Risk in Brief - Risk of Mercury in Fish

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.

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

Adverse Effects of Mercury

  1. Mercury in food may cause adverse effects to the nervous system. Developing nervous system of foetuses and children is particularly vulnerable to mercury, which can cause a decrease in intelligence quotient (IQ).
  2. 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.
  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. Levels of mercury in food in Hong Kong is regulated under the Food Adulteration (Metallic Contamination) Regulations (Cap. 132V) made under the Public Health and Municipal Services Ordinance (Cap.132), which stipulates the maximum levels (MLs) for total mercury and methylmercury in food. The MLs for methylmercury in fish ranged from 0.5 mg/kg to 1.7 mg/kg. Offenders shall be liable to a maximum fine of $50,000 and imprisonment for six months.

Advice to the Public

  1. Fish is an excellent source of protein, essential omega-3 fatty acids, and various vitamins and minerals. Eating 1-2 servings (1 serving = 150g) of different fish per week in a balanced diet offers significant health benefits. Limit the consumption of predatory fish (such as shark, swordfish, marlin, splendid alfonsino, albacore tuna and yellowfin tuna) as these fish accumulate higher levels of mercury.
  2. Mercury levels in canned tuna are lower than in fresh tuna, mainly due to different fish species or smaller fish being used. Since the species of tuna used in canned foods and their mercury levels vary from one product to another, it is prudent for women planning for pregnancy, expectant and lactating mothers to avoid eating too much canned tuna.
  3. Maintain a balanced diet to avoid excessive exposure to contaminants from a small range of food items.

Risk Assessment Section
Centre for Food Safety
September 2025