A carcinogen is any substance or factor which can cause carcinogenesis or the development of cancer. Such substances damage or disrupt the healthy metabolic processes of cells which lead to a number of DNA mutations. Carcinogens are classified as chemicals or substances (asbestos, alcohol, tobacco, etc.), radiation (ultraviolet, radon, even radiation therapy, etc.), viruses (human papillomavirus, Merkel cell polyomavirus, etc.), medications (chemotherapy drugs, oral contraceptives, etc.), lifestyle factors (obesity, smoking, drinking, etc.), and pollutants found in both outdoor and indoor environments.