Risk of Mercury in Fish

Introduction

1. Mercury is an element which exists naturally in the environment in abundance.

2. It can be released into the air and water both naturally and from industrial sources and entered into the food chain.

Dietary Concern of Mercury

1. Mercury exists in three forms, namely metallic, inorganic and organic.

2. 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.

3. As methyl mercury tends to bioaccumulate in the food chain, large predatory fish such as tuna, swordfish and shark would have a greater tendency to accumulate higher amount of the chemical than non-predatory fish species at lower levels in the food chain.

4. Cooking could not appreciably reduce the concentration of mercury in fish.

Harmful Effects of Mercury

1. 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.

2. The Provisional Tolerable Weekly Intake (PTWI) of mercury as recommended by the Joint FAO/WHO Expert Committee on Food Additives (JECFA) is 5 μg/kg body weight.

3. 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

1. 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.

2. The Food and Environmental Hygiene Department (FEHD) has a food surveillance programme in place for mercury in food. In 2002, 444 food samples including 72 samples of fish had been taken for testing of mercury. Predatory fish such as shark, tuna and swordfish had the highest content of mercury. Nevertheless, the results were all below the legal limit.

3. In 2002, FEHD had also conducted a study on dietary exposure of secondary school students to heavy metals. The results showed that the estimated dietary exposure to mercury for an average secondary school student in Hong Kong was 2.98 μg/kg body weight/week, which is below the PTWI as recommended by JECFA. This implies that an average secondary school student in Hong Kong would not experience major toxicological effects. For high consumers, the estimated dietary exposure to mercury was 6.41 μg/kg body weight/week, which is higher than the PTWI (For reference of full report of the study, please click here).
   

Advice to the Public

1. Pregnant women, infants and young children should avoid consuming excessive amount of predatory fish which may contain higher concentration of mercury.

2. Maintain a well-balanced diet to avoid excessive exposure to contaminants from a small range of food items.

3. Moderate consumption of fish is recommended as fish is an excellent source of high quality protein and low in saturated fat.

Risk Assessment Section
July 2003