Chinese carcinogens can be classified into natural carcinogens and synthetic carcinogens according to their sources. Most of the chemical carcinogens need to be metabolized and activated in the body (mainly in the liver) before they can cause cancer, which is called indirect carcinogens. A small number of chemical carcinogens can cause cancer without metabolic transformation in the body, which are called direct carcinogens. Physical carcinogens mainly include ultraviolet light and ionizing radiation (including X-rays and various forms of particle radiation, etc.). Biological carcinogens include viruses, bacteria, molds, parasites, etc. Which are the most dangerous among the carcinogens? The International Agency for Research on Cancer (IARC) classifies common carcinogens into 4 classes according to their degree of danger: Class 1: They are the most dangerous substances that are clearly carcinogenic to human body, such as aflatoxin, tobacco, formaldehyde, alcoholic beverages, arsenic, asbestos, hexavalent chromium, dioxin, betel nut, Helicobacter pylori, etc. Category 2: It can be further divided into two types, A and B. Category 2A refers to carcinogenicity to humans although theoretically, but with limited evidence, such as acrylamide, inorganic lead compounds, chloramphenicol, etc. Category 2B refers to carcinogenicity evidence found in animal experiments is not sufficient, and evidence of carcinogenicity to humans is limited, such as chloroform, DDT, dichlorvos, naphthalene sanitary bulbs, nickel metal, nitrobenzene, diesel fuel, gasoline, etc. Category 3: It means that the evidence of carcinogenicity to human is not sufficient and the evidence of carcinogenicity to animals is not sufficient or limited; or although there is sufficient experimental evidence and sufficient theoretical mechanism to show that it is carcinogenic to animals, it is not equally carcinogenic to human. Such as aniline, Sudan red, caffeine, xylene, saccharin and its salts, Valium, iron oxide, organic lead compounds, static electromagnetic fields, melamine, mercury with its inorganic compounds, etc. Category 4: These are substances that may not be carcinogenic to humans and lack sufficient evidence to support their carcinogenicity, such as caprolactam.