any of a number of agents that can cause cancer, including chemicals, radiation, and viruses. Exposure to such agents, singly or in combination, can initiate cancer under conditions not wholly understood.

Chemical effluents from industry; environmental pollutants from automobiles, residences, and factories; and tobacco smoke bear carcinogens in varying degrees depending upon geographical location and climate. Physical carcinogens include ultraviolet rays from sunlight and ionizing radiation from X-rays and from radioactive materials in industry and in the general environment. A number of viruses are suspected of causing cancer in animals, including man, and are frequently referred to as oncogenic viruses (see oncogenic virus). Repeated local injury or recurring irritation to a part of the body can be carcinogenic.

Some—not all—cancers are heritable in the sense that a predisposition exists, awaiting a convergence of carcinogenic influences for cancer to manifest itself. The identification and timely elimination of carcinogens can reduce the incidence of cancer.