Feeling lightheaded on a hot summer day can be unsettling. Maybe you stood up and suddenly felt dizzy, became unsteady after exercising outside, or felt like you might faint after spending several hours in the heat. While there are many possible causes of lightheadedness, heat exhaustion is one potential explanation, especially when the symptom develops after prolonged exposure to high temperatures. Heat exhaustion occurs when the body loses too much water and salt through sweating ...