SPF - What Is It?
SPF (sun protection factor) is a measurement of the time a sunscreen will protect you from ultraviolet B rays.
Here's how it works: if it takes 20 minutes for your unprotected skin to burn, using an SPF 15 sunscreen would in theory prevent burning 15 times longer - about 5 hours. There are problems with this however - no sunscreen stays effective longer than 2 hours without reapplication. Also, burning or reddening of the skin is a reaction to UVB rays alone and is not indicative of the UVA damage that may be happening. Be sure to look for sunscreens that provide broad-spectrum protection.
To ensure you get the full SPF apply 1 oz (about a shot glass full) 30 minutes before sun exposure to allow the ingredients to bind to the skin. Reapply the same amount every 2 hours and immediately after swimming, toweling off, or excessive sweating.