There are hundreds of kinds of fish in the order Codiformes, which can be called "xx Cod" in a broad sense, but strictly speaking, the most authentic traditional true cod refers to three kinds of fish in the order Codiformes-Codidae-Cod, namely Atlantic Cod, Pacific Cod and Greenland Cod. Because only three kinds of cod can be called cod: Atlantic cod, Pacific cod and Greenland cod. The "deep sea cod" and silver cod we usually eat are not cod. Even if it is white, it is not suitable for frequent consumption, because its heavy metal mercury content is somewhat high. On the contrary, cod is a relatively safe daily aquatic product.

Heavy metal mercury is a kind of metal widely existing in the environment. Most people know that mercury in thermometers is a silvery white liquid at room temperature. However, this metal can also be combined with other substances in nature to form inorganic salts, or with organic substances to form methylmercury.

In the ocean and other water bodies, methylmercury is transformed into methylmercury by bacteria and plankton. Compared with inorganic mercury (without carbon element), methylmercury, as an organic mercury, is more toxic, and has high toxicity to human brain, kidney, liver and cardiovascular system.

The safe dose of mercury is also related to human body weight. The Expert Committee on Food Additives of the Food and Agriculture Organization of the United Nations/World Health Organization has set the safe intake of methylmercury at 0/.6 micrograms per kilogram of body weight per week. China has clear regulations on the limit of mercury in food. For example, the limit of pollutants in food (GB 2762-20 12) stipulates that the limit of methylmercury in aquatic animals and their products is 0.5 mg/kg, among which the limit of methylmercury in carnivorous fish and their products is 1.0 mg/kg, while the total limit of mercury in canned baby food is 0.02 mg/kg.