A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis, the formation of cancer. This may be due to the ability of a substance to damage the genome or to alter cell metabolism. Substances that cause cancer are called carcinogens. A wide range of chemicals and physical agents have been implicated as causal in human cancers. These include industrially produced chemicals such as vinyl chloride and arsenic, naturally occurring substances such as aflatoxin (a food contaminant) and Aristolochic acid (found in some plants), medical treatments such as ionizing radiation and certain dyes, viruses such as hepatitis B virus and Human papilloma virus (HPV), and biological agents including Helicobacter pylori infection.
Carcinogenesis is caused by a complex series of events initiated by chemical or physical agents that interact with cellular macromolecules—primarily DNA—to promote changes at the genetic level that result in malignant transformation. The resulting transformed cells grow uncontrollably, invading surrounding tissues and metastasizing to distant sites in the body; both hallmarks of cancer. Not all agents that interact with DNA can induce cancer; indeed, many are beneficial, serving as diagnostic tools or therapeutic agents (e.g., X-rays used for imaging). It is generally accepted that there is no single “magic bullet” mechanism of action for all carcinogens; rather, each operates through one or more distinct mechanisms depending on its chemical structure