The Sun Protection Factor or SPF value is defined as the dose of UVR ( ultraviolet radiation ) required to produce one MED ( minimal erythema dose ) on protected skin after the application of 2 mg/cm.sq of product divided by the UVR required to produce one MED on unprotected skin.The SPF usually ranges from 2 ( minimal protection ) to 30+ ( higest protection ). SPF 2 gives 50% protection, SPF 15 gives 93% protection and SPF 34 gives 97% protection. Some have recommended that SPF be changed to Sunburn Protection factor,because it is a misconception that application of sunscreen will offer protection against sun-induced skin damage.The use of higher SPF sun screen appears to increase the duration of recreational sun exposure,giving individuals a false sense of security.
Although the general public is starting to wear sunscreen on a daily basis,studies have shown that they don not use it correctly.If a person uses only half of the recommended amount of sunscreen,the SPF can be reduced by as much as a power of two ( therefore,an SPF 15 would become SPF 4 and a SPF 30 would be SPF 8 ). The use of higher SPF can partially compensate for underapplication. For a sunscreen to be effective,a layer of 0.5mm should be applied.This is about a quarter of most standard sized bottles of sunscreen when applied to entire body. Reapplication every 20-90 minutes is necessary to keep most sunscreen effective, depending on the formulation and the activity level of the person. The American Academy Of Dermatologist recommends, regardless of skin type, a broad spectrum sunscreen with SPF of atleast 15 to be worn year round.
Water-resistance labeling indicates that the SPF is maintained after two 20 minute immersion in water,ie 40 minutes. Very water-resistant labeling can be used if the SPF is to be maintained after four 20 minute immersions cycles ie is 80 minutes .Therefore, neither of these labels translates into protection that can last for the entire afternoon at the pool with one application.