Heat exhaustion occurs when you can't sweat enough to cool your body. It can develop when you work or exercise in hot weather and don't drink enough liquids to replace the fluids you lose from sweating a lot. Heat exhaustion can be caused by loss of fluid (dehydration) or loss of electrolytes.

Symptoms of heat exhaustion include fatigue, weakness, headache, dizziness, and nausea. Also, the skin is pale, cool, and moist.

Mild heat exhaustion may sometimes cause fainting. But it doesn't cause a change in your mental alertness and usually can be treated at home.

Moderate to severe heat exhaustion can sometimes lead to heatstroke, which requires emergency treatment.