What about SPF? SPF, or sun protection factor, is a number that denotes how effectively a sunscreen will protect your skin from UVB rays. (It doesn't measure UVA protection.) The higher the number, the longer lasting the protection, no matter what type of sunscreen it is. An SPF of 15, for example, roughly means it will take you 15 times as long to develop a sunburn as it would without wearing sunscreen.
Sheu, Friedman and Hanson agree that an SPF of around 30 is the magic number. Sunscreens with higher SPFs can create a false sense of security and lead users to stay in the sun too long.
Do higher SPFs mean more potentially worrisome chemicals in the sunblock? Higher SPF means a sunscreen has a higher concentration of a given UV-filtering ingredient, or more UV filters mixed together; those are the two ways you get the high SPF.
Whatever you choose and however you use it, any sunscreen is better than none at all, Friedman says. "The bottom line is there's an unequivocal, worldwide rise in the incidence of melanoma, the deadliest form of skin cancer," he says. "So the most important thing we can do is protect ourselves from the sun."