Carcinogen

Any agent capable of causing cancer. Chemicals are the largest group of carcinogens. Major types include polycyclic aromatic hydrocarbons (PAHs), which occur in tobacco smoke, pitch, tar fumes, and soot. Exposure to PAHs may lead to cancer of the respiratory system or skin. Certain aromatic amines used in the chemical and rubber industries may cause bladder cancer after prolonged exposure. The best-known physical carcinogen is high-energy radiation, such as nuclear radiation and X-rays. Exposure may cause cancerous changes in cells, especially in cells that divide quickly: for example, changes in the precursors of white blood cells in the bone marrow causes leukaemia. The risk depends on the dosage and duration of exposure. Over many years, exposure to ultraviolet radiation in sunlight can cause skin cancer. Another known physical carcinogen is asbestos (see asbestos-related diseases). Only a few biological agents are known to cause cancer in humans., one of the blood flukes responsible for schistosomiasis, can cause cancer of the bladder; and, a fungus that produces the poison aflatoxin in stored peanuts and grain, is believed to cause liver cancer. Viruses associated with cancer include strains of the human papilloma virus, which are linked to cancer of the cervix; the hepatitis B virus, which is linked to liver cancer; and a type of herpes virus which is associated with Kaposi's sarcoma.