May is Skin Cancer Awareness month, but I would like to start May with an episode on the lighter side, with a very interesting question a viewer asked: does bright moonlight emit enough UVA light to cause skin damage? And if so, should you wear sunscreen at night?
The answer is definitely no, and I’ll explain why IMMEDIATELY so you don’t wake up tomorrow hearing ads for “Moonscreen” with “MPF” ratings... which of course stands for Moon Protection Factor!
First of all, there is no such thing as “moonlight”… The moon emits no light of its own… it merely REFLECTS a very tiny, inconsequential part of the sunlight that hits it. When that sunlight is reflected to the earth, it actually can hit your skin… but it can’t damage it.
So why am I so sure that that you don’t need “Moonscreen”?
I’ll apologize in advance for the math, but here goes:
The diameter of the sun is slightly greater than 400 times the diameter of the moon. So if the moon were a perfectly reflective mirror reflecting 100% of the sunlight that hits it, it could only reflect 1/400th or ¼ of 1% of the sunlight.
But the moon’s surface is a very poor reflector, instead absorbing most of the visible and UV light that hits it… reflecting only 7 to 12% of visible light, and only about 0.7% of UV light… that’s less than 1% of the UV light.
So between the small size of the moon and it’s poor reflectance, only about 1/40 of 1% of the sun’s UV is reflected to the earth... Said another way, 99.98% of the sun’s UV does NOT get reflected. It's just like wearing a sunscreen that blocks 99.98% of the sun. I wish every sunscreen worked that well!
In fact, if everyone wore a sunscreen all the time that blocked 99.98% of the suns UV, we’d all be in pretty good shape with virtually no photo-aging and almost no sun induced skin cancer.
Oh… One more thing… that inconsequential UV that does reflect to earth is almost certainly completely absorbed by the atmosphere.
So the next time someone tries to sell you moonscreen… Just say no!
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