Testing the efficacy of sunscreens
Avoiding painful sunburn isn’t the only reason to apply sunscreen when you hit the beach. Exposure to the ultraviolet radiation of the sun can lead to other health problems, including immunosuppression, radical formation and skin aging as well as to skin cancer. However, the sun protection factor (SPF) used to determine the efficacy of sunscreen doesn’t necessarily have relevance to all of these risks. Researchers determine the SPF of a sunscreen by quantifying the redness of the skin that occurs with UV irradiation, but this particular biological response is the result primarily of ultraviolet B, or UVB, intensity. A number of injuries associated with sun exposure result from the longer-wavelength ultraviolet A, or UVA, part of the spectrum. Assessments of sunscreens’ ability to protect therefore must take into account the entire UV wavelength range.
Investigators have proposed a variety of methods to characterize sunscreens’ effectiveness when considering the entire UV spectrum: determination of the UVA protection by individual independent methods, measurement of the in vivo UV-induced radical formation, application of spectroscopic methods using specifically prepared carriers, or mathematical model calculations. Still, the latter techniques cannot reproduce accurately the distribution of sunscreens on living human skin.
A team with Charité-Universitätsmedizin Berlin in Germany and with the University of Athens in Greece recently used a tape-stripping method in which both the stratum corneum and sunscreens that have been applied topically are transferred layer by layer onto adhesive strips and the UV/VIS spectra of the tapes measured. The group reported the calculation of a universal sun protection factor (USPF) using values from the complete UV range acquired with the technique described. Thus, the investigators showed that they could quantify the effectiveness of sunscreens under realistic in vivo conditions, taking into account the characteristic inhomogeneous distribution of the UV filter on the human skin.
Researchers applied sunscreen to the arms of volunteers, pressed adhesive strips on the area for 3 s and swiftly removed the tape strips with forceps. They took spectra of the tape using the entire UV-wavelength range. Their method calculates a universal sun protection factor (USPF) based on values obtained with a spectrometer.
They demonstrated the technique using commercial sunscreens from various cosmetic companies. They applied the sunscreens to the forearms of volunteers and pressed adhesive strips 19 mm wide and roughly 6 cm long to the area for 3 s and then swiftly removed the tape strips with forceps. They fixed the strips onto a sample holder and immediately obtained UV spectra in the 280- to 400-nm range using a modified PerkinElmer spectrometer. They took readings immediately after removal to avoid any changes in the distribution of the sunscreens on the tape.
Then they calculated the average sum transmission in the three wavelength ranges in question: UVB, UVA and the entire UV range. Using these values, they calculated the spectroscopic protection factors, which describe how much additional time one can spend in the sun after applying the sunscreen, based on the reduced incident radiation it affords.
The investigators continue to develop the method as a potential standard for determining the USPF and for assessing the efficacy of sunscreens.
“The next task in our research will be to enhance the number and the types of sunscreens and to include other laboratories in the investigation,” said Hans-Juergen Weigmann of Charité-Universitätsmedizin Berlin, an author of the study. “This will be performed in close cooperation with organizations responsible for standardization.
Journal of Biomedical Optics, July/August 2007, 044013.
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