DermTV - Do You Need Sun Protection for Moon Light [DermTV.com Epi #308]


Uploaded by dermTVdotcom on 02.05.2011

Transcript:
[music] Hello, I’m Dr. Neal Schultz
[pause]
and welcome to DermTV.
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!