Change of ultraviolet absorbance of sunscreens by exposure to solar-simulated radiation

Regarding the outdoor behavior of the Caucasian population, modern sunscree ns should provide high and broad-spectrum ultraviolet protection in the ult raviolet B as well as in the ultraviolet A range and should be photochemica lly stable for ultraviolet doses, which can be expected in solar radiation. At present an assessment of the photostability of suncare products is not a general requirement before marketing. In order to evaluate the photostabi lity of suncare products we conducted an in vitro test and measured the spe ctral absorbance of 16 sunscreens before, and after exposure to increasing biologically weighted standard erythema doses (5, 12.5, 25, 50) of solar-si mulated radiation. Seven of 16 suncare products showed a significant dose- and wavelength-dependent decrease of the ultraviolet A protective capacity, whereas the ability to absorb ultraviolet B was not affected. In the ultra violet A range, the decrease of absorbance (photoinactivation), respectivel y, the increase of transmission was 12-48% for an ultraviolet exposure of 2 5 standard erythema dose. Photoinactivation started in the wavelength range between 320 and 335 nm with a maximum above 350 nm. Furthermore, our analy sis showed that the behavior of suncare products was not predictable from i ts individual ingredients. Neither complex combinations of organic filters nor addition of inorganic filters could absolutely prevent photoinactivatio n. The inclusion of a single photounstable filter did not mean photoinstabi lity of the complete suncare product. Photoinactivation of sunscreens appea rs to be an underestimated hazard to the skin, first, by formation of free radicals, second, by increased ultraviolet A transmission.