[inglor_cz]

That depends on your age and cardiovascular health. Overexertion in heat can be deadly.

I am 43, I have no chronic disease, but I would be afraid of more than just sweating if I went on a biking trip in that temperature. Sometimes I feel unwell even when walking in such heat.

[wott]

It is much easier to ride a bike than to walk in the heat. Because you create your own wind.

If I stay at home and walk around I have troubles breathing when it's over 35°C, I have been several times on the verge of fainting when it lasts a bit too long over that temperature.

But I don't have a problem biking at that temperature or above. I've ridden for hours in temperatures near 40°C with no shade. As long as I get plenty of air, it's OK: it lowers the temperatures and I can breath. I mean, I prefer if it is 15°C lower :-), but it feels much better than walking.

Now in such circumstances you'd better not get caught in a climb, when your speed drop under say, 8 mph, because then you don't get any wind, while being on max effort. That's horrible. It happened to me once in a small mountain pass, on a road which looked like it had been painted black to make things worse. I had to climb down the sharp slope on the road side as I could, to find the shade of the few bushes which were around and stay there a few minutes, because I felt my temperature was rising way too much. Never again.

[Kafkish]

Then the problem is not the heat, but your fitness level, which regular biking, jogging will improve. It did for me. After about 20 years of no exercise, just sitting in front of a computer, I decided last November to start exercising (jogging. Bad knees make biking a bad idea)

My 1st day out jogging, I could only go 0.5 miles non-stop before I almost passed out. Today I can do 5.5 miles non-stop. My cardiovascular system is in the best shape it has been in a long time. And I'm almost 60.

[inglor_cz]

I have quite a lot of exercise, but I have hard time tolerating high temperatures in general, and it has been getting worse with age. I should have been born somewhere in Siberia.

[jseban]

I simply just want to do other sports and want to have the choice of what I do for exercise, and have maximum freedom to how I spend my (free) time and energy, and not use most of it on commuting.