What should you know about fainting due to heat?

Heat relaxes blood vessels near the skin so the body can release warmth. Sweating also removes fluid and salt. Together, these changes may lower blood pressure, particularly after standing still, walking in a hot place or rising quickly. Light-headedness, nausea, blurred vision, weakness and clammy skin can come before a brief loss of consciousness.

Move away from the heat and consider other contributors, such as dehydration or low blood glucose. A first faint, repeated episodes or an episode without a clear hot-weather trigger deserves a clinical assessment; fainting can also occur with heart rhythm or blood-related problems.

Lay the person flat and raise their legs if there is no injury that makes this unsafe. Loosen tight clothing, move them to a cool place and use cool, damp cloths. Do not give food or drink until they are fully awake and able to swallow. Once alert, small sips of water may help if they can drink safely. Do not let them stand suddenly; sit first and rise slowly.

Call emergency services if the person is not breathing normally, cannot be woken, has chest pain, severe breathlessness, a seizure, a serious injury or confusion that does not settle. Urgent help is also needed when a faint occurs during exercise, without warning, or with a racing or irregular heartbeat. Arrange medical review for a first episode, repeated fainting, pregnancy, diabetes, heart disease or medicines that affect blood pressure.

Ask whether dehydration, blood pressure changes, blood glucose, anaemia or a heart rhythm problem could have contributed. Tell the clinician what you were doing, how hot it was, how long you were unconscious, and whether you felt warning symptoms.