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How does melanin work?
UV radiation is dangerous because it can damage DNA in skin cells, potentially triggering skin cancer. The skin responds to UV exposure by increasing the production of the dark pigment melanin, which is secreted into the skin by a population of cells called melanocytes. Melanin absorbs UV very efficiently, preventing further UV radiation from reaching the nucleated (DNA-containing) cells deeper in the skin, and hence preventing DNA damage.
The photochemical properties of melanin make it an excellent photoprotectant. It absorbs harmful UV-radiation and transforms the energy into harmless amounts of heat through a process called "ultrafast internal conversion". This property enables melanin to dissipate more than 99.9% of the absorbed UV radiation as heat and it keeps the generation of free radicals at a minimum (see photoprotection). This prevents the indirect DNA damage which is responsible for the formation of malignant melanoma.