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Often, while hiking, my companions and I treat the very early/late sun as being less harmful (as far as skin cancer and UV radiation are concerned). For example, we might not apply sunscreen immediately after waking, but only after an hour or so, or skip putting on another round of sunscreen if it's relatively late (say, an hour before sunset).

My question is- is the sun less dangerous at these times? Is this bad for us? We don't backpack/hike that often, so it's not a very common occurrence, and it hasn't led to sunburn as far as I know. But I would like to know if there is gradual damage being caused to my skin.

I asked this related question on the Fitness Stack Exchange, but it doesn't seem to be getting the attention (or answers) I'd like.

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2 Answers 2

up vote 12 down vote accepted

Around sunrise and sunset, the sun is much less intense. You would get around 5 times less intensity in the first or last hour of sunlight than in the middle of the day. Here is a graph of this effect (It's from a paper, though the paper itself is behind a paywall), and another one which also shows the effect of latitude.

Therefore, while you can’t say that the sun at those times has no effect on the skin, it is much less dangerous than in the middle of the day. Do you put on sunscreen if you are outside for 10 minutes in the middle of the day? If not, then you’re getting about the same damage in 1 hour around sunset.

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And just to add - the main reason for this is the amount of atmosphere the sun's rays travel through before they get to you. When the sun is directly overhead you have around 60 miles of atmosphere above you (based on the usual estimate of thickness of atmosphere) but as the sun sets you can have hundreds of miles of atmosphere directly between you and the sun. Also, more of the rays' path is at lower altitude, where air is denser, attenuating the rays even more. –  Rory Alsop Jun 29 '12 at 13:12
    
Your answer cheers me up! But does the atmosphere dilute all the harmful radiation? Conceivably, the atmosphere could cause a general dilution of radiation while still allowing a relatively high proportion of "harmful" rays to pass. (For example, diluting 90% of UVB rays while only 10% of (possibly more carcinogenic) UVA rays) –  Eyal Jul 2 '12 at 11:06
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@Eyal You probably know that the sun looks redder at sunset. This is because when then sun passes through more atmosphere (as Rory Aslop mentioned), Rayleigh scattering, which is inversely proportional to the fourth power of wavelength, becomes more important. This means that shorter wavelengths (blue) get scattered more than longer ones (red). Since UV light is of shorter wavelength than visible light, it gets scattered even more. The situation is therefore actually even better for you than what those graphs show. –  Big General Jul 3 '12 at 12:39
    
Just be careful at the higher altitudes. A couple of hours before sunset, above 2000m can give you second degree burns (I have seen it happen to the ears of a girl; and it become apparent after the fact, not during the exposure). –  Vorac May 16 '13 at 6:37
    
Also, more important than the time from sunrise or till sunset is the altitude of the sun, like suggested in the second link. This means that when you are used to consider the last hour of sun safe in your homecountry, it does not have to apply for other locations, especially closer to the equator ones were the sun rises and sets a lot faster and therefore is longer at a greater altitude. –  Paul Paulsen May 25 at 21:51
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Let's not get carried away:

http://www.cancerresearchuk.org/cancer-info/cancerstats/types/skin/mortality/uk-skin-cancer-mortality-statistics

Gives a picture of 3-5 deaths per 100,000 per year from skin cancer.

Other studies show that getting a bad sunburn at any point in your life increases your long term risk substantially.

For comparison, car accident fatalities run 9-12 deaths per 100,000/year.

And heart disease runs 800/100,000.

Rule of thumb: Do what it takes to not get a sunburn. Otherwise, enjoy the sun.

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I fail to see how this answers the specific question asked. It reads more like a generic public health advisory about skin cancer. –  Russell Steen Apr 1 '13 at 16:26
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