Sunscreens usually work by using either a physical sun filter, such as talc, titanium oxide or zinc oxide, or chemicals containing active ingredients such as methoxycinnamate, p-aminobenzoic acid and benzophenone that absorb certain UV frequencies while allowing others to pass. Sunscreens normally indicate whether they protect against UVA, UVB or both.
Sunscreens also have a sun protection factor (SPF), which indicates how many times more than normal (no protection) someone would take to burn when using the lotion. For example, if you can normally stay in the sun for 20 minutes without burning, you would be protected for 200 minutes with a lotion with an SPF of 10. Nevertheless, the SPF only indicates the amount of protection from UVB, and not UVA.In practice, however, many users find that the effectiveness of a sunscreen wears off well before the calculated time, thus requiring generous amounts to be continuously applied for protection. This may seduce sunbathers into staying in the sun for much longer than is normally considered wise.
There is also a growing concern that some of the chemicals used in these preparations may be harmful and may, in fact, increase a user's risk of developing malignant melanoma (BMJ, 1996; 312: 1612-3). Such concerns have caused sunscreen manufacturers to discontinue the use of certain ingredients, such as 5-methoxypsoralen (BMJ, 1979; 3 Nov: 1144).