I have fainted many times due to heat and exhaustion. Along with some blurriness I would get horrible nausea. Starting as a kid, I fainted at recess and sports practices multiple times. Into adulthood it continued to happen. Fainted on vacations(to hit climates), at an exercise class, and a few other times sporadically. I usually would wake up with someone holding my legs and a cool towel. To instantly cool off, I try to put ice near my wrist or neck, which helps a great deal. My best tip overall to prevent it is staying well hydrated. I always carry some water on me for this reason- and it actually has become a great habit for other health reasons too! — and of course avoid getting too hot and the heat help. Easier said than done though given I live in a desert. LOL.

I lived over 60yrs in a mostly dry heat desert . City of El Centro, CA. Located 110miles east of San Diego, 13miles from Mexico and 60 miles from Arizona. For 4 months of summer, temperatures seldom drops below 100 degree F. And a few times while living there temps reached as high as 125 F. One has to be very careful when temperatures reach this high. We learned to adapt and I never had a heat related episode. The main reason for heat exhaustion is dehydration . And sodium depletion (loss of salt). One normally sweats a lot when hot as this is nature's way to try to cool our bodies to help maintain our natural core temperature of average about 98. degrees. If you are really hot and not sweating, this is not good. Note the taste of perspiration on one's skin tastes salty. So, one has to allow our bodies to sweat and air to be able to cool us. Many people will wear long sleeves, long pants to keep from getting sunburned. But if you do make sure the material is a lightweight cotton. Something that can let air penetrate. Important that air can reach the skin. Meaning wearing light colored and light weight loose clothes. Nothing heavy, no polyester, etc. No spandex leggings, etc. Good idea if out in the sun is to wear a hat, keep it light weight. A straw type of hat with large brim is best. And drink plenty of water ! Don't wait until you are thirsty as by then you are likely already dehydrated. A serious must in heat is to stay well hydrated. But what many people that are not used to hot weather don't realize not only important to drink plenty of water, but also the importance of replacing the salt. Drinking more water causes us to also pee more, losing salt along with this plus in sweating. As this lack salt (sodium) is what causes one to get dizzy, headaches, muscle cramps, etc. So drinking sports drinks like Gatorade, Powerade, etc. (electrolyes) is good in hot weather to balance water and salt. Don't drink alcohol when hot . There are also many kinds of medications that people take that make being in the sun/heat dangerous. One being diuretics. Some blood pressure pills, etc. And people that are most susceptible to heat strokes and heat exhaustion are people that are not used to hot weather, or at least not used to being outdoors in it very much. Bodies not used to regulating core body temperatures. I moved to San Diego in late 2010. And never complain about being too hot here. LOL