REACTIVITY OF SUNSCREEN COATINGS 129 Figure 2. Effect of light (xenon lamp) on spin trapping of suspensions of treated and untreated titanium dioxide particles. The EPR spectra were obtained with a 9.6-GHz EPR spectrometer using the following instrumental settings: incident microwave power, 20 mW scan time, 2 m time constant, 0.1 s modulation amplitude, 1 gauss scan range, 100 gauss modulation frequency, 100 kHz. The aqueous suspension of titanium oxide, containing 100 mM DMPO, was exposed to the light source for 30 seconds, and the EPR spectrum was recorded immediately. (A) DMPO, ultrafine titanium dioxide (B) DMPO, ultrafine titanium dioxide, white light (C) DMPO, silicone-treated ultrafine titanium dioxide, white light (D) DMPO, polyethylene-treated ultrafine titanium dioxide, white light. of trapped species found with the use of a 455-nm or even a 620-nm filter being only moderately smaller than that observed with no filtration. This apparent difference between the wavelength dependence of titanium dioxide and zinc oxide, however, may reflect the fact that the total yield of trapped species was higher with zinc oxide (Figure 5), and therefore all of the spectra obtained with titanium oxide used higher amplifi- cation, which could emphasize pathways that have low absolute yields. Using high-amplification conditions, ambient light was found to be able to generate trappable species (Figure 6). While the intensity was relatively low, this was an unex-

130 JOURNAL OF COSMETIC SCIENCE A B Figure 3. Effect of sodium formate on spin trapping of suspensions of untreated zinc oxide particles. The EPR spectra were obtained with a 9.6-GHz EPR spectrometer using the following instrumental settings: incident microwave power, 20 roW scan time, 2 m time constant, 0.1 s modulation amplitude, 1 gauss scan range, 100 gauss modulation frequency, 100 kHz. The aqueous suspension of the zinc oxide, con- taining 100 mM DMPO, was exposed to the light source for 30 seconds, and the EPR spectrum was recorded immediately. (A) DMPO, ultrafine zinc oxide (B) DMPO, ultrafine zinc oxide, white light (C) DMPO, ultrafine zinc oxide, 1.5 M sodium formate, white light (D) DMPO, ultrafine zinc oxide, 1.5 M sodium formate (E) DMPO, 1.5 M sodium formate, white light. pected finding, and therefore the experiment was repeated several times, which con- firmed that this was a reproducible phenomenon. When light was rigorously excluded, no radical generation was observed. DISCUSSION These results lead to a number of potentially significant conclusions and also raise a number of questions and potential concerns.