Sunday, 14 September 2008
Another grumpy moment...
Travelling in high altitudes, in warm climates, we are regularly told to use sun block.
But there gets to be a status thing happening over what strength sun block is best. It all gets a bit silly and I decided that it was time to explain what the SPF numbers really mean.
Sun blocks are tested by creating a 0.1mm thick film of the cream, shining UV light of one particular wavelength through this film and recording how much of this light passes through the film. The calculation is then 100 divided by the percent of light that passes through the film, giving you your SPF. *
So, an SPF 25 cream permits 4% (100/4 = 25) to pass and blocks 96% of the UV light. Likewise, an SPF 50 cream blocks 98% of the light. If applied in a uniform 0.1mm thick film, of course.
So the doubling of the SP factor in this example equates to only a 2% increase in the amount of UV light blocked by the cream. It does not mean that it is twice as good.
Even if it is twice as expensive.
Next week I might pick on the current travel 'necessity', hand sanitiser gels.
* Yes, sometimes they test sun blocks in situ but it is rarely done and tricky to do with any sort of accuracy.
Posted by Lee at 07:36