SPF stands for sun protection factor, and the corresponding number (say 30) is supposed to indicate that you can stay in the for thirty times as long and get the same level of sunburn as if you did not apply the sunscreen. However, the SPF rating can be applicable to either UVB and / or UVA (both of which are now being recognized as harmful rays that can potentially cause cancer), and also the amount of sunscreen applied by most people is often inadequate to reflect the protection as stated by the label. A good sunscreen should have a SPF of around 50, and offer both UVA and UVB protection.
