Why are heatwaves dangerous and how does extreme heat affect the body?

Extreme heat brings serious health risks, especially for people who are elderly or have pre-existing conditions. Here’s what you need to know

By Jason Arunn Murugesu

Because of climate change, heatwaves are getting more frequent, more intense and are lasting longer than ever before, with deadly consequences. About 356,000 deaths in 2019 could be linked to extreme heat, according to one model. Here we explain how heat affects the body and how to stay safe.

What is a heatwave?

A heatwave is generally defined as a period of unusually hot weather lasting more than two days. In London, a heatwave means three or more consecutive days of weather above 28°C. In other places, the threshold is different depending on the average for the region.

What does extreme heat do to our bodies?

The human body responds to high temperatures in two key ways: dilating blood vessels in the skin and producing sweat.

Increasing blood flow towards the skin allows more heat to be lost to the environment. Sweat cools down the body when it evaporates off the skin. These responses help maintain core body temperature between 36°C and 37°C, which is necessary to keep metabolic functions working normally.

But they can also have harmful consequences. Dilating blood vessels results in lower blood pressure, which makes the heart work harder to push blood around the body. For people with pre-existing heart conditions, this can lead to heart attacks.

Excess sweat can lead to the loss of salt from the body. In extreme cases, a low level of sodium in the blood can result in nausea and headaches.

What causes people to die in heatwaves?

During heatwaves, there is a higher incidence of death from cardiovascular and respiratory diseases. But it isn’t necessarily clear when an individual’s death was caused by heat, says Ana Nunes at the University of Warwick in the UK. “The link between deaths during or immediately after a heatwave due to exposure to heat is difficult to establish and mortality attributed to extreme heat is most certainly underestimated,” she says.

Who is most vulnerable to extended high temperatures?

The people most at risk from high temperatures are those who are elderly, are isolated, have pre-existing heart conditions and don’t have access to air conditioning, studies suggest. People with pre-existing mental health conditions are also at a higher risk of dying from extreme heat, one study found. This may be because of the effects of medication or because they may not take the precautions necessary to stay cool in a heatwave, researchers have speculated.

Infants are also at risk from hot weather, as they have a much higher body-surface-area-to-mass ratio than adults, which means they can more easily take on heat in hot conditions. In 2019, 53 children died in the US from heat-related illnesses after being left in cars.

What are the risks of extreme heat during pregnancy?

In the US, about 25,000 children a year are born earlier than they would otherwise have been due to exposure to extreme heat, one study found. Another discovered that in lower to middle-income countries, pregnant people who experience extreme heat in the last week before giving birth had a higher risk of preterm birth and stillbirth.

Do you get more used to heat if you grow up in a hot country?

Anyone can physiologically adapt to hot weather over the course of several days – this is termed acclimatisation. Someone who is acclimatised to the heat will sweat more and also secrete sweat with a lower salt concentration. This helps maintain the body’s salt balance. The rate of skin blood flow will also increase to maximise the transfer of heat to the environment.

How do I stay safe and cool in a heatwave?

Drink lots of fluids and stay in the shade for as much of the heatwave as possible. During the day, close the curtains in rooms that face the sun. Wear sunscreen to protect your skin from UV light and put on loose-fitting clothes to keep cool. You should also avoid physical exercise outside during the hottest times of the day.

What is the difference between heatstroke and heat exhaustion?

Heat exhaustion occurs when your body loses excessive amounts of water and salt. This can cause headaches, dizziness and fast breathing. It isn’t usually serious if you can cool down within half an hour.

But heat exhaustion can turn to heatstroke, a serious medical condition that occurs when your body is overheating heavily, which can cause loss of consciousness. If someone is still unwell from the heat after 30 minutes of resting in a cool place, you should immediately call the emergency services.

How do you treat heatstroke and heat exhaustion?

Heat exhaustion can be treated by finding a cool spot to rest and drinking plenty of water. If you suspect someone has heatstroke, immediately call the emergency services.