Photoprotection

The term Photoprotection designates the mechanisms that nature has developed to minimize the damages that the human body suffers when exposed to UV-irradiation. These damages are mostly occurring in the skin, but the rest of the body (especially the testicles) can be affected by the oxidative stress that is produced by UV-light. Photoprotection of the human skin is achieved by extremely efficient internal conversion of DNA, proteins and melanin. Internal conversion is a photochemical process that converts the energy of the UV-photon into small amounts of heat. This small amount of heat is harmless. If the energy of the UV-photon would not be transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlett oxygen, or hydroxyl radical). In DNA this photoprotective mechanism has evolved four billion years ago at the dawn of life. The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damages and indirect DNA damages. The ultrafast internal conversion of DNA reduces the excited state lifetime of DNA to only a few femtoseconds (10-15s) — this way the excited DNA has not enough time to react with other molecules.

For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance, because it dissipates more than 99.9% of the absorbed UV radiation as heat . This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.

artificial melanin
The cosmetic industry claims that the UV-filter acts as a "artificial melanin". But those artificial substances used in sunscreens do not efficiently dissipate the energy of the UV-photon as heat. Instead these substances have a very long excited state lifetime. In fact, the substances used in sunscreens are often uses as photosensitizer in chemical reactions. (see Benzophenone).

This discrepancy between melanin and sunscreen ingredients is one of the reasons for the increased melanoma risk that can be found in sunscreen-users compared to non-users. (see sunscreen)