Heat exhaustion is a heat-related illness that occurs when the body loses excessive amounts of water and salt, usually through sweating. It often happens in hot, humid conditions, especially during physical activity. Symptoms include heavy sweating, weakness, dizziness, nausea, and headache. If not treated promptly, heat exhaustion can lead to more severe conditions like heat stroke. To prevent heat exhaustion, it is important to stay hydrated, take breaks in the shade, and wear lightweight clothing. If symptoms arise, it is crucial to cool down and rehydrate immediately.