What factor sunblock will stop me burning on the ski slopes?

I get sunburned every time I go skiing. What SPF should I be looking for in my sunblock? Dian Goodspeed Albany, New York

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The answer is yes. As in, the question is wrong. Technically speaking, it’s redundant.

“Sun Protection Factor” (SPF) is one of those distractions whipped up by the cosmetics industry to impart a sense of agency within an arbitrary set of choices. Specifically, SPF refers to the number of hours the product is designed to work, not the degree to which it works. Given the length of the day, then, SPF ratings higher than 20 promise more hours of protection than a person could ever use. (So says Paula Begoun, author of the delightful Don’t Go to the Cosmetics Counter Without Me.) More importantly, the real sun protection factor of these products lies in the effectiveness of their active ingredients—not in how long they work.

All sunscreens and blocks will protect you to some degree against burning, provided you apply early and often (which may be the reason you’re burning). Sunscreens contain chemical filters that protect the skin from the sun’s rays; sunblocks, though, also serve as a physical barrier to the rays, so are much hardier in this respect.

So, if you want protection against the real enemy—the long-wave UVA rays that cause aging and skin cancer—look for a sunblock with Parsol 1789 (also known as avobenzone), zinc oxide, or titanium dioxide. These are the only active ingredients that block harmful long-range UVA rays, and they are included in surprisingly few products. Of the various drawbacks to the three, zinc oxide is the Marcel Marceau-esque nose cream you see lifeguards wearing, avobenzone can sweat off easily and possibly cause irritation, while titanium dioxide can block pores and feel heavy. But all will do a better job of keeping your collagen fibers from becoming busted bedsprings than most of the creams designed to keep you from burning.