Sun Protection Factors (SPF) are arrived at by testing in laboratory conditions. Volunteers have their skin exposed to artificial UV radiation until they experience burn and the amount of radiation they receive is measured. The volunteer then has the sunscreen applied to their skin, is again exposed to artificial UV radiation and the amount of radiation required to achieve the same damage is measured. The SPF rating is ascertained by dividing the amount of radiation required to burn with sunscreen, by the amount required to burn without.
Basically, an SPF rating is an indication of how much more UVB radiation you can be exposed to with sunscreen than without before you burn. So, if an individual can withstand ten minutes exposed to UV radiation at a particular level before burning, an SPF30 sunscreen would theoretically enable them to spend 300 minutes exposed to the same radiation before burning
SPF ratings are the subject of much discussion and debate due to a number of omissions, weaknesses and grey areas. Among them;
- SPF ratings are calculated based on an amount of radiation that is controlled and measured, NOT time spent in the sun. As the amount of radiation from the sun varies due to geographic location, weather conditions and even time of day, it is impossible to say with any certainty how much, or little, more exposure sunscreen will actually protect against.
- External factors impact upon an individual’s level of protection from radiation, such as skin type, amount and frequency of application and re-application of sunscreen, activities of the individual (swimming, sweating, etc) and the amount of sunscreen actually absorbed by the skin.
- SPF ratings can not, by design, measure or refer to UVA radiation, as the rating is achieved by the observation of surface skin damage, or sunburn, in which UVA radiation does not play a part.
However, the most concerning research on sunscreen use and effectiveness indicates that a poor understanding of how much sunscreen to use, and when to apply it, is greatly hampering the quality of sun protection being provided.
In the next post we will be discussing the proper of sunscreen and factors that impact upon sun protection effectiveness.
References:
http://www.fda.gov/AboutFDA/CentersOffices/CDER/ucm106351.htm
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