Photoprotection is a group of mechanisms that nature has developed to minimize the damage that the human body suffers when exposed to UV radiation. This damage mostly occurs in the skin, but the rest of the body (especially the testicles) can be affected by the oxidative stress caused 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 were not transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlet oxygen, or hydroxyl radical).
In DNA this photoprotective mechanism evolved four billion years ago at the dawn of life.[1] The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damage and indirect DNA damage. 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 that it dissipates more than 99.9% of the absorbed UV radiation as heat. [2] This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.
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. [3]
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)
| UV-absorber | other names | percentage of molecules that dissipate the photon energy (quantum yield: Φ ) [3] |
molecules not dissipating the energy quickly |
|---|---|---|---|
| DNA | > 99.9 % | < 0.1 % | |
| natural melanin | > 99.9 % | < 0.1 % | |
| 2-phenylbenzimidazole-5-sulfonic acid | PBSA, Eusolex 232, Parsol HS, | ||
| 2-ethylhexyl 4-dimethylaminobenzoate | Padimate-O, oxtyldimethyl PABA, OD-PABA | 0.1 = 10% | 90% |
| 4-Methylbenzylidene camphor | (4-MBC), (MBC), Parsol 5000, Eusolex 6300 | 0.3 = 30% | 70% |
| 4-tert-butyl-4-methoxydibenzoyl-methane | (BM-DBM), Avobenzone, Parsol 1789, Eusolex 9020 | ||
| Menthyl Anthranilate | (MA), Methyl-2-aminobenzoate, meradimate | 0.6 = 60% | 40% |
| Ethylhexyl methoxycinnamate | (2-EHMC), (EHMC), EMC, Octyl methoxycinnamate, OMC, Eusolex 2292, Parsol | 0.81 = 81% | 19% |