In vivo determination of the Infrared-A protection factor on human skin

Chronic exposure to infrared A (IR-A) irradiation causes photoaging. However, daily or acute exposure to IR-A rarely induces erythema or pigmentation. Thus, evaluation of the physiological changes taking place on the skin surface is insufficient for clinical investigations.

We fabricated a novel device to obtain the IR-A protection factor (IPF) on human skin. This device consists of an artificial light source that mimics the actual IR-A intensity of sunlight, and a spectrophotometer to measure the spectral reflectance on the skin surface. The IPF can be determined by measuring the difference in spectral reflectance on the skin before and after the use of products and can be verified by the statistical criterion. A validation study was performed using different light intensities and two experimenters. Finally, we monitored the IPF on 12 commercial cosmetics.

After considering the IPF and L*-values, we selected the optimal sample and performed a validation study. Neither the intensity of IR-A irradiation or the experimenters significantly affected the IPF. 12 commercial products exhibited their own IPF values and were verified by statistical criteria, with one exception.

The present IPF evaluation method was concluded to be robust and reliable. This method is simple and safe for the subjects, and could be helpful for the development of IR-A protection products and the confirmation of product performances.