In Vitro UVA testing of sunscreen products
The level of sun protection attributed to a sunscreen product has traditionally been estimated using the sun protection factor (SPF). The SPF test use the erythemal response of the skin to ultraviolet (UV) radiation. The erythemal response is an inflammatory response resulting in redness due to the dilation of superficial blood vessels to take blood to the exposure site. The SPF value is computed from the ratio of the minimum erythemal dose for protected skin to that of unprotected skin, tested on human volunteers using UV radiation from an artificial source.
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Due to the significant weighting of the erythemal action spectrum in the UVB, it follows that UVA protection offered by sunscreen products cannot be readily evaluated by this parameter, which led to a variety of alternative techniques, including the Boots star rating system. This “lack” has since been addressed by standards with the publication of ISO 24443:2012.
UVA exposure lacks a biological endpoint such as erythema, however it has been concluded that whilst pigmentation is not a marker for UVA-induced damage as is erythema for UVB induced damage, there is a relationship between biological damage to the skin and UVA-protection as assessed in the persistent pigment darkening (PPD) testing procedure. The PPD, the persistent part of the immediate pigmentation observed 2-4h post exposure.
In order to determine relevant UVA protection parameters, ISO 24443:2012 describes a method to provide a UV spectral absorbance curve from which a number of calculations and evaluations can be undertaken, including ultraviolet A protection factor (UVAPF), critical wavelength and UVA: UVB ratio. This method relies on the use of in vivo SPF results for scaling the UV absorbance curve.
The UV spectral absorbance of a sunscreen sample, spread on a roughened polymethylmethacrylate (PMMA) substrate should be measured prior to and after UV exposure to account for product photoinstability over the spectral range 290-400nm in 1nm steps. The spectrophotometer should be arranged such that sample fluorescence be rejected and ensure that and that the measurement area be at least 0,5 cm2 with diffuse illumination or diffiuse collection of the transmitted irradiance through the PMMA substrate.
The UV exposure dose to be applied will be computed in the process of measurement.
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The spectral irradiance at the exposure plane of the UV exposure source should be within the following acceptance limits and have means of controlling sample temperature between 25 °C to 35 °C.
Total UV irradiance (290 nm to 400 nm) 40 W/m2 to 200 W/m2
Irradiance ratio of UVAa to UVBb 8:22
a 320 nm to 400 nm
b 290 nm to 320 nm
The solar irradiance generated by a UV solar simulator is most accurately characterised using a double-monochromator, with input optic diameter closely matched to the exposed area and calibrated with reference to NMI traceable calibration standards.