Mercury is a silvery room-temperature heavy metal and a chemical element. Mercury is found in natural ores deposits and manufactured devices such as barometers, thermometers, dry-cell batteries, switches, fluorescent light bulbs, and other various electronics. Different forms of mercury have different toxicity, in which inorganic mercury and elemental mercury generally do not accumulate in the body, while organic mercury tends to accumulate in the body. Long-term exposure to high concentrations of mercury vapor can cause lung damage and even death. Inorganic mercury is irritating to the skin, and chronic inorganic mercury poisoning mainly manifests as symptoms of kidney damage such as proteinuria and hematuria. Inorganic mercury in water can be converted into methylmercury through methylation reaction under the action of vitamins. Methylmercury is the most toxic form of mercury, has strong neurotoxicity, and can pass through the placental barrier, causing fetal brain damage and developmental damage. This form is rarely present in drinking water but is a very common contaminant in the tissues of fish and causes damage to the nervous system as well as teratogenesis. Both the CAC and the EU stipulate that the maximum limit of methylmercury in carnivorous fish is 1 mg/kg, and other fishes must not exceed 0.5 mg/kg.