As of May 2025, around 4 billion people – almost half the world’s population – had endured at least 30 days of extreme heat over the past year.
“Let’s face facts: extreme temperatures are no longer a one-day, one-week or one-month phenomenon,” UN Secretary-General António Guterres said last year.
“Earth is becoming hotter and more dangerous for everyone, everywhere.”
But hot weather isn’t just dangerous – it’s deadly.
Extreme heat is the leading cause of weather-related deaths, killing almost half a million people every year, according to the World Health Organization (WHO).
And that death toll is expected to grow as the climate crisis drives temperatures to ever greater extremes.
“Climate change is increasing the frequency, intensity and duration of heatwaves, and that is leading to more morbidity and mortality,” says Kristie Ebi, a professor of global health and environmental and occupational health sciences at the University of Washington.
Heat stress is an overarching term for when the human body is suffering from excess heat.
It’s caused by “high temperatures, humidity, lack of airflow and physical effort,” WHO told us in a statement.
As your body warms up, you may experience faintness or dizziness, followed by muscle spasms and pain in the legs, arms or torso.
These are known as heat syncope and heat cramps and come in the early stages of heat stress.
If no action is taken, you may start to lose concentration, become tired or irritable, feel sick and thirsty and sweat heavily.
These are symptoms of heat exhaustion.
If your body still can’t cool down and reaches temperatures above 40 degrees Celsius, you’re likely to feel confused, slur your words, experience seizures or even lose consciousness.
This is the medical emergency known as heatstroke.
If not treated, heatstroke can quickly damage the brain, heart, kidneys and muscles, leading to long-term health problems and even death.
In many parts of the world, temperatures regularly reach or exceed 40 degrees, but that doesn’t mean everyone is suffering from heatstroke.
“The focus is on our core body temperature, not what you measure with the thermometer,” says Ebi.
“Our core body temperature needs to stay within a pretty narrow range to protect our cells and organs.”
Fortunately, the body is relatively adept at keeping cool.
From physiological reactions like sweating to behavioral changes like sitting in the shade, people naturally try to keep their core body temperature in that narrow range, Ebi continues.
However, there are still heat thresholds beyond which humans cannot cope.
“People can acclimatize – we go on vacation to somewhere that’s warm, and the first day feels really hot, but after two weeks, it’s not so bad,” says Ebi.
“But there are upper limits to what people can acclimatize to.”
And those limits are surprisingly low.
Scientists have long believed that humans can only survive for about six hours at 35 degrees Celsius and 100 percent humidity – what’s known as a ‘wet bulb temperature’.
This assumes a person is in ideal conditions: in the shade, unclothed, sedentary, acclimatized and without preexisting conditions.
But this is an unrealistic and crude approach, says Jennifer Vanos, an associate professor at the School of Sustainability at Arizona State University.
To more accurately estimate survivability, you need to take into account different types of climates and different types of people, she says.
When you do, survival limits drop significantly.
Research from 2023, involving both Vanos and Ebi, found that accounting for factors like age and climate, survivability limits were between 0.9 degrees and 13.1 degrees lower than the traditional 35-degree wet bulb estimates.
Heat stress is most likely to affect already vulnerable groups, such as older adults, children, and people who are pregnant or have underlying medical conditions.
It’s especially problematic when a person doesn’t recognize the danger or is unable to take steps to cool themselves down – for example, because they’re working outdoors.
“In many situations in the world, outdoor workers don’t have opportunities to sit under a tree, for example, and may be restricted in how much water they have access to,” Ebi says.
Globally, one in four adults works outdoors, and on average, they spend almost 30 percent more time at risk of heat stress than they did in the 1990s, according to the 2024 Lancet Countdown on Health and Climate Change.
Workers are disproportionately likely to work outdoors in the Global South, especially in agriculture.
In Africa, for example, almost half of all employment is in is in the agricultural sector, according to the Food and Agriculture Organization (FAO), and nearly 83 percent of employment is informal – adding another layer of risk.
“[Informal workers] don’t have the protections that a formalized workforce would be afforded, whether that’s personal protective equipment, having the proper engineering controls or having an organized work schedule around breaks,” says Halshka Graczyk, a technical specialist on occupational safety and health at the International Labour Organization (ILO).
Women are also particularly vulnerable to heat-related illnesses.
