Geeking out

So on the news the other day, they were talking about sunscreen. I liked the program right away because they were asking random passers-by how they think sunscreen works. I knew the answer before they explained it, which strokes my science-y ego (in case you are interested, the active ingredient in sunscreen can absorb the energy of UV rays and store it by exciting its electrons, then slowly release the energy in a more harmless form over time).

Next, they started talking about how SPF is misleading - SPF 15 sunscreen blocks 93% of UV-B rays, while SPF 30 blocks 97% and SPF 50 blocks 98%. In other words, there is a serious diminishing return going on. So, being the nerd I am, I decided to determine the formula. After some messing around, I determined that the amount of rays that are blocked is equal to 1-1/x, where x is the SPF. It turns out this is supported by actual truth (hurray, I love it when I'm right!)

Some of the interesting implications of this:

1) Really, SPF 100 is hardly better than SPF 30. So I guess don't waste money if SPF 30 is cheaper.
2) If you are wondering whether it's even worth it to put on SPF 6, the answer is probably yes, as it will block 83% of the sun's rays - that's a pretty good deal for something that sounds negligible. Even SPF 2 will block half of rays. Not too shabby for something that sounds like a joke.
3) Math and science are cool.

Source:  http://www.personal-care.basf.com/docs/personal-care-pdf/description-of-methods.pdf 
Original news story:  http://www.npr.org/2012/07/06/156380368/whats-your-iq-on-spf