.

The levels of total mercury detected in the three alfonsino samples ranged from 609 to 1370 µg/kg. These samples were also found to contain a higher level of MeHg, ranging from 509 to 1010 µg/kg.

The levels of total mercury in all the other samples ranged from 3 to 469 µg/kg, and the levels of MeHg detected ranged from 3 to 430 µg/kg.

Apart from the alfonsino samples, the study also found comparatively higher average levels of MeHg in samples of yellowback seabream, yellowtail barracuda, and canned albacore tuna, ranging from 205 to 253 µg/kg.

The average level of MeHg for different species of tuna fish samples, including canned albacore, canned yellowfin, skipjack (canned and whole fish), also varied, ranging from 85 to 205 µg/kg. Among these samples, the highest average level was found in canned albacore.

At present, the Joint Food and Agriculture Organisation/World Health Organisation Expert Committee on Food Additives (JECFA) adopts a Provisional Tolerable Weekly Intake (PTWI) of 1.6 µg/kg bodyweight for MeHg.

Based on the levels of MeHg detected in the fish samples in this study, and the data from the risk assessment study "Dietary Exposure to Mercury of Secondary School Students" in 2004, the CFS estimated that dietary exposure to methylmercury of secondary school students was below the PTWI for average consumers (31% to 41% of PTWI). However, the estimated dietary exposure for high consumers may exceed the PTWI (94% to 106% of PTWI).

"The current study shows that most of the fish available in the Hong Kong market have relatively low levels of total mercury and MeHg, while a small proportion contains higher levels. The amounts of MeHg relative to total mercury in different fish species vary considerably," Dr Ho said.

"For the high consumers among secondary school students, their estimated dietary exposure to methylmercury may exceed the PTWI, therefore the possible health risk cannot be ruled out."

The CFS advised consumers to maintain a balanced diet. Moderate consumption of a variety of fish is also recommended, as fish contains many essential nutrients, such as omega-3 fatty acids and high quality proteins.

Pregnant women, women planning pregnancy and young children should avoid eating large predatory fish and those which may contain high levels of mercury, such as shark, swordfish, marlin, alfonsino and tuna (especially the bigeye and bluefin species).

The centre also advised the trade to obtain food supplies from reliable sources, maintain proper records to enable source tracing if necessary, and inform customers of the species of fish sold, served and used in fish products.

Ends/Wednesday, April 16, 2008