Substance that causes or aggravates tumors or other types of cancer. A substance is considered carcinogen if it is (1) found to be a carcinogen or potential carcinogen by the International Agency For Research On Cancer, (2) listed as carcinogen or potential carcinogen in the latest edition of Annual Report On Carcinogens published by the US National Toxicology Program or, (3) is regulated by the US Occupational Safety & Health Administration (OSHA) as a carcinogen. Carcinogens enter a body through inhalation (of asbestos fiber and cigarette smoke, for example), ingestion (of heavy metals and pesticides, for example), and absorption (of dyes and hydrocarbons, for example) through skin. Because the development of cancer is through irreversible cellular damage, no safe dose-limit is assumed for carcinogens. In comparison, toxic substances cause irreversible health damage only when exposure to them exceeds a certain dose limit. Most synthetic chemicals, especially industrial solvents and plastics, are either already-determined carcinogens or suspected carcinogens.