How to Measure UVA Protection Afforded by Sunscreen Products

Dominique Moyal

Disclosures

Expert Rev Dermatol. 2008;3(3):307-313. 

In This Article

PPD Method

The PPD skin response has been selected for the development of a standardized protocol[14,15] because it fulfills the different requirements for selecting a method. The PPD reaction appears to be stable for times greater than 240 min, depending on UVA dose applied/received, classically from 10 to 25 J/cm2 for an untreated skin site, to determine the minimal pigmenting dose. Since PPD response requires doses greater than 10 J/cm2 (approximately 40 min of midday summer sunlight), the photostability of sunscreens is also challenged during a PPD test procedure.

The sunscreen testing protocol developed using this end point is similar to the design of the SPF test. The major points of the protocol include the use of skin phototypes II, III and IV (those readily capable of yielding a pigment response), a xenon arc solar simulator filtered with a WG335 filter 3 mm thick and UG11 filter 1 mm thick (these filters allow a UVA emission spectrum from 320 to 400 nm to be obtained without UVB and visible light), multiple-incremental doses (25% progression) of UVA with and without sunscreen protection and observation of pigment response 2-24 h after the end of UVA exposure. The protection index is determined as the ratio of the dose required to produce a minimally perceptible pigment response on sunscreen-treated skin to the dose for unprotected skin. It has been demonstrated that when using this protocol the results are reproducible on a large selection of products and levels of UVA protection (UVA-PF ranging from 4 to 28).[18,19]

The Japan Cosmetic Industry Association (JCIA) have adopted the PPD method as the official method for assessment of UVA efficacy of sunscreen products since January 1996.[101] Korea and China also adopted this method in 2001 and 2007, respectively.[102,103] The PPD method was recommended by the European Commission in September 2006[104] and has recently been proposed by the US FDA in the sunscreen monograph amendment.[105] The method has been described by these various countries or authorities with some minor differences. Finally, the method is currently in process to be normalized through the International Standardization Organization (ISO).

According to the JCIA standard, no significant differences were found between PFA values of standard samples on skin types II, III and IV. Therefore, skin types II, III and IV have been stipulated. All reported data indicate that skin types II-IV yield equivalent protection factors.[13,17,18,101] However, it is recommended to select subjects with a skin color on the test site characterized by a colorimetric Individual Typology angle (ITA°) from 20° to 41°.[19] Multiple center testing has shown good reproducibility of results between different laboratories for sunscreen formulations with UVA-PFs above 20 ( Table 1 ).[18] A low variability was achieved with a panel size of ten subjects ( Table 1 & Table 2 ), irrespective of UVA protection level. The consistency of these results has shown that the in vivo PPD method is reproducible and reliable.

Photoinstability of sunscreens due to UV exposure is a well-known and common phenomenon.[20] An important point in the assessment of protection against UV radiation is the challenge of product photostability during the test to avoid overestimation of UVA protection level of products that are not photostable. To illustrate this point, avobenzone (butyl methoxy-dibenzoylmethane [BMDM], Parsol®1789), a known photolabile UVA filter, was tested at concentrations of 1.0, 3.0 and 5.0%, respectively, alone and in combination with 10% octocrylene, a UVB filter proved to photostabilize avobenzone.[15] The results of UVA-PF of avobenzone (BMDM) alone ranged from 2.2 with BMDM 1% up to 4.6 with BMDM 5%. When combined with 10% octocrylene, they ranged from 4.6 with BMDM 1% up to 10.6 with BMDM 5%. The UVA protection efficacy of avobenzone is significantly increased when it is combined with octocrylene compared with the products containing BMDM alone at the same concentration. This can be explained by the fact that the PPD method applies realistic UVA doses challenging the photostability of filtering systems while octocrylene is known to photostabilize BMDM. It has been verified under real sun exposure conditions that when photolabile avobenzone applied at 1 mg/cm2 is exposed to a UVA dose of approximately 30 J/cm2 (approximately 2.5 h of noon sun exposure at 40° latitude in summer) there is a dramatic decrease in its UVA absorption properties leading to a sharp decrease in UVA protection efficacy.[21] Thus, the PPD method has been shown to be valid.

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