Confused by Sky-High SPFs? Take a Number

The difference in UVB protection between an SPF 100 and SPF 50 is marginal. Far from offering double the blockage, SPF 100 blocks 99 percent of UVB rays, while SPF 50 blocks 98 percent. (SPF 30, that old-timer, holds its own, deflecting 96.7 percent).

Oh marketing!  How undone you make things!

Consumers should worry more about wearing enough sunscreen, several doctors said, rather than how high their SPF is.

Skimp and you lose. To get the SPF advertised, you must use a full shot glass on your body. That’s an ounce, which means a three-ounce tube should last, at most, a few outings.

Ohhh.. I only sunscreen my face.  I wonder?

“It turns out that if you apply half the amount, you get the protection of only the square root of the SPF,” said Dr. Darrell S. Rigel, a clinical professor of dermatology at New York University, who has done efficacy testing for Johnson & Johnson and the Procter & Gamble Company.

So applying a half-ounce of SPF 70 will not give you the protection of SPF 35, but 8.4, Dr. Rigel said.

Or, “the higher the SPF, the more rapidly the protection falls off with under-application,” said Dr. Gilchrest, who consults for Schering-Plough Corporation, which makes Coppertone.

Eee!