(blastomogens, cancerigens), chemical compounds capable of causing cancer and other malignant tumors, as well as benign neoplasms, when the body is exposed to them. Several hundred such substances, belonging to various classes of chemical compounds, are known. For example, certain polycy-clic hydrocarbons with a phenanthrene group in the molecule, azo dyes, aromatic amines, nitrosamines, and other alkylating compounds show marked carcinogenic activity. Carcinogens are found in certain industrial products, in air polluted by industrial waste, and in tobacco smoke.

The earliest notions of the existence of cancer-producing substances date from the 18th century, when cases of skin cancer in English chimney sweeps were connected with systematic contamination of the skin with coal tar and soot. In the early part of the 20th century skin cancer was induced in animals by smearing coal tar on their skin for a number of months. The carcinogens 3, 4-benzpyrene and other polycyclic hydrocarbons were subsequently isolated from the tar. Urinary bladder cancer frequently developed in workers in the aniline dye industry, who were exposed to the carcinogens β8-naphthylamine, benzidine, and 4-aminodiphenyl before the introduction of appropriate preventive measures. Lung cancer afflicts smokers more often than it does nonsmokers, and it afflicts those living in cities, where air pollution levels are high, more often than those living in rural areas.

The same carcinogen can give rise to tumors of different kinds and locations, depending on the site of application, and a given type of tumor can be caused by different substances. All carcinogens can be divided into three groups, according to the nature of their action: (1) locally active, (2) organotropic, that is, causing tumors not at the site of application but in certain organs, and (3) multiple, that is, producing different tumors in various organs.

The effect of carcinogens depends both on the amount of the substance and on the duration of its action; accumulation (deposition) in a tissue or organ intensifies the effect. Neoplasms do not develop immediately after exposure to a carcinogen. They do so only after a long period of time—one-fifth to one-seventh of the maximum lifetime of the given organism (this period might be 15–20 years for man and four to six months for mice). A neoplasm is preceded by precancerous changes.

The similarity of the chemical structure of carcinogenic hydrocarbons and many biologically active substances, such as sex hormones, biliary acids, and other steroids, suggested that the impairment of steroid metabolism could lead to carginogen formation in the body itself; this assumption was later confirmed experimentally. Such carcinogens, in addition to products of impaired steroid metabolism, include certain amino acid metabolites, such as tryptophan. Carcinogenic action is related to the chemical activity and electronic structure of the part of the carginogenic molecule responsible for the formation of complexes with certain cell components (apparently, nucleic acids and some proteins). It should be taken into account in considering the mechanisms of carcinogenesis that many carcinogens show pronounced mutagenic action.

Prevention of the action of carcinogens is based on a study of their distribution in man’s environment and on the use of prophylactic measures in occupational, public, and personal hygiene. It is important, therefore, to control pollution of the air, water, and soil by industrial wastes and to keep carcinogenic impurities out of food and drinking water. An effective way to prevent tumors is to discover which compounds have carcinogenic properties and remove them from the sphere of man’s activities.