The key word here is always. Get out of the mind-set that sunscreen is only for the beach (90 percent of sun exposure comes during every­day outdoor activity) and smear on that sunblock each day before you leave the house. Even cool temperatures and cloudy days are no protection from sunburns because clouds block only 20 percent of ultraviolet radiation. Damage is possible even in the shade if light is reflected off water, snow, or sand.
Sunblocks are designed to reflect the sun’s rays off the skin; sunscreens are formulated to absorb harmful rays. The difference be­tween them is negligible: What you want to look for is a product labeled broad spectrum, which indicates that it defends against ultraviolet wave­lengths known simply as ultraviolet A (UVA) and ultraviolet B (UVB). At one time, only UVB rays were thought to be harmful to the skin, but scientists now know that both UVA and UVB rays are dangerous.
The AAD recommends daily use of a product with a sun protec­tion factor (SPF) of at least 1 5. The SPF indicates how long you can spend in the sun without being burned. If you usually burn within 20 minutes, for example, a product with an SPF of 15 would allow you to stay in the sun 1 5 times longer, or 300 minutes.
Apply the sunblock or sunscreen liberally to all of the skin that will be exposed, including the lips, ears, scalp, feet, and eyelids (being careful not to get any in your eyes). This should be done 15 to 30 min­utes before going outdoors as the delay allows the chemicals to be absorbed into the skin, where they will be most effective. And if you’re swimming or perspiring, be sure to reapply your sunblock every two hours. Waterproof products may be reapplied less often, though an extra application is smart if you’re in doubt about the need for more.
Don’t apply sunblock to a baby under six months of age the chemicals can too easily be absorbed into the child’s system.