Heatstroke occurs when the body fails to regulate its own temperature, and body temperature continues to rise.

Symptoms of heatstroke include:

• Unconsciousness for longer than a few seconds.
• Confusion, severe restlessness, or anxiety.
• Convulsion (seizure).
• Symptoms of moderate to severe difficulty breathing.
• Fast heart rate.
• Sweating that may be heavy or may have stopped.
• Skin that may be red, hot, and dry, even in the armpits.
• Nausea and vomiting.

Classic heatstroke can develop without exertion when a person is exposed to a hot environment and the body is unable to cool itself effectively. In this type of heatstroke, the body's ability to sweat and transfer the heat to the environment is reduced. A person with heatstroke may stop sweating. Classic heatstroke may develop over several days. Babies, older adults, and people with chronic health problems have the greatest risk of this type of heatstroke.

Exertional heatstroke may develop when a person is working or exercising in a hot environment. A person with heatstroke from exertion may sweat profusely, but the body still produces more heat than it can lose. This causes the body's temperature to rise to high levels.

Both types of heatstroke cause severe dehydration and can cause body organs to stop functioning. Heatstroke is a life-threatening medical emergency, requiring emergency medical treatment.