By Sally Wadyka

Every time we go outside, we face one of life´s unfortunate ironies: What we treasure the most seems to have a nasty habit of exacting revenge on us if we overindulge. Take those spirit-raising rays of sunshine: Soak up too many, and they'll deliver potentially deadly ultraviolet (UV) radiation. According to the Skin Cancer Foundation, sun exposure causes more than 90 percent of all skin cancers. One in five Americans may develop skin cancer during her lifetime.
In an interesting, wouldn't-you-just-know-it twist, however, research has recently emerged about the benefits of sun exposure. One of the body's main sources of vitamin D is sunlight, and a deficiency of that nutrient has been linked to increased risk of several types of cancer as well as to autoimmune diseases. In a study published this year, researchers at Stanford University School of Medicine showed that when sunlight triggers vitamin D synthesis, it causes immune cells to travel to the outer layers of skin where they can help repair cellular damage.

Does that mean you can toss out the sunscreen? Not so fast, warn dermatologists. While this latest research on the benefits of sunlight seems somewhat theoretical, science holds no doubt about the proven dangers of UV rays. Plus, you can get most of the vitamin D you need through diet and supplements, and the amount of unprotected UV exposure required to synthesize vitamin D is actually fairly minimal—about ten minutes two to three days a week.
Not only is the sun the No. 1 cause of skin cancer, it also hastens skin aging. Wrinkles, sagging, uneven skin tone, and brown patches all result from overexposure to UV light. "It's incontrovertible that being in the sun chews up collagen and elastin in the skin," says Alan Dattner, MD, a holistic dermatologist at Integrative Medicine and Dermatology in Manhattan and New Rochelle, New York. UV rays produce harmful particles called free radicals. Left unchecked, these free radicals damage the DNA of skin cells (which can lead to melanoma and basal cell carcinoma) and cause the breakdown of collagen and elastin (the connective fibers that give skin its elasticity and firmness). For proof of the sun's aging powers, compare the skin on your buttock cheek with that on your face. The skin on your derriere still looks relatively smooth, with few if any wrinkles. Your face can show subtle signs of aging even as early as your 20s or 30s.

The ABCs of SPFs
When reading sunscreen labels, pay attention to the sun-protection factor, typically listed as SPF plus a number. A product with an SPF 15 (the minimum recommended by the Skin Cancer Foundation) will block about 93 percent of UVB rays—the ones that cause tanning and burning. SPF 30 will block about 97 percent, and even lotions with an SPF of 50 or higher probably block no more than 99 percent, according to the Skin Cancer Foundation. "Higher doesn't necessarily mean better," says Joel L. Cohen, MD, chair of patient education for the American Society of Dermatologic Surgery. "Because to get that higher SPF number, the product may contain a higher concentration of potentially irritating chemical ingredients."
The other caveat? SPF refers only to the amount of protection you're getting from UVB rays. While ample proof exists that UVA rays