That's highly dependent on humidity. If it's dry your body has means to cool itself through perspiration, but if the air is saturated in water vapor, temperatures as low as 35 or 36C can be deadly.
It reminds me a bit of when I would go on expeditions in the southwest deserts. I am constantly drinking prodigious quantities of water throughout the day but rarely need to urinate. But with more sweat than the southwest deserts, which have extremely dry air (you mostly don't sweat at all because it has no liquid phase).
As long as people drink enough fluid, they should be fine.
Then I visited the southwest, and noticed that I was perfectly comfortable on 100F days. Then I noticed that I wasn't slimy, either. And then it all clicked into place: the basic physics explanation doesn't apply if the air is already saturated.