How harmful is the sun in a late afternoon to a run?
I live in a hot and humid climate (Tel Aviv), and usually I run at night. But some days I have plans in the evening, and my only chance to get a run in is by starting at around six, six-thirty (which in summer is about an hour before sunset).
How bad is this, in terms of exposure to the sun? I don't normally apply sunscreen for such runs. By the time I finish (about an hour) the sun is almost setting (or has set).
A final detail: let's say I do this about once a month - run at six in the evening, without applying sunscreen. Is that below the threshold that I should be concerned about?
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Short answer: you should not be too worried about the sun in the later afternoon.
Long answer: there are several risk factors to consider when running in the heat, and if you properly stick to them you will remain healthy and minimize the potential health risks of sun exposure.
Carcinogenic exposure
Scientifically speaking there is no consensus regarding which component(s) of sunlight (UVA/UVB/UVC) are carcinogenic. Since UVA/UVB/UVC proportions can change drastically based on the time of day, that means we really have no way of knowing what time of day presents the highest risk to our body as far as skin damage and cancer are concerned. Because of this, you should not consider UV exposure when deciding what time to run.
That being said, whatever time you do choose to run you should make sure to take adequate precautions by applying sun screen before leaving. If the sun is out--even if it isn't very hot--you are at risk and need to use sun screen.
Heat stroke
To avoid heat stroke you want to stay properly hydrated. That means drinking water before, during, and after your exercise. Listen to your body and drink as needed. If you are doing a prolonged exercise, consider bringing a sports drink like Gatorade to help replenish the salts and minerals that your body is craving.
Some additional pointers to avoid heat stroke are:
Run in shade
Wear light clothing
Splash water on your face
Don't over-exert yourself
Exhaustion
If you are accustomed to running in fair conditions and you transition suddenly to hot weather exercise, you will notice that it is much harder to keep up with your previous pace. Running in hot weather takes more effort from your body and as a result you cannot expect to perform at your peak the way you would in fairer conditions. Keep the heat in mind when doing exercise and make sure not to exhaust yourself.
Summary
Taking all of this into account, you can see that there is nothing intrinsically dangerous about running in the heat... so long as you take the proper precautions. Therefore, whether you choose to do morning, afternoon, or evening runs, as long as you plan accordingly you will do fine.
I'd rather do this as a comment on Moses' answer, but I don't have the necessary SE seniority to do so.
While there may not be consensus on what components of UV radiation are carcinogenic, it would be far more useful to present all the opinions so you can better draw your own conclusions. I myself am unaware of dissenting opinions to the following; UVA is carcinogenic, UVB is associated with Vitamin D development and Vitamin D is believed by some to be anti-carcinogenic. Others of course believe this to be hogwash, but since those people are either militant skeptics or associated with the pharmaceutical industry (there's not patent money in selling Vitamin D, and it's freely available to anyone who gets sunlight) their their claims I would think are worth less than those suggesting Vitamin D is very beneficial. Also, it's turned out that people working indoors but near windows have higher skin cancer rates than those who are outdoors all the time, apparently because glass blocks UVB but not UVA.
Some European regulatory body (I don't live in Europe so I'm not familiar with the organisations t here) has approved sun screen that only blocks UVA and lets UVB through on those grounds. So absent some very convincing information otherwise, I would lean towards believing that UVA is responsible for skin cancer and UVB helps protect against it (and other types of cancer). I have no idea about different types of UV exposure according to time of day, but I would go for some of that European UVA blocking sun screen if I were regularly exposed to a lot of sun.
Hydration is also contested, and some argue that overhydration is just as bad as dehydration (it is, either hypernatremia or hyponatremia is bad for you). Waterlogged by Timothy Noakes (published by Human Kinetics) goes into this issue, and if you're planning on drinking a lot even if you're not thirsty, you should probably look into what the consequences of doing so would be. I would personally not go any further than keeping water with me to drink only when I'm thirsty. Your call of course based on which researchers you believe more.
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