In the SPF numbers game, Neutrogena has taken a decisive lead with its new Ultra Sheer Dry-Touch Sunblock SPF 100+ ($12, drugstores). But really, does anyone need an SPF that high? The answer depends on how much sunscreen you apply. If you slather on the same amount used to establish SPF values (about half a teaspoon on your face alone), 97 percent of skin-reddening UVB rays are blocked by an SPF 30, 99 percent by an SPF 100. But here's the rub: "Most people apply 25 to 50 percent of the amount of sunscreen used to determine SPF," says James M. Spencer, MD, associate clinical professor of dermatology at Mount Sinai School of Medicine. Half the necessary dosage will yield only about one-third of the listed SPF. So a higher number is a nice insurance policy: Go easy on an SPF 100, and you're still left with an SPF of about 30.
Even a triple-digit SPF, though, must be reapplied every two hours (all sunscreen eventually degrades in the sun) or after swimming. And SPF measures protection only against UVB rays, not UVA, which also contribute to skin aging and cancer. The FDA has proposed revising its labeling requirements so that every sunscreen also has a one- to four-star UVA protection rating, but the new rules won't be finalized before the end of 2009 (manufacturers will then have 18 months to comply). In the meantime, sunscreens that contain Helioplex (like Neutrogena's Ultra Sheer line) are a great bet, says Spencer—they have a well-stabilized (meaning it stays active longer) form of avobenzone, a highly effective UVA filter.