The recent European recommendation on the efficacy of sunscreen products requests now a minimum ratio of UVA/UVB protection. However, the visible and the infrared (IR) parts of the sun spectrum have received little attention concerning their possible contribution to skin damage. A common biophysical answer for the different wavelengths of the sun spectrum can be found in the creation of excess free radicals – mainly reactive oxygen species (ROS). Thanks to electron spin resonance spectroscopy applied to skin biopsies, we determined for the first time the free radical action spectrum covering UV and visible light (280–700 nm). Convolution of the action spectrum with sunlight spectral irradiance showed that 50% of the total skin oxidative burden was generated by visible light. Creation of ROS by visible light was experimentally confirmed by varying the illuminance of a spotlight. We also evidenced the creation of excess free radicals by near-IR radiation. In that case, free radical generation does not depend exclusively on the dose, but also on the skin temperature increase initiated by near-IR light. Some phenomena which are still unclear, such as the question about the deleterious or beneficial role of sunlight, are reviewed, implying the research on new protection strategies for the prevention of skin cancer.

The purpose of this study is to demonstrate the influence of plate roughness on well‐known absolute or relative UV spectroscopic indices such as the in vitro SPF, in vitro UVA PF (PPD), UVA/UVB ratio, critical wavelength and SPF/UVA PF (PPD) ratio. Firstly, a roughness index was measured by non‐contact surface topography analysis on different PMMA plates, which are available today on the European market. A large difference between maximum and minimum plate roughness values could be measured ranging from 1.88 to 6.76 μm. Then, the PMMA plates were grouped into five different roughness classes. To measure the different spectroscopic indices, the same sunscreen preparation was applied at the same amount rate (1 mg cm–2) to each of the selected plates. Large differences were noted for the absolute in vitro indices SPF and UVA PF (PPD), with the SPF varying in one instance from 13.5 to 40.7. On the other hand, the relative absorbance indices UVA/UVB ratio and critical wavelength showed a small but significant variation in relation to roughness. The SPF/UVA PF (PPD) ratio index based on the direct in vitro values was found to be very sensitive to roughness variation. However, adjusting the UV absorbance curve to reach equality between the in vitro SPF and in vivo SPF is a recognized and efficient mathematical process to limit the variation. All these results can be interpreted using the irregular sunscreen film theory. Mathematical developments can be achieved by modelling the height distribution with a gamma function.

The present work uses the initial step of the whole cascade of biological effects in the skin, the creation of free radicals by means of UVA/UVB radiation, to develop a total sun protection factor. Until now, existing in vivo indices have not been fully satisfying: SPF only reflects protection from UVB light, and persistent pigment darkening is restricted to the UVA part of the sun spectrum. The quantitative measurement of free radicals generated in human skin biopsies by means of electron spin resonance X-band spectroscopy allows to determine a new total SPF. This new sun protection index covers all UVA/UVB wavelengths taking into account their effects in the epidermis as well as the dermis. Use of skin biopsies avoids exposure of human volunteers to potentially harmful radiations. The new index is always practically equal or lower than the in vivo SPF depending on the level of a product’s UVA/UVB photoprotection balance. With this, we propose to name this new protection index ‘integrated sun protection factor’.

There is a continuing need to measure and communicate reliably the UVA protection offered by commercial sunscreens. To that end, the COLIPA (European Cosmetics Trade Association) 'In Vitro Sun Protection Methods' group has developed a new in vitro method for measuring UVA protection in a standardized, reproducible manner. The method is based on in vitro UV substrate spectrophotometry and convolution of resulting absorbance data with the action spectrum for the in vivo Persistent Pigment Darkening (PPD) endpoint to provide an in vitro UVA protection factor (UVAPF) which is correlated with an in vivo measure. This method has been published as a COLIPA guideline, used currently in European geographies for testing and labelling sunscreen products. This article summarizes two 'ring' studies, involving eight separate testing laboratories, which both defined critical parameters for the method and validated it. In Ring Study 1, eight laboratories tested the in vitro UV transmission of a total of 24 sunscreens and, from the data, a unit dose of UVA (D(0) of 1.2 J cm(-2)) was defined to provide a single irradiation step which, by taking into account potential sunscreen photo-instability, gave the closest agreement with in vivo UVAPF values. In Ring Study 2, eight laboratories tested the in vitro UV transmission of a total of 13 sunscreens using this single irradiation step and established a very good correlation (r(2) = 0.83; slope = 0.84, P < 0.0001) between resulting in vitro UVAPF values and corresponding values derived from the in vivo PPD method. This new method, therefore, can be used to provide a reliable in vitro metric to describe and label UVA efficacy in sunscreen products, in line with the EU Commission recommendation 2006/247/EC.

Spectroscopic in vitro assay has recently gained importance for UVA protection, or even for SPF assessment, as manufacturers are increasingly keen to replace in vivo by in vitro tests. These assays are based on transmission UV spectroscopy of a known amount of sunscreen product spread onto an ultraviolet-transparent substrate. In the most recent European publications or methods, the preferred substrate is a roughened PMMA plate. We demonstrated in a previous work the importance of a strict control of plate roughness to ensure reproducibility and repeatability of in vitro results. In the present paper, we studied the effect of varying the thickness of applied sunscreen on low or high plate roughness. Two optimal sunscreen amounts, 0.75 mg cm(-2) for low roughness and 1.2 mg cm(-2) for high roughness, could achieve in vitro SPF close to in vivo SPF. The UV absorption spectra were measured and analyzed, via a mathematical model of irregular film geometry based on a well-known statistical distribution, the gamma law function. Our calculations show the relevance of the model to describe correctly the sunscreen film geometry evolution according to different in vitro spectra changes. Inversely, adjustment of the film geometry to fit with a set of in vivo SPF results allows us to extrapolate the real UV absorption spectra of sunscreens applied on the skin at 2 mg cm(-2). A high plate roughness, preferably a moulded plate for reproducibility, is required to obtain the closest agreement between in vitro and in vivo sunscreen absorption spectra over the entire UV waveband.