Personally I've always sworn by the old "listen to your body" adage. It's pretty effective, even in temperatures exceeding 45 degrees C where the old heart rate can be a little volatile.
Cant argue with that....
Dont fight what you body is telling you.
When you body overheats your vasculature dialates. This vaso-dilation cause a decrease in blood pressure. Baro-receptors in your carotid arteries sense this then send a message to your brain telling it blood pressure is low. The brain now sends a message to the heart telling it to pump harder to get your blood pressure up. But theres a problem. Your so dialted up you have no blood returning to your heart. Its all pooled up in your dialated venous system. Passing out will occur from the lack of blood pressure in yuot brain.
What do you do? Raise the persons legs to transfer volume to the heart. This is a nasty viscious cycle.