When shopping for sunscreens, consumers will no doubt notice each bottle lists its SPF number. Numbers tend to be as low as 4 or as high as 100. But what is SPF? And what does it have to do with protecting the skin from the sun's harmful rays? According to the Skin Cancer Foundation, SPF, which stands for sun protection factor, is a measure of a sunscreen's ability to prevent the skin against ultraviolet B, or UVB, rays from the sun. The Skin Cancer Foundation notes that SPF works in a way that might surprise even the most devoted of sun worshippers. If it takes 20 minutes for unprotected skin to start redding, then a sunscreen with an SPF of 15 will theoretically prevent redding for 15 times longer than that — or about five hours. While that's impressive, the Skin Cancer Foundation notes that the SPF model does spark some concern. For example, no SPF sunscreen, regardless of its number, should be expected to remain effective for longer than two hours without reapplication. In addition, reddening of the skin is a reaction to UVB rays alone and indicates little about any damage caused by ultraviolet A, or UVA, rays. To protect themselves against both UVB and UVA rays, the Skin Cancer Foundation recommends consumers use only broad-spectrum sunscreens with an SPF of 15 or higher. Wearing protective clothing, staying out of the sun between the hours of 10 a.m. and 4 p.m. and reapplying sunscreen after sweating or going into the water are other ways to protect the skin from the sun's harmful rays.