Sunscreens and SPF

Sunscreens can be both chemical absorbers and physical blockers. Chemical absorbers work by absorbing UV radiation and can be further differentiated by the type of radiation they absorb, UVA or UVB, or both. Physical blockers work by reflecting or scattering the UV radiation (DermNet NZ 2009c).

Sun protection factor is a worldwide system, stating how much protection a sunscreen provides, applied to the skin at a thickness of 2 mg/cm². The test works out how much UV radiation (mostly UVB) it takes to cause barely detectable sunburn on a given person with and without sunscreen applied. For example, if it takes 10 minutes to burn without a sunscreen and 100 minutes to burn with a sunscreen, then the SPF of that sunscreen is 10 (100/10). There is no recognised international measurement of UVA sun protection factors (DermNet NZ, 2009c). In the UK, a star system is used. Sunscreens can have anywhere from 0 to 5 stars. The number of stars is not an absolute measure and depends on how much UVB protection the sunscreen offers (Cancer Research UK, 2009f). In due course, a new EU symbol is being introduced to convey the UVA and UVB protection offered by some UV protection products. A sunscreen with an SPF of 15 provides greater than 93% protection against UVB. Protection against UVB is increased to 97% with SPF of 30₊. The difference between an SPF 15 and an SPF 30 sunscreen may not have a noticeable difference in actual use as the effectiveness of a sunscreen has more to do with how much of it is applied, how often it is applied, whether the person is sweating heavily or being exposed to water. Hence, a sunscreen with SPF15+ should provide adequate protection as long as it is being used correctly. However, most people apply their sunscreen at about one-third the thickness used for testing; they fail to apply it to all exposed areas of skin; and they forget to reapply it every couple of hours. Therefore, the actual protection may be a lot less than the tests indicate. The BBC Weather internet pages provide guidance on the UV Index provided with weather reports and also give updated information on UV Index, offering a basis for guiding to protective behaviour (BBC Weather Centre, 2009).