What is SPF?
For example, if you usually start to burn after 10 minutes in the sun,
an SPF 10 sunscreen would theoretically let you stay out for 100
minutes before getting red. Of course, the protection factor is
dependent on using generous amounts of the product and re-applying it
after swimming, sweating, or prolonged exposure.
wearing a sunblock of at least SPF 15 every day. Applied adequately, a
lotion with SPF 15 stops about 92 percent of UVB (U-V-B) radiation.
SPFs higher than 15 give slightly more protection, but the gains are
small and incremental. An SPF 30 keeps out about 96.7 (ninety-six point
seven) percent of UVB rays, while a product with SPF 40 absorbs around
97.5 (ninety-seven point five) percent.
However, the SPF rating refers
only to the amount of protection from UVB radiation, the wavelength
most likely to burn you and most often associated with skin cancer. SPF
doesn't address the amount of UVA (U-V-A) protection a given product
may offer. To guard against UVA rays and the premature aging they can
cause, a sunscreen should also contain zinc oxide, titanium dioxide
(dye-AWK-side), or avobenzone (av-oh-BEHN-zone).