How extreme heat is testing the human body's limits

Malaysian Reserve

15-07-2025

Dangerously high temperatures are becoming more common, putting more people at risk of heat stress
by LAURA MILLAN
EXTREME heat is becoming more frequent and intense due to climate change, testing the limits of the human body. Thousands of people across the world die each year from excessively high temperatures.
A failure to mitigate global warming could result in more than 5.8 million extra heat-related deaths across European cities by the end of the century, according to a study published in Nature Medicine. That's equivalent to roughly twice the current population of Rome.
Europe has already experienced dangerously hot conditions this year, starting the summer with a searing heatwave that pushed temperatures above 40°C in some places. The US was similarly sweltering in late June, prompting extreme heat alerts that at one point covered areas home to over 120 million Americans.
In both cases, the stifling temperatures were caused by a 'heat dome', an area of high atmospheric pressure that settles over a region and traps warm air underneath.
What Makes Extreme Heat So Dangerous?
Heat stress occurs when the human body can't get rid of excess heat. It manifests in a variety of symptoms, including dizziness, nausea and cramps, and can culminate in heatstroke — life-threatening hyperthermia that can damage the brain, heart and kidneys.
People are more likely to become dehydrated in high temperatures, raising the risk of heart attack and stroke, and heat can worsen breathing problems, especially in places with elevated pollution levels. Women and senior citizens are the most vulnerable in extremely hot weather.
High temperatures also make for more challenging labour conditions and the physical and cognitive strain can increase the likelihood of workplace injuries. Following an upward trend in such incidents over the past few years, Japan's government introduced fines for companies that fail to adequately protect workers during heatwaves.
Built-up urban areas are particular hotspots as materials like concrete and asphalt trap heat during the day and release it at night, creating what's called the urban heat island effect. Higher temperatures overnight, when the human body normally tries to recover from heat, contribute to heat stress.
Heat can be fatal. Globally, the World Health Organisation (WHO) estimates that there were almost half a million heat-related deaths annually between 2000 and 2019.
It's hard to know exactly how many people die from extreme temperatures each year. Official tolls are almost certainly an undercount because of the different ways heat impacts the human body and aggravates underlying illnesses. Pre-existing chronic conditions like heart disease and diabetes often end up being listed as the cause of death.
People cool off under water mist spray in the Asakusa district of Tokyo, Japan
How Is Extreme Heat Measured?
Forecasters are increasingly using gauges of heat stress and discomfort to understand the health risks posed by high temperatures. These include the humidex, the heat index and 'wet-bulb' temperature, all of which measure how hot the weather feels to the human body when taking into account the effect of humidity. Humidity makes it harder for the human body to cool itself by sweating. That's because there's more water vapour in the air, meaning sweat doesn't evaporate as quickly.
An ambient air temperature of 42°C with 40% humidity — think Phoenix, Arizona, in July — equates to a wet-bulb temperature of around 30°C. A lower air temperature of, say, 38°C, but with higher humidity of 80%, gives a wet-bulb reading of about 35°C.
That 35°C level is considered the upper threshold for human survival, enough to trigger heatstroke even for healthy people with unlimited shade and water. Such high readings have already been recorded in some coastal subtropical locations.
Heat can kill at much lower wet-bulb temperatures too. A 2020 study published in the journal Science found that regions affected by the heat waves in Europe in 2003 and Russia in 2010 — which proved deadly for thousands — experienced wet-bulb values no greater than 28°C.
There's also another more advanced measure of heat stress known as the wet bulb globe temperature which, on top of temperature and humidity, factors in wind speed, the angle of the sun, and cloud cover. This can help assess when it's dangerous to work or exercise on hot days in direct sunlight. Tennis tournament Wimbledon, for example, has a heat rule that allows 10-minute breaks when the wet bulb globe temperature is at or above 30.1°C.
Where Is Extreme Heat a Problem?
Heat and humidity have traditionally been highest in South Asia and subtropical climates. In India, maximum air temperatures soared above 45°C for nine consecutive years through 2024, according to data from the nation's weather department.
The country has notched wet-bulb temperature readings higher than 32°C in some places. The United Nations (UN) predicts India will be one of the first nations to surpass the survivability limit of a wet-bulb temperature of 35°C. Places that are usually temperate are also increasingly experiencing incredibly hot days. Japan, Hungary and Croatia were among the nations that experienced their warmest July on record in 2024.
In the US, heatwaves are becoming more common, tripling in frequency from an average of two per year across major cities in the 1960s to six in the 2020s. Heat is responsible for more deaths in the country than any other weather phenomenon, including floods and hurricanes.
What Is the Economic Impact of Extreme Heat?
Workers are less productive during hotter conditions and this could weigh on economic growth. An individual operating at a moderate intensity loses half of their work capacity at a wet bulb globe temperature of 33°C to 34°C, according to a report from the International Labor Organisation. It estimated that heat stress could cost the global economy US$2.4 trillion (RM10.49 trillion) in annual GDP by 2030.
Higher temperatures boost power demand for air conditioning and fans, which can place a strain on electricity grids and increase the risk of blackouts. The grid can also suffer thermal damage, such as melted cables and sagging overhead wires, as can other vital infrastructure like railway tracks, which can buckle as they get hotter.
Heat exacerbates drought and this can constrain electricity production by depleting reservoirs used for hydropower and restricting the availability of water to cool nuclear reactors. Drought can threaten the transport of goods via key waterways too, such as the Panama Canal and Rhine River, and lower agriculture yields, pushing up food costs. Cocoa, coffee and olive oil are among the commodities that have seen prices surge as scorching heat reduced crop harvests.
How Is Extreme Heat Related to Climate Change?
Global warming is making heatwaves more brutal, particularly in the tropics. A study published in May concluded that every country encountered more extreme heat days over the past year as a result of human-caused climate change.
Nearly half the world's population experienced at least 30 additional days of extreme heat thanks to climate change, according to the research from London-based World Weather Attribution, US nonprofit Climate Central, and the Red Cross Climate Centre. The analysis found that Barbados, Haiti and a number of other Caribbean and Pacific islands saw more than 100 extra hot days.
Those scorching conditions were a reflection of the planet experiencing its hottest year ever in 2024 — a record that could swiftly be broken as there's an 80% chance that at least one of the next five years will be warmer than 2024, according to the United Nations' World Meteorological Organisation. — Bloomberg
This article first appeared in The Malaysian Reserve weekly print edition