Photocarcinogen

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Photocarcinogenic substances are those which have a destructive effect on DNA when they are illuminated. This destructive effect often results from free radicals generated by the photocarcinogen. Many chemicals that are not carcinogenic can be photocarcinogenic. This can easily be understood from a photochemical perspective: The reactivity of a chemical substance itself might be low, but after illumination it transfers to the excited state. This excited state is chemically much more reactive and therefore potentially harmful to biological tissue.

Melanin is not photocarcinogen, because it dissipates the excitation energy into small amounts of heat. (see photoprotection)