Any substance or agent that can cause cells to become cancerous or that increases cancer risk.
From Greek 'karkinos' (crab, used metaphorically for cancer) combined with '-gen' (something that produces or causes). The term was coined in the late 1800s as scientists began understanding what triggers cancer development.
Tobacco, asbestos, and UV rays are carcinogens because they damage DNA in ways that make cells ignore their 'stop dividing' signals—essentially hacking the cell's safety instructions.
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