What does SPF on sunscreen actually mean?

SPF, or Sun Protection Factor, is a measure of how well a sunscreen protects the skin from UVB rays, the type of radiation that causes sunburn, damages the skin and can contribute to skin cancer. SPF does not measure how well a sunscreen protects against UVA rays, which are also harmful and dangerous. 

If your skin would normally burn after 10 minutes in the sun, an SPF 15 sunscreen would allow you to stay in the sun for approximately 150 minutes without burning (a factor of 15 times longer). This is a rough estimate that depends on skin type, the intensity of the sunlight and the amount of sunscreen used. SPF is actually a measure of protection against the amount of UVB exposure and is not intended to help you determine the length of exposure. 

For the best protection, experts recommend using a minimum SPF 15 sunscreen, applying the correct amount (2 mg/cm2 of skin or about one ounce for full body protection) and reapplying every 2 hours.