A photocarcinogen is a substance which causes cancer following illumination. 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 a photocarcinogen, because it dissipates the excitation energy into small amounts of heat (see photoprotection). Oxybenzone (a component of some sunscreens) is suspected owing to its skin penetrating qualities and its production of free radicals.
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