SPF

Sun Protection Factor, or SPF, is a rating given to sunscreens.  It was developed by the Food and Drug Administration in the 1970s.

SPF is defined as the ratio of how long it takes for skin to redden with sunscreen compared to without sunscreen.  The reddening of your skin is called erythema and is due to UVB.

For example, with a SPF 30 sunscreen, it will take your skin 30 times longer to redden than it would without the sunscreen, assuming the same UVB exposure. So if your skin would redden in 10 minutes without sunscreen, then it will redden after 300 minutes (5 hours) with a SPF 30 sunscreen. It's easy to see why most people interpret SPF as telling them how much longer they can stay in the sun, but that’s not how it should be interpreted.

Here's another way to understand SPF.  A SPF 30, when applied properly, protects you against 29 out of 30 parts of UVB or 97% of UVB rays.  The 30th part of UVB (the last 3%) is still absorbed by your skin and will affect it.  That’s why, when you use a sunscreen with SPF 30, it takes 30 times as long for your skin to redden as without the sunscreen.

Similarly, a SPF 15 protects you against 14 out of 15 parts of UVB or 93% of UVB rays, a SPF 50 protects against 98% UVB, and a SPF 100 would protect against 99% UVB.  When you move from a SPF 15 to a SPF 50 you are not actually tripling your protection - you are improving it from 93% protection to 98% protection.  That's a more realistic way to compare the Sun Protection Factor you get from different SPF-rated sunscreens.

Above SPF 50, there is an even smaller increase in % protection.  That’s partly why the FDA has wanted to put a limit on SPF numbers – to avoid consumers misunderstanding the increase in protection from going above SPF 50.  On the other hand, a higher SPF rating can partially compensate for not applying enough sunscreen, which is the most common error made by individuals using sunscreen.