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How to avoid passing out in the summer heat

January 9, 2023

It’s summer time and while you may be looking forward to your holiday in the sun, there are some people who just can’t take the heat. Heat can cause fainting or syncope which is a temporary loss of consciousness, usually related to an insufficient flow of blood to the brain. Neurologist Dr Dougall McCorry tells us why heat causes some people to faint and the steps we can take to avoid fainting this summer.

Heat overwhelms some people and is a trigger for fainting. In hot weather, our bodies sweat more, so we lose water and can easily become dehydrated, if we don’t keep drinking water throughout the day. The warmer we are, the more we sweat and the more dehydrated we can become. Dehydration removes water from the blood, meaning less blood flows in our veins, causing a drop in blood pressure. Together the heat and low blood pressure combined make a person faint or “pass out”.

We can refer to fainting from heat as heat syncope, although normally there is more than one trigger involved in fainting, so syncope alone is a more correct term to use.

A common scenario is people fainting while on an aeroplane, particularly if they have been drinking alcohol before their flight. Alcohol dehydrates and because the air on a plane has no moisture and you’re not moving for long periods of time, blood isn’t circulating around your body, causing low blood pressure. This has resulted in numerous people fainting on board a flight. It’s important to understand that fainting is an automatic response to an environmental stimulus or trigger.

So common triggers for fainting can be:

  • You feel too warm
  • You see blood
  • You’re in a hospital environment
  • Blood tests
  • Dehydration
  • Standing for too long
  • Sitting for extended periods of time
  • Severe pain
  • Anxiety

How can the temperature of the weather affect blood pressure?
As we’ve seen, hot weather causes people to sweat a lot which can lead to dehydration. Dehydration, together with the heat results in less blood flowing around the body that can lead to a drop in blood pressure and can cause fainting. It’s more common to see people faint in the summer months because when the weather is warmer, our bodies don’t need to pump so much blood around, resulting in lower blood pressure.

People who are on blood pressure lowering medication are more at risk, because in the heat their blood pressure might drop too low.

It’s not common to see people fainting in the winter, however, our bodies don’t like extremes, and so if somebody is uncomfortably cold, and is experiencing other symptoms like severe pain, fainting is of course possible.

What are the other possible causes of passing out?
Aside from the triggers what we’ve seen above, there may be more sinister causes to black outs such as an underlying heart condition or neurological disorder such as epilepsy.

The majority of black outs are caused by fainting due to a trigger such as hot weather and dehydration. However if you experience the following, you should see your doctor right away:

  • You’re fainting with no provoking factor
  • Fainting after exercising
  • Fainting with chest pain or palpitations
  • Fainting without coming around quickly

How can I prevent fainting, especially during a heat wave?
To prevent yourself from fainting during a heatwave, the best steps to take are:

  • Keep hydrated
  • Consume electrolytes
  • Wear cool clothing
  • Eat regularly
  • Keep walking and don’t be physically inactive for extended periods of time

Most people will receive warning signs before they pass out, common warning signs can include:

  • Feeling light-headed
  • Dizziness
  • Blurred vision
  • Muffled hearing
  • Feeling sick

If you feel like you’re about to faint, lie down with your legs elevated and wait for the feeling to pass. It’s advisable not to go anywhere and to stop what you’re doing immediately.

Who is most at risk of passing out during the summer?
Older people with health problems and younger teenagers are most likely to faint due to heat. Older people who are taking blood pressure tablets, treatment for angina or diuretics are more likely to suffer from low blood pressure and pass out. Anything that dehydrates the body or causes low blood pressure can result in fainting. Anyone with co-morbidities like diabetes are also at greater risk.

Younger people may faint in the presence of blood for example, if they’re getting a blood test done or if they suffer from anxiety. More often than not, fainting is not a serious concern.

If fainting is persistent and interfering with your everyday life, then you should see a cardiologist and neurologist to rule out any heart condition such as heart arrhythmia or a brain disorder.