ISO 24443 Determination Of Sunscreen UVA Photoprotection in vitro

The test is based on the assessment of UV transmittance through a film of sunscreen sample spread on a roughened substrate, before and after exposure to a controlled dose of radiation from a defined UV exposure source.  Because of variables that cannot be controlled with typical thin film spectroscopic techniques, each set of sunscreen transmission data is mathematically adjusted so that the in vitro SPF data yields the same measured in vivo SPF value that was determined by in vivo testing.  Samples are then exposed to a specific measured dose of UV radiation to account for the photostability characteristics of the test product.  The resulting spectral absorbance data have been shown to be a useful representation of both the width and height of the UVA protection characteristics of the sunscreen product being tested. The mathematical modelling procedure has been empirically derived to correlate with human in vivo (persistent pigment darkening) test results.

Procedures:

A glycerine treated “control blank” HD2 plate shall be placed in the UV-2000S Transmittance Analyser and used to take a blank scan.

The test material will be applied to a new untreated roughened HD2 plate, at a rate of 1.3 mg/cm2 in a series of small dots over the surface using a positive displacement pipette and will then be spread evenly over the surface area (25 cm2) using a fingertip (without finger cot). To ensure an accurate rate of application, the pipette will be weighed without test material and then again with test material.  The test material treated plate will then be allowed to equilibrate in the dark for 30 minutes at ambient conditions.

Transmission measurements through the product treated plate will then be taken using a Labsphere UV-2000S Ultraviolet Transmittance Analyser. The plate will be measured at nine locations which will be regulated by a custom template.

The test material will then be irradiated at a fixed rate determined by the UV 2000S software using an Atlas Suntest CPS+.

After the irradiation step, the treated test material will then be measured for post-exposure readings using the UV 2000S Transmittance Analyser. The locations shall be as close as possible to the previous sites in order to reduce any variability in measurement due to inhomogeneity of sunscreen distribution across the plate.

The Labsphere computer system performs all the mathematical calculations necessary to produce the UVA Protection Factor.

This process shall then be repeated to produce three further test material treated plates (4 in total) and to establish the protection aspects of the test material.  Additional plates shall be added to the sampling if the 95 % confidence interval (CI) is greater than 17% of the mean value of the UVAPF value, until the 95 % CI is less than 17% of the mean UVAPF value.