Mixing Sunscreens

mixing sunscreens

I was asked an interesting question on Twitter today.   Not Anita asked me “If I apply a SPF50 suncream to my face, then a SPF15 mineral foundation, what SPF protection do I have?”

It is an interesting question when you think about.  People are mixing sunscreens every day.  Do the two products blend and give you an SPF half way between the two, of 32.5. Or does the second one to be applied cancel out the first one, which would leave her with an SPF of only 15. Or does the bigger number trump the lower one?

My reply was that in theory the two sunscreens should be additive, so you would end up with an SPF of 65. That was a good enough answer for Twitter. But I thought it was interesting enough to do a blog post going into a bit more detail. Because when I said in theory, I was referring to a very specific theory.

The theory in question is the Beer Lambert Law, and it is a very old theory. As is often the case, the name is misleading. It wasn’t discovered by somebody called Beer Lambert. It wasn’t even discovered by two people called Beer and Lambert. it was in fact originally discovered way back in 1729 by Pierre Bougeur. Lambert simply quoted it later. Beer added a bit more detail in 1852. So what does this law state? It is simply that the amount of light absorbed by a material is proportional to its thickness, its absorbency and its concentration.

This might sound obvious, but in science being obvious isn’t enough. You need to test it out. And if possible, you need an equation to describe it. This is certainly possible in this case, and as equations go it is a pretty simple one. But being simple doesn’t stop it being very useful. The Beer Lambert relationship enabled chemists to work out a great deal of information simply by passing light through samples. In the twentieth century this became mechanised and systematic, and machines were built called spectrophotometers that could be used to do a lot of testing very quickly and easily. It became practical to do research into basic chemistry at a speed that would have astonished chemists from an earlier era.

To give one example, spectrophotometers could identify chemicals that had particularly strong absorption in the UV range. It would have taken a very long time to test every potential sunscreen ingredient by applying it to the skin of volunteers and wait for them to see how well it protected them. Placing a sample in the spectrophotometer gave you an answer in seconds.

But the Beer Lambert las also meant that once you knew the amount of UV light a particular agent absorbed, you could work out what concentration should work in the formulation – again saving a great deal of legwork.

And last of all, knowing the theory you can answer Not Anita’s question. All that from really quite a simple theory. It is surprising sometimes just how useful science can be.

Thanks to Bill Branson via Wikipedia Commons for releasing the copyright to his photo of a woman applying a sunscreen.


7 thoughts on “Mixing Sunscreens”

  1. Hmmmmmm bit too simple Colin. What if the two formulations are incompatible (a cationic plus an anionic emulsion)? That would certainly disrupt any film on the skin and change things completely! And I am sure there are many other factors that would make Beer Lambert unsuitable. Sorry, just saying!

    1. All theories are too simple. The whole point of them is to tease out of reality some useful underlying truth. If Beer Lambert was the whole story in sunscreens you’d be able to work out SPFs on a spectrophotometer.

      But in this particular case Not Anita did specify that the second product was mineral makeup so that shouldn’t disrupt the emulsion of the first product. And I don’t think eumulsions remain as emulsions on the skin for very long, so your example incompatibility is quite an unlikely one. And even if it did disrupt the emulsion, so long as the chromophors are already distributed evenly they should still possess their UV absorbing power.

      But yes you are right, things are bit more complex than I let on in the post. Which is just as well for us – we wouldn’t have jobs if it was too simple.

  2. Colin, many thanks for your very informative blog. I’ve been trying to find your twitter account handle but have come up with nowt… can you put it somewhere prominent?

  3. Thanks Rebecca. I had a link on there ages ago, but I seem to have misplaced it. I have just added a new one.

  4. Thank you for this informative blog, Colin. I enjoy reading it for quite some time now.

    I am wondering about mixing/layering sunscreens on skin since I read this post by the “The BeautyBrains”: http://thebeautybrains.com/2012/08/06/what-happens-when-you-mix-sunscreens/

    In that post they explain that by mixing sunscreen one gets the divided-by-number of products SPF-factor (for example: lotion 50SPF + foundation 15SPF –> 32.5SPF). That seems logical when actually mixing the fluids but in that post it sounds that even layering on skin doesn’t make much difference (“best you can hope for”).

    If that is true, every product with a lesser SPF diminishes the protection given by a high SPF-product, which means that layering skincare, sunscreen and make-up can actually reduce the intended effectivity.

    Up until then I assumed the more layers of SPF, the better and what you describe above supports this, if I understand correctly.

    I guess, layering different products with the same SPF is probably the way to go, but that’s often not very realistic, especially if you need SPF 30 protection or higher.

    So, what’s the deal? Does applying products on skin equals mixing products? How do I layer products for maximal benefit in general? What’s the best way to apply different products with SPF?

  5. Rodolfo Baraldini

    There are many antagonistic relations between different sun filters.
    Mixing 2 sun filter systems, not formulated in order to be mixed toghether, may produce many undesired results.
    A SPF lower than the sum of the SPF, but some time also lower then the major of the 2 SPF.
    A worst photostability.
    A worst photoreactivity.

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