After the Bronze Age
McManus, Reed, Sierra
Call it "Revenge of the Freckled." It's no longer just the blue-eyed and fair-skinned who must take sun protection seriously. Whether it's because of a thinning ozone layer, mass migrations to the Sunbelt, or simply a populace that spends too much time outdoors, ultraviolet (UV) radiation is having an effect: more than 600,000 cases of skin cancer are diagnosed each year in the United States, almost all the result of overexposure to the sun. It's now suspected that even a brief history of sunburn in your first 10 or 20 years doubles your risk of skin cancer as an adult.
The obvious remedy is to keep skin out of the sun. Shade your face with a wide-brimmed hat and wear tightly woven clothing (otherwise, solar radiation leaks through). Keep in mind that solar radiation is stronger the closer you get to the equator and the higher in altitude you go; that water, snow, and sand reflect as much as 85 percent of the sun's rays; that UV rays penetrate several feet underwater; and that even overcast days are dangerous, because clouds don't block ultraviolet rays.
The key is to think like the researchers at the U.S. Food and Drug Administration: to them, tans and sunburns are only superficial indicators of the cellular damage caused by prolonged sun exposure. They consider sunscreens and sunglasses medical devices, not fashion accessories.
A sunscreen's all-important "sun protection factor" (SPF) indicates how much longer a person can stay in the sun before burning when using a sunscreen than without any protection at all. Each individual's base figure depends on skin type, location, and the time of day and year. (Dermatologists use a six-point scale of skin types to determine the figure.) In the tropics a fair-skinned person can slide for only about ten unprotected minutes of midday sun before starting to broil; sunscreen with an SPF of 15 will extend that safe time to two and a half hours. (Lathering on more sunscreen doesn't extend one's allowable exposure time, it merely replenishes the existing protection.)
Sunscreens with SPFs as high as 50 are available. But because the FDA devised its testing methods before manufacturers started concocting high-digit formulas, the agency recommends an SPF of 15. …