In Vivo SPF 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.
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 technique, 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.
Click here for our applications note on In Vitro UVA testing of sunscreen products
Click here for our measurement guide of Erythemal Dose
In the determination of SPF, a filtered xenon arc lamp solar simulator (or equivalent) having defined spectral distribution and of known and variable irradiance should be used to expose a number of small sites on volunteers' skin (phototypes I, II, or III ) between the waist and shoulder line in incremental erythemal doses. Areas exposed include an area of unprotected skin, area(s) of skin protected by the sunscreen(s) under test and an area of skin protected by an SPF reference formulation. By incrementally increasing the UV dose, varying degrees of skin erythema are generated. The delayed erythemal responses are visually assessed for redness 16 to 24 hours after UV radiation, by the judgement of a trained evaluator to determine the MED for unprotected skin (MEDu) and protected skin (MEDp). An individual sun protection factor (SPFi) is calculated as the ratio of MEDpi/MEDui, the sun protection factor for the product (SPF) is the arithmetic mean of all valid SPFi results (10-20 volunteers) from each and every subject in the test and should be expressed to one decimal place.
The UV solar simulator shall be temporally stable, uniform across the exposure site, and emit a continuous spectrum. The source should be filtered to create a spectral distribution that complies with the required acceptance limits below and ensure appropriate amounts of UVA radiation. The source spectral specification is described in terms of cumulative erythemal effectiveness by successive wavelength bands from 290 nm up to 400 nm. The erythemal effectiveness of each wavelength band is expressed as a percentage of the total erythemal effectiveness from <290 to 400 nm, or as the Relative Cumulative Erythemal Effectiveness (%RCEE).
%RCEE acceptance limits for the UV solar simulator output
|Spectral Range (nm)||Permitted range %RCEE|
UVA Irradiance Requirements
|Region||Spectral Range (nm)||Percentage total UV Irradiance (290-400nm) (%)|
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.