What does the SPF number mean?

All sunscreens will display a number that is followed or preceded by the letters SPF (sun protection factor). Remember the SPF only refers to the sunscreens potential to block UVB rays and not UVA.  SPF numbers range from 2 to 50+.  The numbers tell you the time the skin will take to redden with the sunscreen versus the amount of time it will take to redden without the sunscreen. So if you have appropriately applied an SPF30, it would take your skin 30 times longer to go red as compared to having no sunscreen on. 

So if your skin normally reddens after 10 mins in the sun, applying an SPF30 sunscreen would allow you stay in the sun for 300 mins.  The higher the number the longer the protection. However SPF is actually a measure of the degree of protection it gives you from UVB rays and should not be used to determine the length of sun exposure.

An SPF of 30 allows about 3% of UVB to penetrate the skin and SPF50 about 2%.  This does not seem much but can make a big difference in certain skin types.