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# How to measure combined clothing and sunscreen?

2018-03-13 17:03:53

As I understand it, you can't combine multiple suncreens to combine their SPF values. This makes sense as they all work on the same concepts and your skin can't simply "hold more" because its two different brands.

But how do you measure combined sunscreen and clothing?

Or for that matter, combined layers of clothing?

My instinct is that you would multiply them:

Clothing: UPF 3, passes 33% of UV. C=0.33

Sunscreen: SPF 5, passes 20% of UV. S=0.20

Total = C * S = 0.066, or SPF 15

Can anyone confirm this?

Bonus question: What about tan + sunscreen? Would they add, multiply, or just be the maximum of the two? I'm guessing it's not the last because black skin with a natural SPF of around 13 still benefits significantly from an SPF 15 sunscreen.

Purely theoretical (in practice sunscreen isn't as strong as it says on the bottle to begin with) you multiply it.

If 1/3rd of the UV photons make it through the first layer, and of those that made it through 1/5th

• Purely theoretical (in practice sunscreen isn't as strong as it says on the bottle to begin with) you multiply it.

If 1/3rd of the UV photons make it through the first layer, and of those that made it through 1/5th get through the second layer as well, then 1/15th of the original photons made it through both layers, and 3*5=15.

It's the same for tan, that's melanin, which is a pigment that's great at absorbing light. That's why it looks dark. Any light that gets absorbed does not get through to the living layer of the skin where it can be harmful, because it would need to be not absorbed to do that, so the same idea as for clothing and sunblock applies. So with three layers of protection the total factor would then be 3*5*13(=15*13=10*13+5*13=130+65)=195.

Furthermore it does not matter for the protection factor whether the protecting layer reflects light away, which makes it look light or white or in special cases like a mirror, or absorbs it, which makes it look dark or black. As

2018-03-13 19:43:05