The acronym SPF, Solar Protection Factor , is a measure that indicates through a number as a sunscreen can protect the skin from solar radiation, particularly UVB rays, which cause sunburn, skin damage and can contribute to cancer of the skin. The SPF number is indicated on all products that contain sunscreens and although there is no standard definition for measuring its effectiveness, a sunscreen with a higher sun protection factor, allows sun exposure with greater security than a with low sun protection factor.
Another important thing to remember is that it is not linear as the number, a protection factor 15 absorbs about 93 percent of harmful rays, while an SPF 30 absorbs about 97 per cent. Of course these numbers are influenced by many other variables such as the time zone in which you are exposed to the sun, the dose of sunscreen you use, the parts of the body that are exposed to the sun and the number of times the sunscreen is applied.
For if one is exposed during times when the concentration of UVA and UVB rays is very high duration of protection decreases, as well as if you apply a little sunscreen or after bathing or after sweating is not reapplied protection.