Carcinogen

A carcinogen is any substance, energy form, or exposure that increases the risk of cancer by inducing cellular changes that lead to malignant transformation. Carcinogens can act through multiple mechanisms, including direct damage to DNA, induction of mutations in oncogenes or tumor suppressor genes, chromosomal alterations, and epigenetic changes that alter gene expression. Some agents require metabolic activation to become carcinogenic, while others act directly. Carcinogenic effects may involve initiation, promotion, and progression phases of carcinogenesis, and the resulting cancer risk depends on the dose, duration, and context of exposure, as well as individual susceptibility.

Carcinogens are typically classified by international agencies such as the International Agency for Research on Cancer (IARC) into groups reflecting the strength of evidence for human carcinogenicity: Group 1 includes agents with sufficient evidence of carcinogenicity in humans; Group 2A and 2B include probably or possibly carcinogenic agents; Group 3 consists of agents that cannot be classified due to insufficient evidence; Group 4 contains agents that are probably not carcinogenic to humans. Examples commonly cited as Group 1 carcinogens include tobacco smoke, asbestos, ultraviolet radiation, certain chemicals such as formaldehyde, aflatoxins, and some ionizing radiations. Other agents are classified in Groups 2A or 2B depending on the strength and consistency of supporting evidence.

Assessment and management of carcinogenic risk involve epidemiological studies, animal testing, exposure assessment, and regulatory measures to limit exposure, monitor workplace hazards, and inform public health guidance. While exposure to a carcinogen can elevate cancer risk, the magnitude of risk depends on exposure level, duration, and individual factors, making risk assessment a nuanced, context-dependent process.