REACTIVITY OF SUNSCREEN COATINGS 133 Figure 6. Effect of room light on spin trapping in suspensions of untreated titanium dioxide and zinc oxide particles. The EPR spectra were obtained with a 9.6-GHz EPR spectrometer using the following instru- mental settings: incident microwave power, 20 roW scan time, 4 m time constant, 0.2 s modulation amplitude, 2 gauss scan range, 100 gauss modulation frequency, 100 kHz. The aqueous suspension of metal oxide, containing 100 mM DMPO, was laid on the laboratory bench and exposed to room light for 15 minutes, and the EPR spectrum was recorded immediately. (A) DMPO, ultrafine zinc oxide, room light for 15 minutes (B) DMPO, ultrafine titanium dioxide, room light for 15 minutes. other pathways that can lead to the same adduct, this in itself is not conclusive evidence that the reacting species is a hydroxyl radical. The competition experiment (Figure 3) in which formate was added into the reaction system to compete with DMPO for the reacting species provides supporting evidence for our assignment of the adduct as the hydroxyl radical. The observation that this is generated even by a relatively long- wavelength light suggests that the chemistry may be complex and could involve several different intermediates. While taking into account some of the uncertainties noted above, there still appear to be a number of practical considerations that can be derived from the results of these experiments. Most obviously and perhaps most importantly, the effectiveness of the surface treatments with silicone that were used appears to be sufficient to prevent biologically significant damage from the reactive species that were detected by spin trapping. It therefore might be prudent to use such materials wherever possible and to avoid the use ofuntreated zinc oxide or titanium dioxide. The methodology used in these experiments is relatively straightforward and could be applied to other materials, and therefore it might be prudent to consider using this approach more widely to assess cosmetics that might have significant exposure to sunlight or strong room light. It also might be useful to consider the use of a variant of these techniques in which EPR spectroscopy is used directly in living subjects (8,9). In vivo EPR spectroscopy has lower

134 JOURNAL OF COSMETIC SCIENCE sensitivity than conventional EPR as used in these experiments, but it has the potential advantage of making the measurements in the most complex and realistic situation: the live subject. The results obtained here indicate that it would be useful to carry out further studies in order to understand more fully the extent of the generation of the reactive species and to determine their potential for causing damage in living systems. One key to the assessment of the potential damage is to obtain more definitive data on the dependency of the phenomena on the wavelength and intensity of the light. Another important variable that should be studied is the dependency of the effects on the physical form of the oxides, including particle size and structure of the material. Also, while the silicone coating that was used in this study was quite effective, more limited studies with a polyethylene surface treatment suggested that it was not as protective, and this suggests that a more thorough investigation of various surface treatments would be useful. It probably also would be useful to use other physical-chemical approaches to characterize the reactive species that are generated in these systems. Clearly it also would be very desirable to obtain more information on the biological consequences of the generation of the reactive intermediates. CONCLUSIONS 1. In the presence of light, both zinc oxide and titanium oxide can generate reactive species that can be spin trapped, and the resulting spin adduct is similar to that observed when hydroxyl radicals are present. 2. Coating the particles with a silicone surface treatment eliminates the generation of detectable spin adducts. 3. The relative yield for light-induced spin adducts in higher in zinc oxide than in titanium dioxide. 4. The exact wavelength band that drives the photochemistry of these materials cannot be derived from the current study due to limited experiments. For zinc oxide it is less than 435 nm, and less than 620 nm for titanium dioxide. 5. There are at least two potential scenarios in which these reactivities might be biomedically important: (a) when the materials reach live cells through transport or a lack of the usual dead layer of skin (e.g., in an abraded area), and (b) if the materials generate secondary reactive species that can penetrate through the dead layers of the skin. 6. There remain some potentially important aspects of this photochemistry that it may be prudent to understand for those involved in the use of these materials for cosmetic purposes, especially for sunscreens. 7. The biological consequences of the generation of the reactive species should be more thoroughly studied. 8. The methodology used for these studies could (and probably should) be applied to study the potential generation of reactive intermediates by sunscreens and other cosmetic preparations that may receive substantial exposure to light. ACKNOXWLEDGMENT This work was supported by NIH Grant P41 RR11601, an NIH-supported resource center.