(Graph courtesy of remixyourhealth)
SPF, or Sun Protection Factor, seemed to come out of nowhere in the 1980s as people started to become aware of the need for sunscreen. (Before that, "suntan oil" appeared to be a derivative of cooking oil that helped broil skin.)
The original explanation I heard for the meaning of SPF was that it allowed you to stay out in the sun longer by a certain factor. My friends and I initially interpreted this as, if you were going to stay out in the sun an hour, you could stay out two hours with an SPF of 2. Or 10 hours with an SPF of 10. An SPF over 24? That meant you could extend your life by adding hours to your day.
Like MPG, SPF is a hard number to build good intuitions around. The right interpretation of SPF is that, if you want sun damage of X amount, you can now stay N times as long to get the same X sun damage. You want a an hour of sun damage? At an SPF of 10, you can stay out10 hours and get the same amount of sun damage as one hour.
But most people don't have in mind a level of sun damage they want. Instead, they have in mind a time that they want to spend in the sun. As with MPG, SPF is better understood once it is flipped. Flipping it reveals that there are surprising diminishing returns. As SPF numbers start to sound very impressive, growing from 30, to 45, to 60, to 100--and the price grows as well--the actual increase in skin protection is negligible. More after the jump.