# Potential Metal Contamination in Foods of Animal Origin--Food Safety Aspects
## 2 Major Toxic Metals in Food
### Mercury (Hg)  
The average mercury content of the Earth's crust is 80 \(\upmu\)g/kg, but its actual concentration can vary significantly from area to area. In shale soil, for example, it can reach 10 mg/kg. Important natural sources of mercury are volcanic activity and ocean evaporation. However, human activities also release significant amounts into the environment through the burning of fossil fuels, steel and cement production, metal processing, and gold and mercury mining. Mercury released into the air spreads widely and can persist for years. The concentration of dissolved mercury in aquatic environments varies as follows: 0.5-3 ng/L in oceans, rivers, and lakes and 2-15 ng/L in coastal waters [111]. Inorganic mercury in natural waters and soils is strongly bound to sediment or organic matter. In this form, living organisms cannot absorb it. However, in aquatic environments, bacteria living in sediment methylate inorganic compounds, and the resulting methylmercury, due to its strong lipophilic properties, enters the food web. The best-known example of this is Minamata disease. Since methylmercury accumulates in animals faster than it is eliminated, even at low levels of environmental contamination, bioaccumulation is a possibility, leading to potentially dangerous concentrations in fish, fish-eating wildlife, and humans. The highest mercury levels are found in long-lived predatory fish living in the oceans (e.g., swordfish [_Xiphias gladius_] and shark species [_Selachimorpha_]) and in freshwater pike and perch. In fish, mercury accumulation increases with age and size. It is found in the highest concentrations in the kidney, spleen, and liver and, to a lesser extent, in the gills, reproductive organs, brain, and muscle [112; 113]. Mercury biomagnifies and thus causes toxic effects in consumers at the top of the aquatic food chain. The bioconcentration factor is 1000-8000 in algae, \(<\)0.1 in plants, \(>\)1000 in invertebrates and fish, and 2 in birds [111]. Mercury concentrations in food are generally below the detection limit (20 ng/g), but they can be significant in aquatic organisms (fish, marine mammals). Mercury levels in the edible tissues of various fish species range from 50 to 1400 \(\upmu\)g/kg, but in polluted environments, they can reach 10 mg/kg [114]. The liver is the largest source of mercury, followed by the kidney and then the muscle. In skeletal muscle, methylmercury is found primarily (70-90%) bound to cysteine or in chemically related compounds, while in the liver, it forms a complex with selenium [115]. Mercury levels are relatively low in fish at the lower levels of the food chain (e.g., anchovy, herring) and in foods made from them, while mercury levels in products made from the offal of fish at higher levels (e.g., shark, tilefish) are 1-2.4 mg/kg, and in whales, it can be as high as 10 mg/kg [109]. Generally, low total mercury content has been detected in the flesh of aquatic animals (marine and freshwater fish species, shellfish species, oysters, squids) [23; 24; 25; 26; 27; 28; 29]. The mercury concentrations measured in the muscle of roe deer (_Capreolus capreolus_) were 0.24-1.46 mg/kg w.w. [32]. Basically, mercury has a very limited solubility in soil and has a low uptake capacity for plants. Generally, the main source of mercury in plant leaves is air pollution with Hg\({}^{0}\), not soil pollution. Different plants, e.g., mushroom, rice, and wild plants, have been identified as organisms that accumulate more mercury than other plants. Certainly, these plants can contribute to the mercury burden of human and animal bodies after their consumption [116; 117; 118]. Mercury is not an essential compound for the body. However, according to the literature data, at low doses, it has been shown to enhance growth in rodents, pigs, and chickens, but this has not been demonstrated in fish [119]. Its absorption depends largely on its chemical form. Elemental mercury is absorbed from the gastrointestinal tract in humans and animals at a rate of about 0.01%.