SPF is an acronym for Sun Protection Factor. SPF is actually a measure of protection from amount of UV-B exposure and it is not meant to help you determine duration of exposure. Sunbathers often assume that they get twice as much protection from SPF 100 sunscreen as from SPF 50. In reality, the extra protection is negligible. Properly applied SPF 15 blocks 93% of UV-B rays; SPF 50 sunscreen blocks 98 percent of sunburn rays. Dermatologists recommend using a SPF15 or SPF30 sunscreen. Higher SPFs do not actually give much more protection.
Sunblock and sunscreen block the rays from the sun being absorbed by
our skin. Ninety five percent of the UV (Ultra violet) energy
hitting the earth’s surface is UV-A. The other 5% is UV-B. Most of
UV-B radiation is absorbed by our atmosphere. UV-A penetrates the
skin more deeply than UV-B. However, UV-B causes more problems
generally associated with exposure to the sun’s rays, like skin
cancer, aging, and DNA damage. UV-B waves are primarily responsible
for sunburned skin. Scientists know less about the dangers of UV-A
radiation, but the general consensus is that it is less obvious than
UV-B damage, but possibly more serious.
Sunscreens generally only block UV-B rays, and not UV-A. To get
broad spectrum protection, sunscreen must contain both the organic
compounds associated with UV-B absorption and an inorganic
associated with UV-A reflection.
Sunburn reactions usually begin about 4 hours after exposure and
peak between 8-24 hours, so what we feel while being exposed is just