The number on the bottle (SPF 15, SPF 30) indicates the amount of time you can stay out in the sun without burning, compared to how long you could have stayed outside without burning if you weren't wearing sunscreen. For example, a sunscreen with an SPF of 15 will allow you to stay outside 15 times longer than if you weren't wearing sunscreen.
An SPF 30 sunscreen doesn't provide double the protection of an SPF 15 sunscreen, however. An SPF 30 sunscreen filters out about 97% of UVB rays, while an SPF 15 filters out about 93 percent of these rays. Even if you wear a sunscreen with a higher SPF, you can't just apply it and forget about it. Sunscreens wear off, especially if you're swimming or sweating. No matter what SPF sunscreen you're wearing, reapply it every two hours, and more often if you're swimming or sweating. Also, look for a sunscreen that provides broad spectrum protection from both UVA and UVB rays. Talk to a dermatologist for more information. for more info go to www.droz.com