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Wed Dec 29, 2010 8:20 am  |
| I think the PPD measures the UVA rays which are the ones that cause the most aging to the skin and the SPF measures more the UVB rays which cause burning. ( it may be the other way around) Makeup Alley website skin care board talks a lot about the PPD and there are some very knowledgeable posters like Haruka. My impression is that the highest PPD sunscreens are the ones from many Asian countries and Europe as they contain the most UVA blocking ingredients. And we should all be mindful of the PPD ratings especially when using certain skin care actives like lightening products. HTH Harriet |
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Wed Dec 29, 2010 10:45 am |
http://sun1.awardspace.com/Sunscreens/Standards/Standards.htm
Persistant Pigment Darkening (PPD, UVA-PF)
The Persistent Pigment Darkening (PPD) method is a method of measuring UVA protection, similar to the SPF method of measuring UVB light protection. Theoretically, a sunscreen with a PPD rating of 10 should allow you to endure 10 times as much UVA as you would without protection. People are irradiated with a UV-A light source (320 to 400 nm). Two or three hours after irradiation with UVA, the skin has changed yielding a persistent pigment darkening. The advantage of the persistent pigment darkening is that the residual colour that has developed after exposure to the radiation allows a more precise reading to be taken. They compare the sunscreen protected (MPDp) skin and the unprotected (MPDu) skin to determine the results. The results were expressed as UVA-protection factors UVA-PF. [Tinosorb] [Nora80] [Ciba]
The PPD response is stable and reproducible; however its clinical significance is questionable, because the action spectrum for PPD is not defined for wavelengths shorter than 320 nm, and the response is masked during outdoor sun exposure by other skin responses to UV. Thus it is impossible to relate the PPD protection factor directly to the degree of protection is sunlight. [Stanfield] |
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