# How can sunlight cause skin cancer? [closed]

If the atmosphere absorbs the UV wavelengths, then what’s left is visible, IR and maybe a really small amount of UV that transmits it’s way to earth. IR and visible are non ionising, so is it the case that such a small amount of UV irradiation can ionise skin cells? And cause cancer?

## closed as off-topic by StephenG, The Photon, GiorgioP, John Rennie, ZeroTheHeroApr 21 at 0:13

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• I'm voting to close this question as off-topic because this is a medical question. It is also well documented and discussed online and would breach our guidelines on doing basic prior research to questions. – StephenG Apr 19 at 17:11
• UV definitely reaches the surface of the earth. Also, UV doesn't "ionize" skin cells ... it breaks molecular bonds in DNA. Over time, the biological functions that fix this damage can't fix all of the damage, and some skin cells turn cancerous as a result. – David White Apr 19 at 17:27
• @David White This is an answer not a comment... – my2cts Apr 19 at 17:30
• @StephenG It is not entirely a medical question - the OP is interested in the physics part of it. – KV18 Apr 19 at 17:35

The atmosphere does not absorb all the UV from sunlight. At the top of the atmosphere about $$10\%$$ of the Sun's power arrives as UV; at the surface, with the Sun directly overhead and a clear sky, it is about $$3\%$$, with shorter wavelengths getting blocked more completely. The solar flux at the surface (again, with the Sun directly overhead and a clear sky) can be about $$1\,\mathrm{kW/m^2}$$, so the UV flux at the surface can be up to $$30\,\mathrm{W/m^2}$$.