Zealous advertisers advocate sun­screen products with sun protection factors (SPFs) ranging from 15 to 100+. But how much of this is scientifically validated, and how much is spin-doctoring taking advantage of fear­ful and gullible consumers? Is an SPF of 100 really twice as good as one of 50, or even 30 or 15 for that matter? The short answer is no. The numbers do not compute that simply, as we will see.