Heat stroke can come on very suddenly and it is deadly. And heat exhaustion is sneaky, too. And they can cause permanent damage.
A friend of mine who is very familiar with desert hiking, and who is an experienced nurse, was telling me she had a close call last spring. She lives in Iowa and had been doing a lot of 10 mile hikes in below zero weather all winter. She goes to AZ a lot, and she flew out here and went on a 10 mile hike, temp about 100 degrees. She said the last 5 miles she stopped 9 times to find what little shade there was, poured water over her head, and drank a lot of water, and she said she was a little delirious, and very, very hot. She said it was as if her blood was too thick to provide any cooling mechanism. But she knew about heat exhaustion, and had plenty of water with her. In the long run, she has sustained what seems to be a permanent problem now of heat regulation. She says her body's heat mechanisms and metabolism are totally messed up ever since then.
There is a point of no return unremarked at the time in most lives. Graham Greene The Comedians
Steal your face right off your head!