So, where I live, summers are really hot, and we - a couple of million of people here - are really adapted. i.e. we are here well into the boreal winter, and w

So, where I live, summers are really hot, and we - a couple of million of people here - are really adapted. i.e. we are here well into the boreal winter, and we had a hefy 35° at midday, today. Well, then running 10-15 km. in 60 min. at 45-48° (humidity at 80-90%), is quite possible, and yeah, you end up not having any health issues. Let's take a look how it actually felt to me - several times - to run at 48° and 90% of humidity - actual data from my cellphone - for those who have not tried it. First you go out to the street, yeah, it's not the Sahara, it is just your neighborhood. You hit the street and the heat hits you, heavily, it is like getting yourself into a body of water, but it is hot air, really hot air. Then magic happens, you get entirely covered by sweat, full-body, your legs (hopefully you're in shorts), arms, everything under clothes is fastly covered in sweat. The first minutes you just take a very slow pace, almost walking fast through the heat, your heart starts to rapidly fire if you try to go faster than this, so you take it slowly, maybe a couple of hundred of meters, at some point the sweat (probably? could be the skin), start to make its magic: you stop feeling the intense heat and start to feel how the sweats cools itself while you are moving through the air. Yep, heavy sweat over your body doesn't heat itself and the air current actually cools it off a bit (probably, I'm not measuring it by any means, just running like many run at 48-50° here). That's a shield against atmospheric heat, and your body begins to work better, your heart rate slows down A LOT, you keep moving, breathing a bit slower every meter, till you get to an equilibrium, and then you start to actually run. This happens in maybe the initial 500 to 800 meters of the several km. you're naturally able to run. Then you usually run the 30-40% of what you are usually capable in sub-48° At some point you'll need to restart the process, you'll feel exactly when, because the sweat over your body begins to be too hot, and your heart rate start to climb wildly. Many runners just slow down to a fast walk till the heart cools it off, or just go to full stop and call it a day or they take cool water and restart the race after that. I think these studies should be done in places where actual high temperatures have existed for hundreds of years, and there you'll know how able to handle it are real humans who have experience intense heat first hand their entire life. i.e those that do not consider 40-50° as "heat wave", but just regular summer temperatures.