How to choose a sunscreen

Read the label before you buy. The SPF number is an indication of protection against UVB rays only. Sunscreen products labeled "broad-spectrum" protect against UVA and UVB radiation, but there is no standard system for measuring UVA protection.

Look at the ingredients. Products with an SPF of 15 or higher that contain avobenzone (Parsol 1789 ), zinc oxide , or titanium dioxide are likely to be effective against the entire spectrum of UVA rays.

Check the expiration date: Most sunscreens expire within two to three years.

Settle on an SPF: Choose an SPF of at least 15. The SPF represents the level of sunburn protection provided by the sunscreen. SPF 4 blocks out 75 percent of burning UV rays, while SPF 30 blocks out 97 percent. The FDA is reviewing ways to measure how much UVA protection sunscreens provide.

Apply it correctly: The average person is supposed to apply an amount of sunscreen equal to the size of a golf ball.

When using SPF 15 correctly, you will get the equivalent of one minute of UV rays each 15 minutes you spend in the sun. So, spending eight hours in the sun wearing SPF 15 is the same as spending 32 minutes unprotected.

Reflective surfaces like water, snow, and sand increase UV exposure.

Sources: The American Cancer Society and Deborah A. Scott, M.D. director of the Center for Laser Dermatology and Skin Health at Brigham and Women's Hospital.