The main risks that eating fish can pose to human health come from bacteria, viruses, dioxins, pesticides, medicine residues in aquaculture products, parasites and heavy metals. In recent years, the presence of mercury has been the most worrying. Mercury is an element present in nature in the soil, rocks, lakes and oceans. It is freed into the environment by climatic changes, volcanoes and forest fires. It is also released by certain human activities, such as deforestation and the burning of gas and oil. The concentrations in the air and water are very low and are not a significant source of exposure for humans.
Mercury contamination in humans generally comes from food. Traces of mercury can be found in dairy products, meat, poultry, eggs, pasta, fruit and vegetables. However, the concentrations in these foods are negligible compared to what is found in fish. According to the World Health Organization, swordfish and shark are the most significant sources of mercury in food.
Methylmercury is an organic form of mercury which can become very toxic at high exposure rates. It is the most common form in fish, and in some species it is found in concentrations potentially harmful to human health. In humans, methylmercury is easily absorbed by the blood stream and spreads throughout the body, concentrating in certain points such as the brain and, in pregnant women, the developing fetus.
Following exposure to this substance, humans can suffer a wide range of symptoms. Their seriousness depends on the intensity and length of exposure. The principal effect on human health is alterations in the functioning of the central and peripheral nervous systems. For example, high exposure in fetuses and young children can cause motor difficulties, sensory problems, mental retardation and blindness. Adults can suffer from changes in personality, tremors, disturbances of vision, deafness, loss of muscle sensitivity and coordination, damage to the memory, and in extreme cases, death.
Unlike intense, short-lived exposure, long-term (chronic) exposure to small doses can cause symptoms that are hard to recognize.
According to some studies, omega-3 fatty acids and trace elements such as selenium, iodine, iron and choline found in marine organisms could neutralize the toxic action of methylmercury in people who eat fish. The fetus, however, is very sensitive to prolonged exposure, which can interfere with the development of the nervous system at much lower doses than for adults.
There is no cooking method which can reduce the concentration of the substance, which is bound to the protein in the fish muscle tissue and is resistant to high temperatures. In general, the national agencies concerned with food safety recommend that the most susceptible sectors of the population (pregnant or nursing women and small children) eat a range of fish in moderation and if possible exclude predatory fish such as tuna, swordfish, skate, monkfish and ling. The research group Zero Mercury believes that methylmercury contamination from fish constitutes a serious public health problem.
The recent report, The State of the Marine Environment, produced by the United Nations Environment Programme (UNEP) highlights that good results have been seen in the Northeast Atlantic and the Mediterranean. Over the last decade, the concentration of cadmium, mercury and lead in mussels and fish has decreased. However, the situation remains worrying in the Caspian Sea, where an estimated 17 tons of mercury and almost 150 tons of cadmium are dumped every year. In the seas of East Asia, the growing amount of often-toxic electronic waste is becoming a serious problem: It is estimated that up to 9 million batteries are dumped in these waters every year.
For more information:
Blue Ocean Institute
French Ministry of Health (in French)
Washington State Department of Health
Natural Resources Defense Council