-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 few 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. What are the most dangerous carcinogens among them? The International Agency for Research on Cancer (IARC) classifies common carcinogens into 4 classes according to their degree of danger: -Class 1: The most dangerous substances with clear carcinogenicity to human body, such as aflatoxin, tobacco, formaldehyde, alcoholic beverages, arsenic, asbestos, hexavalent chromium, dioxin, betel nut, Helicobacter pylori, etc. -2 category: It can be further divided into two types, A and B. Among them, 2A category refers to carcinogenicity to human body although theoretically, but the evidence is limited, such as acrylamide, inorganic lead compounds, chloramphenicol, etc.; 2B category refers to the evidence of carcinogenicity found in animal experiments is not yet sufficient, and the evidence of carcinogenicity to human body is limited, such as chloroform, DDT, dichlorvos, naphthalene sanitary ball, nickel metal, nitrobenzene, diesel fuel, gasoline, etc. . -3 categories: It means that the evidence of carcinogenicity to human is not sufficient, and the evidence of carcinogenicity to animals is also not sufficient or limited; or although there is sufficient experimental evidence and sufficient theoretical mechanism to show that it is carcinogenic to animals, but 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. -4 categories: These are substances that may not be carcinogenic to humans and lack sufficient evidence to support their carcinogenicity, such as caprolactam.