The likelihood of sunburn in sunscreen users is disproportionate to the SPF.

BACKGROUND/OBJECTIVE: Sunburn is a common feature in sunscreen users. The purpose of this paper is to estimate the expected frequency and magnitude of sunburn resulting from typical use of sunscreens labelled SPF15 and SPF30 by people spending long periods outdoors in strong summer sunshine. METHODS: By combining the probability distribution of the measured sun protection factor (SPF) in vivo with those for the average application thickness and the uniformity of application over the skin surface, a simulation model was developed to estimate the variation in delivered protection over the exposed skin surface from consumer use of sunscreens. RESULTS: While either sunscreen, if delivering the nominal SPF over the entire exposed skin, would be sufficient to prevent any erythema, the simulation indicates that the combination of the average quantity applied with the variability in thickness over the skin surface will lead to erythema, especially in SPF15 sunscreen users. CONCLUSION: People who intend spending long periods outside in strong sunshine would be better advised to use SPF30 labelled sunscreens than SPF15 sunscreens, and to apply the product carefully over exposed skin if they wish to minimize their risk of sunburn and, by implication, skin cancer.

How a calculated model of sunscreen film geometry can explain in vitro and in vivo SPF variation.

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.

Investigation of the homogeneity of the distribution of sunscreen formulations on the human skin: characterization and comparison of two different methods.

The efficacy of sun protection, mostly realized by the application of sunscreen formulations, is commonly described by the sun protection factor (SPF). Previous investigations have shown that the efficacy of the sun protection inter alia depends on the homogeneity of the distribution of the topically applied sunscreen formulation on the human skin. Therefore, suitable methods are required to determine the homogeneity of topically applied substances on the skin surface. This study provides and compares two different methods, which enable this determination. Laser scanning microscopy allows the analysis of tape strips removed from skin treated with a sunscreen. These reflect the inhomogeneous distribution on the skin that can complementary be determined directly, utilizing a dermatological laser scanning microscope. For the second method, a chromatic confocal setup was utilized, which enables the study of the microtopography of skin replicas before and after the application of a sunscreen product. The two methods were applied for the evaluation of three different sunscreen formulations for each method. A correlation of the homogeneity of distribution with the in vivo SPF could be confirmed. Both methods are suitable to investigate the homogeneity of the tested sunscreen formulations, although they provide different advantages and disadvantages.