Sunscreen is only one part of a smart plan for sun protection. Sunscreens absorb, reflect or scatter the harmful UVA and UVB rays of the sun by providing physical or chemical protection. Physical sunscreens contain ingredients such as zinc oxide or titanium oxide that form a film to reflect or scatter UV light before it contacts the skin. Chemical sunscreens contain ingredients such as avobenzone or osybenzone that absorb UVA or UVB rays.
Every sunscreen has a sun protection level (SPF) that is a measurement of the amount of UVB protection. The higher the number, the greater the protection. An SPF of 15 filters out about 93 percent of the sun's UVB rays; SPF 30 filters about 97 percent of UVB rays.
The effectiveness of sunscreen depends on several factors, including how likely your skin is to burn, how intense the UV rays are, and the amount of sunscreen you apply and how often you apply it. Sweating, swimming, showers and even high humidity, can decrease the effectiveness of your sunscreen. While some sunscreens are water resistant, none are truly waterproof.
When you're choosing a sunscreen, look for broad-spectrum protection from UVA and UVB rays. And make sure it actually contains sunscreen: Tanning lotions, accelerators and oils don't, so if one of those products ends up in your beach bag, you may come home with a nasty burn.