We've had a fair number of notes from readers confused about information on sun protection factor numbers in the story "Burned Up Over Sunscreen" and a related sunscreen quiz (July 5). Only our government could have created a more confusing SPF rating system. We'll try to explain here with a little help from the people who know best, experts at the American Academy of Dermatology.
We stated that SPF 30 does not represent a doubling of sun protection over SPF 15. Several readers correctly pointed out that if SPF 15 provides 93% protection and SPF 30 provides 97%, that is a doubling of protection--only 3% of the sun's rays are getting through, compared to 7%.
To try to clarify this, we quote the academy's "Facts About Sunscreens Questions & Answers."
"Does SPF 30 have twice as much sun protection as SPF 15?
"SPF protection does not actually increase proportionately with a designated SPF number. In higher SPFs, such as an SPF 30, 97% of sunburning rays are absorbed, while an SPF of 15 indicates 93% absorption and an SPF of 2 equals 50% absorption."