I myself have gotten heat exhaustion once but that was while sitting still in a sauna. I knew to get out when I was dizzy. I gently collapsed onto the floor, still conscious. I noticed something odd happened. Well first off I repeatedly blinked slowly which I expected. But also, at first, even lifting my head an inch would make me dizzy. But as time passed, I could lift myself up further until an hour after collapse, I was back to normal.
But I was wondering, if someone is running to exercise and it is hot outside, how hot does it have to be before heat exhaustion sets in and they need to stop running? For me in the sauna, it took a temperature over 120 degrees Fahrenheit to make me dizzy and even then, the amount of time it took was 45 minutes between getting in the sauna and getting dizzy. But I know that exercise would lower the temperature and time needed. I think it would be closer to 100 degrees ambient temperature before it sets in and here is my hypothesis as to why:
So, assuming the person in question does not have a condition that prevents him/her from sweating and is hydrated and has snacks or has eaten a meal not all that long before he/she started running so that dehydration and hunger aren't a factor here, the sweating and temperature play a role.
Sweating helps initially because it cools the body. But while you are running, you emit heat from your muscles working. Running involves most of the muscles of the body so that is a lot of heat. This heat goes into the cooler air, up to a point.
At some point the ambient temperature right around you will be in equilibrium to the amount of heat from the muscles. This is when the heat will start building up. At this point, sweating isn't sufficient to cool down much, if at all. Once sweating becomes insufficient, it is only time before that person gets dizzy.
If that person gets dizzy and collapses in consciousness, that person has heat exhaustion. Becomes unconscious and body temperature rises, heatstroke. In either case, cooling down and more importantly getting out of the heat is a priority but heat exhaustion is less urgent than heatstroke is.
So am I right about how someone would get heat exhaustion while running and also, am I right that near 100 degrees Fahrenheit would be the air temperature at which this will start happening?