Photochemical formation of singlet molecular oxygen in illuminated aqueous solutions of several commercially available sunscreen active ingredients

Evidence is presented for the photochemical formation of singlet molecular oxygen (O-1(2)) in air-saturated aqueous solutions of several sunscreen active ingredients using sunlight-range illumination. This is of significance because (1) O-1(2) is known to be cytotoxic, and (2) there have been several reports of toxic effects associated with the use of some sunscreens; most notably, with p-aminobenzoic acid (PABA). Illuminated aqueous solutions of PABA, 2-ethylhexyl p-(dimethylamino)benzate (ODPABA), 2-hydroxy-4-methoxybenzophenone (BZ3), 2,2'dihydroxy-4-methoxybenzophenone (BZ8), 2-ethylhexyl 2-cyano-3,3-diphenylaclylate (OCR), 2-ethylhexyl p-methoxycinnamate (OMC), and 2-ethylhexyl salicylate (OCS) were evaluated individually for O-1(2) formation. Furfuryl alcohol (FFA), a well-known chemical trap for O-1(2), was added to each of the aqueous sunscreen solutions. The FFA was consumed when solutions of PABA, ODPABA, OMC, and OCR were illuminated, but no loss of FFA other than by direct photolysis occurred in solutions of BZ3, BZ8, or OCS. There was also no significant loss of FFA in any of these solutions kept in the dark. Further evidence for the formation of O-1(2) in illuminated aqueous sunscreen solutions is provided by the results of experiments in which individual solutions containing sunscreen active ingredients and FFA that were diluted with D2O exhibited an increased rate of FFA consumption while the addition of azide ion (N-3(-)) reduced the rate of FFA consumption. Continuous sunlight-range illumination of aqueous PAPA solutions produced significantly higher steady-state concentrations of O-1(2) than in solutions containing any of the other sunscreen active ingredients evaluated. The substituted benzophenone compounds (BZ3 and BZ8) and the salicylate-based compound (OCS) not only appear to produce no O-1(2), but they also appear to produce no other reactive oxidant species that are capable of consuming FFA. This indicates that BZ3, BZ8, and OCS may be preferable, from the standpoint of toxic oxidant formation, for use as sunscreen active ingredients when compared to the other compounds evaluated in this study.