In Vitro Assessment of Sunscreen Photostability: The Effect of Radiation Source, Sunscreen Application Thickness and Substrate

Abstract

The photostabilities of four sunscreen products have been assessed in vitro by applying sunscreen to a substrate and measuring the spectral transmission prior to, and after exposure to a source of ultraviolet (UV) radiation. Results were independent of whether an application thickness of 1 or 2 mg/cm ; 2 was used, and whether the UV source was natural sunlight or a xenon arc solar-simulator. There were significant differences, however, between results obtained on a roughened quartz substrate and those obtained on excised human epidermis. It is unlikely that any substrate will give an exact representation of the in vivo situation and, indeed, both quartz and excised human epidermis have disadvantages associated with their use. However, the ranking of the four products in terms of their photostability was the same for both substrates. This implies that transmission spectroscopy, with either a quartz or a human epidermis substrate, can be used successfully to compare the photostabilities of different sunscreen products.

Sunscreen tests: Correspondence between in vitro data and values reported by the manufacturers

“…In case of four exposure times, these irradiances guarantee exposition doses of 3, 9, 15 and 30 SED respectively. These doses are in accordance with values used by others authors [5]. From data provided by Lot Oriel, an output irradiance of about 2 kW/m 2 in the spectral range 400-2500 nm have been derived.…”

“…Studies performed using Excised Human Stratum Corneum and Excised Human Epidermis have been reported, but the biological nature of these materials introduces practical problems. The choice of the substrate is a very critical issue as sunscreen tests performed on different substrates are not always reproducible [4][5][6], especially when the substrate/sunscreen is irradiated before or during the tests. Moreover, when a particular substrate is used other parameters can affect the final in vitro SPF value.…”

Effectiveness of different substrate materials for in vitro sunscreen tests

“…The resulting curve gives two important attributes: the amplitude of the transmittance curve that reflects the degree of protection and the shape of the curve that underlines the capability of the sunscreen to protect the different spectral regions. As already demonstrated [5][6][7], the use of a deficient instrumentation can be responsible of discrepancies in absorbance measurements and, hence, in SPF evaluation. Moreover, to perform test on high SPF products, a spectrophotometer with a sufficient dynamical range is essential.…”

“…With regard to the technique and the amount of sunscreen application, measurements performed on a single type of substrate changing the quantity of the product have been reported in many studies [7,11]. While in vivo test guidelines impose the uniform application of 2 mg/cm 2 of product quantity on the substrate, there is not a clear indication for in vitro SPF analyses.…”

Determination of the UV solar risk in Argentina with high-resolution maps calculated using TOMS ozone climatology

“…It can be chosen in the software and should be done by the dermatologist. However since patients usually do not apply the required quantity of sunscreen per each square centimeter of skin, especially on particular parts of the body (Stokes and Diffey, 1999), the software internally uses an ''effective SPF'', which is about 40% smaller than the nominal one.…”