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Enhancing Sunscreen Efficacy for Realistic Application
By: Elsa Jungman, University of Paris XI, and Howard I. Maibach, MD, University of California
Posted: June 30, 2010, from the July 2010 issue of Cosmetics & Toiletries.
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Another reason for lack of sunscreen efficacy relates to incomplete coverage. Fifty volunteers applied a lotion containing a fluorescent marker that Loesh et al. examined using Wood’s lamp to identify areas missing sunscreen.5 Thus, where there is no fluorescence, there is no lotion. Each volunteer applied 15g of a lotion and they were told to apply it as if it were a sunscreen. Only the missing spots were studied with fluorescence; the authors did not measure the quantity or applied on the face. The investigation revealed incomplete sunscreen coverage areas such as the eyelids, hairline and ears. Azurdia also showed that some anatomical sites were completely missed, especially the neck and V of the chest.4
Lynfield and Schechter further showed that on average, only 22 g of sunscreen is applied to the entire body, where standard textbooks estimate the amount required to cover the entire body at around 30–60 g.6 To obtain the SPF labeled on the package, 35 mL is required.1
The unevenness of application and incorrect dosage of sunscreens could also be the result of the formulations used. Diffey and Grice investigated the use of chemical and physical sunscreens labeled SPF 25.7 The median physical sunscreen amount was 0.94 mg/cm2 and the median chemical sunscreen amount was 1.48 mg/cm2. The physical sunscreen was used in a smaller quantity than the chemical sunscreen since subjects found the physical sunscreen more difficult to spread all over the body.
Ivens et al. evaluated skin coverage using a fluorescent cream, solution and ointment formulations.8 While these formulations were not sunscreens, they demonstrated the skin coverage of different types of basic formulations. The fluorescent marker helped to localize the product on the body under Wood’s lamp; therefore, the researchers were able to determine which formulation spread best on the body. Results indicated that only the ointment was equally distributed on the skin. Other formulations were spread sparsely, compared with the application center. This is because ointment does not immediately dry and become tacky. Rather, it remains in a flowing state with the active substance dissolved in its carrier ingredient. In comparison, creams and solutions rapidly increase in viscosity after application due to rapid alcohol/water evaporation.
In addition, Barr studied the efficacy of sunscreens in a spray form, observing that they provide protection only in the area where the mist is applied.9 Since spray is transparent and fast-drying, users reported difficulty in determining whether all exposed skin was protected.