Thursday, May 14, 2009

SPF 100

Neutrogena now offers an SPF 100, sun block. What does that mean? The SPF number doesn't measure percentage of protection. An SPF of 30 already screens nearly 97% of the sun's UVB rays.

The SPF number is a multiplying factor that calculates the increased amount of time you can endure sun exposure without burning. If it normally takes you 15 minutes to burn, an SPF of 15 would protect you for 15 times that amount, or 3 hours and 45 minutes. Of course with sweating and rubbing, and maybe swimming you'd want to reapply anyway.

So an SPF of 85 (another common product) would mean a person who burns in 15 minutes without sun block, could theoretically stay in the sun for 85 times that long, or 21 hours and 15 minutes. If you put on the sunblock before dawn you would be protected until well after midnight. An SPF of 100 actually gives you protection for 25 hours, more than a full day. Even if you live above the arctic circle and never go indoors you don't need that much SPF.

1 comment:

ogre said...

If you burn in 15 minutes without it.

My wife does. I assume that there are people who are fairer still (I've seen and worked with some, so I think it's safe to assume).

So yeah, it's probably overkill, but just maybe for those who are incredibly fair and have burned seriously once or twice, being more than certain might be appealing (but what it probably is is an appeal to the superficial logic that SPF 100 sounds like the whole enchilada...). Or maybe it's for people who will be exposed to solar flares while mountain climbing at high altitudes...