There's mercury on my plate! Not a nice idea, but reality: In fact, most foods contain a percentage of the toxic heavy metal. One must be careful when considering certain food.
Where does the mercury in food come from?
Elemental mercury is released, for example, during the decomposition of gold and silver. It is part of various everyday objects, for example batteries and fluorescent lamps, and also found in dental fillings and pharmaceuticals. However, most mercury is released during the combustion of coal. It first enters the atmosphere and settles on the ground. After about a year, it is converted into soluble methyl mercury by bacteria. This soluable methyl mercury accumulates in the earth and in the water, where it is absorbed by plants and animals. The higher up on the food chain a living thing is, the more mercury is accumulated within it. Therefore, predatory fish, such as shark and tuna, can carry particularly large amounts of heavy metal.
Mercury on a Plate: Why is this a problem?
Mercury is poisonous when ingested, because once ingested by the body, it destroys tissue, specifically affecting the nervous system, the kidneys and the cardiovascular system. Mercury poisoning manifests itself in a variety of symptoms, including headache, nausea, dizziness and kidney failure. It can cause irreparable damage.
Mercury is also problematic because it is able to pass both the blood-brain barrier and the placental barrier and can damage a fetus in the womb. At worst, it can attack the central nervous system of an unborn child, which could later lead to neurological abnormalities that affect (for example) speaking and walking. Another consequence is paralysis. Furthermore, mercury can be passed to a baby through the mother's milk, which is why expectant mothers who have ingested heavily contaminated food should not breastfeed.
Keep in mind: The mercury levels that we absorb from our food are not life-threatening as the quantities are too small. However, huge amounts of food contamination can lead to life-threatening levels. For example, in the 1950's, in the Japanese city of Minamata in the Kumamoto prefecture, large quantities of mercury was dumped into the sea. Mercury contaminated the algae, fish, shellfish and the drinking water, and thus entered the human food chain. The resultin mercury poisoning claimed about 3,000 fatalities in addition to 17,000 seriously debilitated. Since then, chronic mercury poisoning has also been called Minamata disease.
How much mercury can a human being tolerate?
The recommended doses (TWI - Tolerable weekly intake, sometimes broken down to the daily dose) were adopted by the European Food Safety Authority and have been revised downwards over the past decades. Currently, the agency does not recommend consuming more than 1.6 micrograms per kilogram of body weight in one week. Pregnant women should consume even less, due to the fact that long-term consequences for a fetus remain undetermined. In addition, women who breastfeed as well as children should refrain from eating heavily contaminated food.
How high are the mercury levels in individual foods?
Since many factors play a role, the mercury content of individual foods varies. Fruits, vegetables, cereals and mushrooms are said to have low amounts. The same applies to dairy products as well as meat. However, in many fish and seafood, mercury accumulates more easily. Extremely high concentrations are found in swordfish, shark and butter mackerl. They all contain over 50 micrograms per 100 grams, shark even around 90 micrograms. Despite this, there are also fish that are considered safe in terms of mercury levels. These include salmon, cod and trout with a maximum of only about 5 micrograms of mercury per 100 grams.