Photosensitive patients often comment that sunscreen products seem of little benefit. We used fluorescence spectroscopy to assess quantitatively their sunscreen application technique. A dose response relationship for sunscreen skin surface thickness and fluorescence intensity was determined for an intrinsically fluorescent sunscreen, NeutrogenaŽ sun protection factor (SPF) 15.
Ten women with long-standing photosensitivity conditions were asked to apply this sunscreen in the manner they would normally on a bright sunny day. Fluorescence measurements were taken from all unclothed body areas, comprising 17 sites of the head, neck, upper and lower limbs.
Geometric regression analysis of the dose-response data showed a high level of correlation (r = 0.99) between sunscreen thickness and fluorescence intensity, allowing fluorescence measurements to be converted to an equivalent sunscreen thickness. The overall median sunscreen thickness was 0.5 mg/cm², with median thicknesses of individual sites ranging from 0 to 1.2 mg/cm². The most frequently missed sites were the posterior neck, lateral neck, temples and ears, all of which had median thicknesses of 0mg/cm².
Hence, photosensitive patients fail to apply sunscreen in some prominently exposed sites and use average thicknesses far less than the manufacturers recommendation (2 mg/cm²). The level of protection is much lower than anticipated from the stated SPF of the product.
Azurdia RM, Pagliaro JA, Diffey BL, Rhodes LE. Sunscreen application by photosensitive patients is inadequate for protection. British Journal of Dermatology, 140, 255-258.
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