Most people know that sunbathing takes a toll on the skin's appearance, often in the form of a slow conversion to leather. It makes sense -- anything that can cause skin to blister is probably doing a fair amount of long-term damage. The more surprising fact is that the sun's rays can harm your skin in the 20 minutes it takes to walk the dog -- even on a cloudy day. (Eighty percent of the sun's rays make it through those clouds [source: AAD].)
Much of what we characterize as "premature signs of aging" is caused by sunlight [source: SkinCarePhysicians]. Exposure to the sun's two types of ultraviolet radiation, UVA and UVB, contributes to freckles, wrinkling, age spots, spider veins, blotchiness, thickened patches and leathery texture. UV light damages both collagen and elastin, which help support the structure of the skin so it doesn't sag. With extended UV exposure, the skin is less and less able to repair itself, so over time the skin becomes irreversibly loose, tough and discolored.
All in all, this "photoaging" makes us look older than we really are, and it can be avoided with regular sunscreen use. Doctors recommend using at least SPF (sun protection factor) 15 for the most basic protection from sun damage, and it's important to make sure the sunscreen is broad-spectrum, meaning it protects against both UVA and UVB. Fifteen is just the minimum, though: In general, the fairer the skin, the higher the recommended SPF. Someone with a very fair complexion might go for an SPF 50 (and wear a hat at all times).
Perhaps the most important element in a sunscreen, though, is suitability for your skin type and lifestyle, since you have to be willing to apply it every day. For dry skin, look for a moisturizing sunscreen or a moisturizer with built-in SPF. For an oily complexion, try a product that contains silica, which helps give skin a matte finish. Sensitive skin will do best with a hypoallergenic product. If you sweat a lot, choose a "sport" sunscreen -- it won't sting if it drips into your eyes.
While lots of people use sunscreen to save their skin from photoaging, that's really just the beginning. Sun protection can save a lot more than your looks.