Metal Contaminants in Food: Mercury


Fish is rich in protein, which is an essential nutrient required for human body, and it is indispensable food in a balanced diet, especially for pregnant women and children in their body developmental stage. However, some certain species of fish contain higher levels of mercury compared with other fish, such as tuna, swordfish and marlin. It may affect our health after consumed. According to the World Health Organization (WHO), it indicates that there are four metals causing a significant public health concern, namely arsenic, cadmium, lead and mercury. Metal contaminants can enter our food through the environment (e.g. air, soil and water) or in the food production processes. These may be present in foodstuffs in trace amounts. In Macao, the Administrative Regulation No. 23/2018 ‘Maximum Limits of Heavy Metal Contaminants in Food’ aims to regulate and limit the maximum amount of heavy metal contaminants detected in foodstuffs, including of arsenic, cadmium, lead, mercury and tin, so as to safeguard the food hygiene and safety.


What is Mercury?

Mercury is a natural metallic element found in the Earth's crust and exists ubiquitous in the environment. It has three forms – elemental mercury, inorganic salts and organic compounds. Methylmercury is the most common form of organic mercury and the most toxic. It releases to the environment and causes pollutions, through volcanic eruptions, weathering and erosion of rocks as well as industrial activities (e.g. refuse incineration, electroplating and mining). In addition, it bio-accumulates in the body of organisms and piles up via the food chain.


Sources of Mercury Intake

Generally, mercury is ingested by eating contaminated fish and shellfish. It is the most common route of mercury exposure to humans. Aquatic microorganisms are able to transform inorganic mercury to methylmercury which accumulates in the body of organisms in the food chain; thus, the large fish has a high level of mercury. Cooking cannot effectively destroy or reduce the concentration of mercury in fish.


Health Effects

Excessive intake of mercury can harm human’s nervous system, especially the developing brain. It negatively affects the brain development of fetuses, infants and young children. In the case of adults, it is detrimental to the hearing, vision, memory ability, muscle coordination, etc.


What Should We Note in Our Daily Diet?

Women planning pregnancy, pregnant women and young children are known to be more susceptible to the toxic effects of mercury. They should avoid eating large fish (e.g. tuna, swordfish and marlin) so as to minimise health risks. Whereas the levels of mercury in fish species vary considerably by its origin and habitat, it is advisable to eat a variety of fish. Moreover, it is important to maintain a balanced and diversified dietary pattern to avoid excessive intake of metal contaminants from a limited range of fish species.