It does successfully prevent sunburn, but what about the evidence for sunscreen protecting you from skin cancer later in life?
The answer: Nobody is really sure. Last year, I wrote a short piece for BoingBoing that looked at this a little bit. The key point: Cancer takes a long time to happen and we haven't been using sunscreen long enough to have much evidence about it.
But, at Discover's The Crux blog, Emily Elert expands on some of the other problems in play. One of the key things—and something that will hopefully be fixed by this time next year—there's nothing on the sunblock you buy to tell you how protective it is against skin cancer. SPF is all about the burn. So even if some sunscreens do protect against cancer, you don't have a good way to know whether or not you're using one of them.
First of all, the way sunscreen’s effectiveness is measured—its SPF rating—basically only describes its ability to block UVB rays. That’s because UVB is the main cause of sunburn, and a sunscreen’s SPF stands for how long you can stay in the sun without getting a sunburn (a lotion that allows you to spend 40 minutes in the sun rather than the usual 20 before burning, for example, has an SPF of 2).
UVA rays can cause cancer but not sunburn, so they don’t factor into the SPF calculation. That means that if you slather on a high SPF sunscreen that only protects against UVB, you’d still absorb lots of UVA radiation, potentially increasing your long-term cancer risk.
Soon it will be easier to tell which sunscreens include ingredients that block or absorb UVA as well as UVB. According to FDA regulations passed last year, products that pass a “Critical Wavelength” test—meaning that they block wavelengths across the ultraviolet spectrum—will carry the label “Broad Spectrum” alongside the SPF, while sunscreens that don’t pass the test will be forbidden from claiming they have such capabilities. However, those regulations don’t go into effect until December, so for this summer, you’re still stuck with SPF. And, by the way, you probably need to apply twice as much sunscreen as you think to actually get an SPF as strong as that marked on the bottle: manufacturers test their products’ SPF with the assumption that you will slather on obscene amounts. This discrepancy could be contributing to the fact that the NIH, when looking the connection between sunscreen use and skin cancer in large populations, doesn’t see clear evidence that sunscreen is effective in reducing the risk of skin cancer. (It’s worth pointing out, too, that there is a clear genetic component in some skin cancers, so just avoiding sun or using sunscreen regularly are not the only factors that determine whether someone gets it.)
Image: Beer, cigarettes and sun block: Roskilde Festival 2009 essentials., a Creative Commons Attribution (2.0) image from wouterkiel's photostream