I've had a few of these scares as well. Type 'heat index chart' into Google Images and even print it out if needed. Any time the index is 90 or higher I try not to do strenuous activity like run or play sports. I also sweat profusely even in air conditioned climates when exercising and so hydration can be tough to maintain any time of the year and it doesn't matter if it's indoors or outside. Heat stroke is an extreme version, but I'd imagine most people have probably at least had some form of heat exhaustion before and especially if you live in a very hot and humid area like I do. Keeping electrolyte drinks on hand really helps and you could also keep a thermometer on you to check your body temp as well.