What causes heat exhaustion?

Heavy exercise, outdoor work, crowded spaces, or poor air circulation can increase heat gain and sweating. Dehydration makes cooling harder because the body has less fluid available for sweat and circulation. Risk also rises when someone is not acclimatised to hot weather, wears heavy clothing, or has an illness that causes vomiting or diarrhoea. Typical features include marked thirst, heavy sweating, weakness, headache, dizziness, nausea, muscle cramps, and cool or clammy skin. Heat exhaustion can worsen into heat stroke if cooling and fluids do not correct the problem.

Move to a cool or shaded place, loosen unnecessary clothing, and rest. Sip cool water or an oral rehydration drink if the person is awake and able to swallow. Place cool, wet cloths on the skin and use a fan if available. Avoid alcohol and strenuous activity while recovering. If symptoms do not start improving promptly, arrange urgent medical assessment; read more about [Dehydration](/conditions/dehydration/) and [Heat Exhaustion](/conditions/heat-exhaustion/).

Call emergency services for confusion, fainting, a seizure, severe weakness, trouble breathing, inability to drink, or very hot skin. These can signal heat stroke, which needs immediate treatment. Do not give drinks to someone who is drowsy, confused, or vomiting repeatedly. Seek medical care if symptoms persist after cooling and drinking, or if the person is pregnant, very young, older, or has a serious medical condition.

Ask what may have increased your risk, whether dehydration or another illness contributed, how to replace fluids safely, and when you can return to exercise or outdoor work.