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Photostability And Its Effect on UV Protection

Suncare products should provide high, broad-spectrum protection, and should be photochemically stable for ultraviolet doses which can be expected in normal sun exposure. Currently, an assessment of the photostability of sunscreens is not a general requirement before marketing. Photostability refers to the ability of a molecule to remain intact with irradiation. Photostability is potentially a problem with all UV filters because they are deliberately selected to be UV radiation-absorbing molecules.

Studies of some sunscreen products have shown photodecomposition of various UV filters that result in a decrease in their absorptive (protective) capacity. Following exposure to UV radiation, some suncare products demonstrated a decrease in UV-A protection (photoinactivation / photoinstability), whereas the ability to absorb UV-B was not affected. Photoinactivation occurred in the UV-AII range and continued into the low UV-AI range. This photodegradation is dose-dependent and wavelength-dependent and can occur at low UV doses.

This photostability issue has been raised specifically with avobenzone (Parsol 1789); photolysis has been demonstrated in in vitro studies. The photoinstability of avobenzone may degrade other sunscreens that are mixed with it in a formulation; this has been observed with octyl Methoxycinnamate (OMC) and octyl dimethyl PABA (Padimate O). In comparison, oxybenzone was shown to be relatively stable when combined with avobenzone. Other studies have shown that certain ingredients may have a stabilizing effect on others; octocrylene has been shown to photostabilize avobenzone and is often used with it in many currently available suncare products.

Higher SPF sunscreen products have led to the use of multiple chemical UV filters, often used in combinations at maximum concentrations. If not properly formulated these individual chemical filters may interact in a way that actually decreases a product?s sun protection. Photoinactivation of UV filters may damage human skin by two mechanisms. First, the photolysis of photounstable UV filters may produce free radical intermediates which have toxic effects, or may bind to proteins or DNA altering function. Second, a decrease of the UV-A absorptive capacity of a UV filter results in an increase in UV-A-induced skin damage.

The assessment of photostability is a complex issue. The photostability of UV filters depends upon the solvent or the vehicle used during the study or in the sunscreen product. Photostability tests of individual UV filters do not take into account the possible interactions of ingredients within complex finished sunscreens. Studies have shown that the behavior of a complete product, following exposure to UV radiation, could not be predicted from the behavior of the individual filter substances of which it was composed. In addition, the presence of a single photounstable UV filter among the list of ingredients in a certain suncare product does not necessarily result in photoinstability of the complete product. These chemical interactions are often studied in vitro, and the relevance of these observations to the in vivo situation is not often clear. Significant research is required to improve our understanding in this area. Certainly, a required assessment of complete product photostability prior to marketing would be of additional benefit to consumers.


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