Mercury poisoning has only been prominent news in the last few years, but evidence of the negative effects of high-level mercury in the body have been around for more than 50 years. In the mid 1950s, a Japanese chemical factory near the village of Minamata dumped 27 tons of mercury into the water over the course of decades. Eventually the villagers began experiencing nerve damage effects such as dizziness and numbness. Babies were being born with severe birth defects. Local wildlife was becoming ill and dying. Fish like tuna were dietary staples and were contaminated with mercury.
Mercury is a common byproduct of the burning of fossil fuels, which is the major contributor of mercury into the air. When it falls into water, it is transformed by bacteria into methylmercury, which is its most toxic form. It accumulates in the tissue of fish, especially in ones higher up in the food chain such as sharks, tuna and swordfish, which absorb the mercury found in prey fish.
According to the Environmental Protection Agency (EPA), mercury poisoning results in lack of coordination of movement, peripheral vision impairment, speech impairment, muscle weakness, memory loss, mood swings, mental disturbance along with the impairment of sensation experienced by the villagers of Minamata. The effects tend to increase over time as your body stores more and more mercury. Foetuses and infants, as well as children, can suffer neurological impairment and adverse effects to the brain and nervous system. Mercury is passed to a foetus and infant by the mother and her breastmilk.
Mercury and Fish
Regulatory limits on mercury air pollution can help to minimise the risk of mercury poisoning, but most of the mercury consumed by fish has accumulated and will not disappear. The highest levels of mercury are found in fish such as tuna. The varieties of tuna used for sushi and steaks, like yellowfin, are the largest of the species and are the most contaminated. They frequently contain levels higher than the Food and Drug Administration (FDA) danger level of one part per million. They should be avoided by pregnant or breast-feeding women or eaten on occasion by everyone else.
Smaller species such as skipjack and albacore, which are used for canned tuna, are less contaminated. The Center for Science in the Public Interest (CSPI) has stated that up to one can of albacore or two canned of light tuna per week is safe for most people, though men and women past menopause can safely consume from two to three times this amount.
For those who are trying to consume more fish as part of a healthy diet, choices such as salmon, trout, catfish, flounder shrimp and crab are all low-mercury options. You can also take fish oil supplements in order to get the essential fatty acid benefits of eating fish while avoiding the mercury threat altogether.