Sunday, 14 September 2008

Another grumpy moment...

Travelling in high altitudes, in warm climates, we are regularly told to use sun block.

Fair enough.

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. *

Simple, no?

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.


  1. Living the dream!!! I really like your observations on exagerations by guides, it gets to me too. Suz

  2. a question fir the chemist - if zinc oxide is one of the ingredients in sun block (no matter the SPF number) is it more effective at blocking harmful rays?

  3. Zinc oxide physically blocks the rays while the chemical variants rely on the chemicals absorbing the rays. ZnO tends to be safer and more effective but not always the most pretty.

  4. thanks for that, lee. I'd heard that the higher numbers didn't much matter - I've experienced that they don't much matter - but I didn't understand the "why."

  5. Reapplying frequently would have to be important...

  6. Thanks for all that info - I feel better now! Love all the photos- Maus

  7. Wow Lee and Margaret, I just caught up with all your pics of your travel again and it's just awesome. What great photos and to imagine you guys are experiencing it all!!!
    I'm so envious. Wow!
    So tell me, are you guys gaining weight or loosing it during this holiday? LOL Feeling fitter from walking alot? etc etc.


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