I. Definition of Heavy Metals 1. General Definition: Metals with a density of 4.5 g/cm3 or more are called heavy metals. From the sense of density, there are 54 kinds of metals are heavy metals. 2, industrial definition: in the industry is really classified as heavy metals for the 10 kinds of metal elements: copper, lead, zinc, tin, nickel, cobalt, antimony, mercury, cadmium and bismuth. 3, food nutrition and hygiene definition: nutrition to copper, zinc, nickel, cobalt classified as trace elements. Food hygiene often lead, mercury, cadmium and arsenic defined as toxic metals. Usually close to the health of toxic metals mainly refers to lead, mercury, cadmium and arsenic. Second, the pathway of pollution 1, industrial waste containing metal toxic industrial waste discharged into the environment, can be directly or indirectly contaminated food, and contaminated water and soil metal toxins, but also through the role of bio-enrichment, so that the content of its food in a significant increase. 2, food production and processing pollution production and processing contact does not meet the hygiene requirements of mechanical equipment, containers or packaging materials, under certain conditions, its harmful metals can be dissolved and contaminated food. In the process of food transportation, due to contamination of the means of transportation, can also contaminate food. 3, pesticides and food additives contamination of certain pesticides, such as organic mercury, organic arsenic, etc., or pesticide impurity, containing metal impurities, in the use of the process can contaminate food. Food in the production process, the use of food additives containing metal impurities, can also cause contamination of food. 4, the natural environment of toxic elements in the background content of high elemental content of living organisms and the survival of the air, soil, water, the content of these elements was significantly positive correlation Third, the pollution of food by toxic metals 1, mercury mercury and its compounds are widely used in industrial and agricultural production and health care industry, can be through the wastewater, waste gas, waste residue and other pathways to contaminate food. In addition, the use of organic mercury seed mixing, or in the growing period of crops applying organic mercury pesticides can contaminate crops. Mercury into the human body mainly from contaminated food, aquatic products, especially fish, shrimp, shellfish. 2, cadmium cadmium-containing industrial wastewater contamination of water bodies, so that the cadmium content of aquatic products increased significantly, the absorption of crops and cadmium residues in food increased, with cadmium-containing metal as a container for storage of acidic foods or beverages when a large number of cadmium dissolved, resulting in serious contamination of food. Seafood, animal food (especially kidneys) is higher than plant foods, and plant foods to cereals, roots and tubers, legumes content is higher. 3, lead food processing machinery and equipment, food containers and packaging materials, food additives can cause lead contamination of food; ceramic tableware glaze, tin cans and boxes of tin plating lead; printing food packaging materials, inks, pigments, children’s toys, paint is also a source of lead can also contaminate food. 4, arsenic use of arsenic-containing pesticides in excess or the use of crops can lead to a significant increase in arsenic content. The toxicity of arsenic in food is related to the form of its existence, food arsenic inorganic arsenic and inorganic arsenic two categories. In general, the toxicity of inorganic arsenic is greater than organic arsenic; trivalent arsenic is more toxic than pentavalent arsenic. Prevention and control measures 1, understand the common sources of contamination of toxic metal contamination of food, due to the long residual period, try to avoid choosing food that is easily contaminated by the source of contamination, do not use food additives that do not meet the hygiene standards, containers and packaging materials and so on. 2. Understand the maximum permissible limits of toxic metal elements in various types of food, and strengthen the ability to choose food. 3. Prevent the accidental consumption and misuse of contaminated food and food containers.