Standard sunscreen has been considered a good defense against cancer because it's known to block UV radiation with a one-two punch: Inorganic compounds such as titanium oxide and zinc oxide scatter or reflect UV light, while organic ingredients such as oxybenzone absorb these rays and convert them into heat, which then dissipates into the air.
But, the Nature study found that UV radiation interacts with our skin in a way that can outdo even the best efforts of the highest SPF creams. In experiments on mice, researchers at the University of Manchester found that UV rays target the gene that is responsible for suppressing tumor formation. In 40% of the subjects, a single dose of UV radiation caused mutations on the gene. And, while the SPF 50 sunblock they gave the mice (to protect against both UVA and UVB rays) delayed the onset of melanoma, study authors say it "only provided partial protection" against tumor formation.
Although sunblock plays an important role in protecting our skin, these findings make it clear that no matter how much SPF we're wearing, limiting our exposure to direct sunlight is more important than we ever thought. Of course, you shouldn't toss the Coppertone — just maybe invest in a sun hat, too.