But remember that UV light also produces these damaging molecules after penetrating unprotected skin. And researchers argue that if you re-apply sunscreen, as you're supposed to, these potentially damaging effects of reactive oxygen species could be mitigated because re-application prevents UV rays from reaching the sunscreen molecules that have penetrated deepest into the skin. Sunscreen manufacturers also try to optimize their formulations so that they stay on the surface of the skin rather than penetrating deep as they did in some of these experiments.
Some sunscreen companies add antioxidants like vitamin E and vitamin C to their formulations to absorb some of these reactive compounds. One 2011 study found that the addition of antioxidants to SPF 15 or 50 sunscreen formulations reduced the numbers of reactive oxygen species in skin more than two-fold. (To find sunscreens with added antioxidants, look for inactive ingredients such as sodium ascorbyl phosphate, tocopherol, tocopheryl acetate or diethylhexyl syringylidenemalonate.)
But antioxidants don't always behave as expected, bringing us to another sunscreen controversy. Retinyl palmitate, a form of vitamin A, is an antioxidant added to many sunscreens and cosmetics (it's also used to fortify some dairy products and cereals). Although the FDA considers retinyl palmitate to be safe, research suggests that upon interaction with UVA light, the compound produces reactive oxygen species. Yes, that's correct: An antioxidant added to organic sunscreens to help quench these potentially damaging reactive molecules is actually producing them instead. When's winter?
Retinyl palmitate has gotten an especially bad reputation lately because of a series of studies (yet to be published, but available online) conducted by the National Institute of Health's national toxicology program. Researchers there slathered mice with a retinyl palmitate lotion and then exposed them to different amounts of UV light. Some of the treated mice developed a greater number of malignant tumors than mice that weren't given the cream, but it's hard to know how to interpret the results in part because the mice had all been genetically engineered to be predisposed to cancer. Mice also have extremely thin skin, so compared to humans, UV light probably penetrates more easily into their lower skin layers.