Sun protection works in two basic ways. Sun blocks are inert substances, which contain metallic pigments such as titanium dioxide and zinc oxide that scatter the sun’s rays so they don’t penetrate the skin. Sun screens are substances that absorb the photons from sunlight and react with them chemically so that they cannot damage the DNA in skin cells. Many products now on the market contain both sun screens and sun blocks.
Although it used to be thought that only ultraviolet light (UVB) could cause skin cancer, scientists now know that both UVB and UVA can do so, said Dr. John Williams, a dermatologist at Brigham and Women’s Hospital.
That means you have to protect yourself against both, and that’s where things get complicated. The SPF (sun protection factor) number on the package label refers only to UVB protection. (An SPF of 30 absorbs 95 percent of the sun’s UVB rays; higher numbers confer a modest additional amount of protection – an SPF of 50 absorbs 98 percent of the rays.)
So, to make sure you’re protected against both UVB and UVA, you need to pay attention not just to the SPF number but to the ingredients in the whole product. Look for titanium dioxide and zinc oxide, the only two FDA-approved sun blocks that protect against the whole spectrum of UVA light. Alternatively, look for a substance called avobenzone (also known as Parsol 1789), which is a relatively new sun screen that also protects against the entire UVA spectrum.
Bottom line? Choose any product that protects against both UVA and UVB and that does not irritate your skin on a daily basis. Also, be sure to apply sun protection half an hour before going out in the sun to allow the product to be absorbed by the skin, said Dr. A. David Rahimi, the CEO of Forever Young, a dermatology practice in Los Angeles. Then re-apply every few hours, especially after sweating or swimming. People with fair skins should use products with an SPF of at least 30.