542. What does the SPF on sun screens mean? - RC
Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can't see them when they're on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen's SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light's energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.