Out here in the southeastern part of the US, it's a VERY real deal. I've had heat stroke once and heat stress a couple times. Stroke when I was playing ball in school, just being a stupid nothing-can-hurt-me brat kid. Well guess what...you still have to take care of your body no matter how good of condition you're in physically.
I work outside daily, I mean every single day (even days off now that I'm doing a little fishing guiding). The heat and just as importantly the humidity are VERY real. I've seen what water poisoning will do. With the humidity, you sweat but it doesn't evaporate very well so you're thinking the entire time (among other things) that you need to drink more water. Too much is not good either so you have to be mindful of what your body is doing. I find myself actually really slowing down during the afternoon heating at work. Yes it affects my ability to think clearly and to perform a physical job efficiently. It does everyone, well everyone but those who have the blessing of a/c in the workplace. I don't have that luxury. We go in at 0800, its just nasty humid. Not hot but it feels about 90 by opening time. By noon the heat index is generally well over 100. By 1600, it's 110. The highest heat I've evern worked in was actual air temp (in the shade) of 114° and a heat index of 127°. Great right? Go to the parts dept or showroom to cool off--but the big metal building will only cool so much, even it gets up around 82 or so on hot days like that (from 65 in the morning). But 82's better than 110+.
I've had occasions where I was working on something and completely forgot where I was at, and what I was doing...disoriented from the heat. It happens, and in my line of work, can be real dangerous. It can creep up on you & you don't really know it. We've had machines dropped off where folks have gotten heat stroke on them while using. Had one guy a number of years ago that died due to heat stroke. Guy was push mowing his yard (small yard mind you) out in the hot summer heat and humidity. His wife said she thought it was weird that he was mowing part of the neighbor's yard but wrote it off as him being 'nice'.
Trust me...this stuff happens.
Another thing that happens is carbon monoxide poisoning. I've personally coded when I was 9 because of CO. It happened OUTSIDE, me floating around in a raft behind dad's old Carver boat. He had just changed the oil in the old Chevy 250 6 cylinder that powered it, started the engine and it was sitting there idling. That was the last thing I remembered until waking up at a place I wasn't familiar with, having a big bubble plastic thing stuck over my face and my mom standing over me crying her eyes out--and I barely remember that. There was no wind, so CO just kept building up until my young body couldn't take any more of it. Those of us who live in areas, particularly here in the South where there is NO wind during the hot summer evenings should really be careful with running engines. Just because your tractors have a DPF and emit nice clean exhaust gases doesn't mean that those gases are safe. There's still CO being blown out. CO has no odor, no color so you can't see, smell, feel, or taste it. But you know when you've had too much. In my case, the damage is permanent with the right side of the brain experiencing some damage, mostly in the eyesight department. One shop I worked at, during the winter months you HAD to raise a door to let the CO out. They had those Nat Gas infared heaters, they were ventless. If you were to start a machine up in the shop for any length of time, say a minute, with the heaters running, you'd die. I don't see how those things got an OSHA approval but the boss said that they were approved and they had a sticker on the back of the gas valve. Many times had headaches and disorientation. I absolutely despise those things.