To measure what SPF factor is right for your skin, measure the length of time it takes for your skin to start burning. eg 10 min. then multiply the SPF rating EG SPF 15 by the 10mins. = 150min - this means that by using a SPF 15 you would be protected from sun burn for approx. 2.5hr, This measure is a rough figure as there are other factors that also influence as the light intensity, skin type. The SPF is a measure of protection from UVB rays, UVA rays are higher in the spectrum and are more dangerous to the skin.