Heat syncope occurs when a person faints suddenly and loses consciousness because of low blood pressure. Heat causes the blood vessels to expand (dilate), so body fluid moves into the legs by gravity, which causes low blood pressure and may result in fainting.

Symptoms that could lead to heat syncope (fainting) include:

• Feeling faint or lightheaded.
• Pale, cool, and moist skin.
• Lightheadedness when changing position, such as moving from a lying position to a standing position (orthostatic hypotension).

Heat syncope can be caused by blood pooling in the legs if a person has been standing still for a long time in a hot environment. It can also be caused by vigorous physical activity for 2 or more hours before the fainting happens.

A person's risk of developing heat syncope increases when the person has not adjusted (acclimated) to a hot environment. Being dehydrated may also increase the risk for heat syncope. Recovery is rapid after the person lies down in a cool environment.

Heat syncope is sometimes a symptom of a nervous system, metabolic, or cardiovascular problem that needs further medical evaluation.