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.
Read the rest
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.