The ‘silent killer’: what you need to know about heatwaves

 

Explainer

The ‘silent killer’: what you need to know about heatwaves

 

How spells of hot weather put us in danger and what measures we can take to protect ourselves

 

Ajit Niranjan

Mon 22 Jun 2026 16.00 BST

https://www.theguardian.com/environment/2025/jun/27/the-silent-killer-what-you-need-to-know-about-heatwaves

 

Heatwaves have grown hotter and stronger as the planet has warmed, making what doctors call a “silent killer” even more dangerous. How worried should we be about heat – and how can we stay safe as the climate changes?

 

How many people die from heat?

Hot weather kills an estimated half a million people each year. The average annual death toll is greater than that from wars or terrorism, but smaller than that from cars or air pollution.

 

Despite this, heat is rarely listed as the cause of death. That’s because extreme temperatures are largely indirect killers. Most heat victims die early from illnesses – such as heart, lung and kidney disease – that are made worse in warm weather.

 

How does extreme heat hurt your health?

High heat stresses the human body, sending the heart and kidneys into overdrive as they work to keep the body cool. The added strain – particularly for those with chronic illness – can prove fatal even before heatstroke hits.

 

There are also secondary health effects from high heat. Heatwaves lead to more accidents, dirtier air, bigger wildfires and more frequent power outages, all of which can increase the burden on health systems.

 

Why do warm nights matter?

When days are too hot to function and nights are not cool enough to recover, the body is unable to rest. This compounds the damage done during scorching days.

 

In many European countries, meteorologists describe nights with temperature minimums above 20C as “tropical”, while in Spain, which is more familiar with extreme heat, they call nights above 25C “equatorial” or “torrid”. In recent years, they have informally introduced a new category for night-time temperatures above 30C: “hellish”.

 

Who is most at risk from extreme heat?

People who are forced to be outdoors in scorching weather – builders, farmers, rough sleepers etc – are most likely to suffer from heat exhaustion and the heatstroke that can follow.

 

But older people, and particularly those with underlying illnesses, make up the bulk of heat-related deaths. Women are more likely to die from heat-related causes than men. Poorer people – who are less likely to have air conditioning, well-insulated homes or access to green spaces – are also at greater risk.

 

Why does humidity make it feel hotter?

Sweat is the body’s best defence against heat, lowering internal temperatures as it evaporates. But when humidity is high and the air hot and sticky, the body struggles to cool down because sweat clings to the skin. The effect this has on perceived temperatures can be equal to several degrees, enough to spell the difference between life and death.

 

Why are heatwaves getting hotter?

More than a century’s worth of fossil fuel pollution has clogged the atmosphere, trapping sunlight and heating the whole planet. Average global temperatures have risen by about 1.3C since preindustrial times – and land temperatures by even more – which has pushed the baseline higher and made punishing extremes far more common.

 

There is also some evidence that the climate crisis is making heatwaves worse by weakening the jet stream. Scientists think this is increasing the occurrence of heat domes, which are areas of high pressure and heat that get stuck over a region for days or even weeks.

 

Won’t climate change mean fewer people die from cold?

Cold weather kills far more people than hot weather today, even in warm regions such as sub-Saharan Africa and south Asia. But as temperatures rise, the number of deaths from heat is projected to grow much faster than the number of lives saved from milder cold. When scientists modelled this in 854 European cities, they found a net increase in temperature-related deaths under all emissions scenarios, even accounting for how people adapt.

 

How can we adapt to heatwaves?

Cutting fossil fuel pollution is the biggest step that can be taken to stop heatwaves from getting even hotter, along with protecting forests and wetlands that suck carbon dioxide out of the atmosphere.

 

Urban planners have called for cities to be redesigned so they have less concrete and fewer cars, and more parks and water. This can negate the urban heat island effect, which makes cities hotter than their rural surroundings.

 

Buildings with air conditioning or passive cooling can bring down death tolls, as can strong healthcare systems and swift emergency warnings.

 

Does air conditioning make heat worse overall?

Air-conditioning units increase planet-heating emissions if the power they consume is generated by burning fossil fuels, as it mostly is today, but their pollution is falling as countries clean up their electricity grids. Some experts cite the scale of the heat-related death toll as a worthy reason to use more air conditioning – particularly for the most vulnerable groups – even if it pushes temperatures higher.

 

This year, the UK’s Climate Change Committee (CCC) recommended that air conditioning be installed in all care homes and hospitals within the next 10 years, and in all schools within 25 years.

 

How can I stay safe in a heatwave?

The simplest advice is to stay out of the heat: avoid going outside during the hottest parts of the day, and stay in the shade if you have to. To keep your home cool, close windows during the day and open them after dark, when outdoor temperatures fall below inside temperatures. Cover windows with blinds or curtains to block out direct sunlight.

 

Doctors also recommend drinking water frequently, wearing loose clothing and checking on vulnerable people in your community.