This is a helpful article from the NY Times (May 14, 2009) on sunscreens and Sun Protective Factor. Sunscreen use is a religion to some. There is no evidence that it prevents melanoma when used in adults; but it may prevent some nonmelanoma skin cancers and may help some "at-risk" individuals. Sunscreens are a huge market for the PIGs (pharmaceutical industry giants) -- so be an informed consumer! This article from the NY Times should make you feel better about sticking to a sunscreen with a lower SPF.
"A sunscreen’s SPF, or sun protection factor, measures how much the product shields the sun’s shorter-wave ultraviolet B rays, known as UVB radiation, which can cause sunburn. It used to be that SPF topped out at 30. No more. These days, a race is on among sunscreen makers to create the highest SPF that R&D can buy.
"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). Thus, SPF 30 is all one really needs to prevent burning.
"A sunscreen’s SPF number is calculated by comparing the time needed for a person to burn unprotected with how long it takes for that person to burn wearing sunscreen. So a person who turns red after 20 minutes of unprotected sun exposure is theoretically protected 15 times longer if they adequately apply SPF 15. Because a lot of sunscreens rub off or don’t stay put, dermatologists advise reapplication every two hours or after swimming or sweating.
Here is the full article: Confused by SPF.
(Photos from the Times articles)