Across much of Africa and Asia, agriculture makes up the bulk of female employment. What’s more, women can be physiologically more susceptible to heat because of things like hormonal differences and a greater proportion of body fat, Graczyk says.
In many parts of the world, women continue with manual outdoor work during pregnancy, especially in activities such as subsistence farming.
These women are “particularly vulnerable to heat-related illnesses” and have at least twice the risk of miscarriage and adverse outcomes at birth, according to a 2024 ILO report that Graczyk helped author.
“We already know that pregnant women in the rural economy are at very high risk,” says Graczyk.
“But women are likely to continue working because they must feed their families, especially in subsistence agriculture.”
What’s more, many of these vulnerable outdoor workers “live where their work and work where they live,” Graczyk adds, meaning they are exposed to heat after work, throughout the night and on days off.
“There’s really no escape – there’s no refuge.”
Beyond the immediate impacts, extreme heat can also undermine the wider systems on which our health depends.
“Productivity will decline due to heat, whether it’s because more breaks are needed or because people just can’t work that hard,” says Vanos.
In 2023, this led to around 512 billion potential work hours being lost to extreme heat – up by almost half since the 1990s, according to the Lancet Countdown.
Poorer regions are disproportionately affected: countries that rank ‘low’ on the Human Development Index (HDI) lose an average of 221 potential hours per worker, compared to 41 hours for those ranked ‘very high.’
Taking time off work means lost income for workers and their dependents, which threatens their access to housing, medical care, drinking water and food.
Their food security is particularly at risk as the majority of lost labor is in agriculture, especially in poorer countries, where it accounts for more than 80 percent of lost work.
Poor health in turn makes people more vulnerable to heat stress.
“In the average population, being a healthier person is so important for being able to handle the heat that you’re exposed to,” Vanos says.
“Eating well, exercising and generally having higher well-being as a person is going to protect you more from the heat.”
Despite the dangers, the negative health impacts of heat are predictable and largely preventable, according to the World Health Organization.
“Governments, employers and event organizers must act now to protect workers,” WHO told us in a statement. “Heat-related risks should be part of national climate and workplace health plans.”
Heatwave early warning and response systems are a key part of this, says Ebi.
“My emphasis is always on the response: it’s not just the meteorologist telling us it’s hot; it’s knowing what you need to do,” she says.
This includes recognizing and responding to the symptoms of heat stress, as well understanding what infrastructure is available, such as cooling shelters.
There is also a need for better urban planning, such as green roofs and tree planting programs, Ebi adds.
Planting trees in and around workplaces, for example, can reduce ambient temperatures and provide shade, creating a cooler environment for outdoor workers.
Trees also reduce heat absorption by buildings, which can improve conditions for indoor workers who don’t have active cooling.
A 2023 study led by the Barcelona Institute for Global Health suggested that European cities could prevent a third of heat-related deaths by increasing tree cover from 14.9 percent to 30 percent.
Some technological innovations could also help, such as incorporating heat-conducting materials into buildings or mechanizing processes to reduce physical labor.
But it’s vital to look beyond heatwaves. After all, eight out of 10 occupational injuries happen during periods of excessive heat even before a heatwave hits, says Graczyk.
“We’re facing more and more risks just from changing weather patterns, longer summer seasons, higher daily temperatures – not just more heatwaves,” she says.
“You can’t just implement measures for workers when you hit a heatwave.”
One way to adapt to long-term heat changes is adjusting working hours to protect workers during the hottest times of the day.
In Qatar, for example, work is prohibited in summer between the hours of 10:00 and 15:00, Graczyk tells us.
Other countries, such as Mozambique, have set maximum heat limits – meaning workers must stop work when the temperature hits a certain level.
Without adaptation measures, heat-related deaths are projected to increase by 370 percent by mid-century, assuming global temperatures continue to rise to just under 2 degrees.
At the same time, heat-related labor loss is projected to increase by 50 percent.
Mitigation and adaptation measures are vital in saving the lives of outdoor workers – and people more generally – but there are limits to how far we can adjust to heat while still leading full and active lives.
Ultimately, if we want to curb the rise in heat-related illnesses and deaths, we need to tackle the underlying cause.
“Extreme heat is increasingly tearing through economies, widening inequalities, undermining the Sustainable Development Goals and killing people,” said Guterres last year.
“We know what is driving it: fossil fuel-charged, human-induced climate change.”
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