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If SPF 30 filters 96.7% of the sun and SPF 50 only 1.3% more (98%) what is the point?

January 8, 2013

Back in November I wrote a little article entitled “Inconvenient Fact Of The Day:  SPF does NOT just refer to UVB” on the back of a factually confusing press release about Australia’s move to allow the marketing and sale of SPF 50 sunscreens. Well it is now time for the sequel to that article.

The move to SPF 50 has prompted some to look at their SPF 30 products and say ‘oh bugger’. Now this is not because there is anything technically wrong with SPF 30 products, it is more a case of them having been numerically demoted. That and the fact that the standard (or goal posts) have now changed to ensure that UVA and UVB protection is always balanced – before while it was crucial that a product had some UVA protection to reach SPF 30 the ratio between the UVA and UVB had less baring on the result.  That said, almost all brand owners opted for a broad-spectrum analysis anyway as when it comes to sun protection quality is as important as quantity (when talking UV spectrum filtering).

Anyway, that aside it is time to look at and test these numbers.

30 – a reasonable number.  A number that when pre-fixed with the letters “SPF” means something quite useful.  It means that the product in question, when used as directed will give you thirty times more protection than your un-protected skin can offer you.

That means that if you usually get redness appear after 5 minutes in the sun, with this product on you have ‘bought’ yourself up a maximum of 150 minutes.

150 minutes?  That’s 2 ½ hours of pure unadulterated sun baking.

Sounds like a lot but let’s look at it from the other side.

We usually think of sunscreens as either chemical or physical shields that stop the sun from getting through and while we recognize that they must let something through (they are no longer allowed to be called sun blocking agents) we prefer not to think about it because we like to look on the bright side – especially when buying or choosing something.  I mean have you ever seen a product sold like this:

“3% defective”                  VS   the usual                     “97% effective”


“5% fat”                               VS  “95% fat free!”

But playing down or even ignoring the flaws is probably not the best plan when it comes to sun protection as this type of flaw is rather like having a hole in your bucket as these videos illustrate…….

Simulating SPF 30:

Simulating SPF 50: 

The ‘bucket’ with one hole is like the SPF 50 sunscreen. It takes a long time (approx. 250 minutes) for 95% (the maximum possible due to the positioning of the holes and shape of cup) of the water to filter through to the other side.

But every drop of water that makes it through the ‘bucket’ counts – if that were your skin it would be getting wet and you would ‘feel’ it to one or another degree.  You can’t tell me that the only point in this experiment that matters and can be ‘felt’ occurs when 95% of the water has leaked?

The ‘bucket’ with two holes is like the SPF 30 sunscreen. Again it does take some time for the water to leak through but this time it is closer to 150 minutes to become 95% wet (again, 95% wetness is a limit of my experiment and bares no other significance).

If I told you to take your favorite novel and place it less than one or other of the buckets (your choice) for any length of time which one would you choose?

What if instead of a book I told you to take the gamble with your Iphone?

Or a tray of your favorite biscuits?

Now, what if we repeat this exercise with sunscreen and the sun.

The difference in percentage terms between SPF 30 and SPF 50 may be small, insignificant even when you think of it as an exam pass – who but a perfectionist would care if their ‘pass’ were at 95, 96 or 98%?   But we are not talking exam grades we are talking about a known risk and our skin.

Keeping that in mind plus the fact that the average person still uses less than ½ of the recommended application of sunscreen each time they apply and the stakes are higher.  Go back to our bucket experiment and imagine how quickly the water would leak through if we made double the amount of holes.  The WPF 50 would leak in less than 125 seconds – less than the WPF 30 and the WPF 30 would leak through in only 75 seconds – that’s only the same as a standard, well-applied SPF 15 on MED 5 skin.

It seems that while it is easy to visualize and recall that feeling of being slightly damp, wet, very wet and sodden it is terribly difficult for us to see sun exposure in that way – we are either burned or not burned.

Well, I’d like to challenge that and suggest that it is only logical to accept that there are many states in between.  Further I am not convinced that we (the scientific community) fully understand when those shades tip the balance in terms of health-giving and become un-healthy – 50% of our MED?  75%, 90%, 95%.  Will we ever know?

So does the gap matter?

Well, to my mind it does but it only matters that it is considered and weighed up.  Now we all know that a bit of sun is good for us – we are just not sure how big that bit is.  If you are the kind of person that is diligent in sunscreen application and re-application, practices sensible sun behavior, has a naturally strong sun-constitution (darker skin, few moles, healthy and un-broken barrier, younger rather than older) then the chances are that for 99.9% of your life you could get away with any SPF of 15 or above. However, if you are not that person then you might choose to stack as many of the controllable odds as you can in your favour.  That sounds reasonable to me.

SPF 50 products are more efficient at preventing the sun from touching your skin all other things being equal and while the difference in percentage terms looks small, for some people the reality may mean the difference between skin health and decline and only you know if you are one of those people.

Next – SPF 50 VS SPF 30 – but aren’t the extra chemicals going to negate the SPF benefit?  

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