Unlike the visible devastation of hurricanes, floods, and tornadoes, extreme heat leaves no trace behind, making it a silent and deadly threat, deadlier than all of these extreme weather events combined. Countries where heat was never a problem are suddenly confronted with rising illnesses and casualties, with authorities struggling to keep count as the public often misjudges and underplays the risks. As temperatures rise globally, Earth.Org takes a look at how extreme heat affects the human body and what socio-economic and physiological factors play a role.
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This is part 1 of a two-part explainer on extreme heat.
Every year between 2000-2019, approximately 489,000 people died from extreme heat around the world. 45% of these casualties happened in Asia, the world’s most disaster-hit region from weather and climate hazards; 36% were in Europe, the world’s fastest-warming continent. Here, heat-related mortality has increased by around 30% in the past two decades.
These estimates are often much higher than official data. This is because accurate tracking of heat-related deaths is challenging and many countries still lack proper record-keeping. Unsurprisingly, heat is often referred to as a “silent killer” and it has quickly become the deadliest extreme weather event in many parts of the world.
Understanding Extreme Heat
To understand why extreme heat is dangerous, we must first look at how our body reacts to it – a more or less straightforward process.
For optimal health, the human body requires an internal temperature of around 36.5C (97.7F). When exposed to heat, our body initiates cooling mechanisms to maintain its temperature stable, including sweating to dissipate heat through evaporation and dilating blood vessels to release heat.
In extreme heat conditions, when the environmental temperature exceeds our body’s temperature, these physiological processes are compromised. Elevated humidity levels can further complicate matters, as they hinder the evaporation of sweat from the skin and thus the body’s cooling down process.
A runner sweating in Hong Kong’s heat. Photo: Kyle Lam/hongkongfp.com
For people living in places like Hong Kong and Singapore, elevated temperatures and high humidity are nothing new. Both regions see temperatures soar above 30C (86F) for most part of the year, which feel even higher when coupled with humidity. And yet, heat-related illnesses here are on the rise. A yet-to-be-published study conducted by researchers at the University of Hong Kong found that heatwaves in the city over the past decade, 18 in total, may have contributed to 1,677 excess deaths.
As of 2022, Hong Kong had already warmed 1.7C compared to pre industrial times, according to Berkley Earth. The city just witnessed an “exceptionally hot” July, with its Observatory forecasting that 2024 will be one of the hottest years it has ever seen. The same is happening in most places around the world.
The increase in extreme heat is a direct result of our warming planet, which is driven by greenhouse gasses that trap heat in the atmosphere. This raises Earth’s surface temperature, leading to longer and hotter heatwaves.
Prolonged exposure to high temperatures without sufficient rest or cooling breaks can result in the accumulation of heat within the body, overwhelming its cooling mechanisms and leading to severe, sometimes deadly heat-related illnesses. Some of the most common are heat exhaustion – with symptoms including heavy sweating, weakness, dizziness – and heatstroke – which is marked by a high body temperature, confusion, and loss of consciousness and can be life-threatening if not promptly treated.
People living in urban areas, which often lack cool environments or natural shading, are particularly vulnerable. But in a rapidly warming world, another issue is emerging: nighttime heat.
High nighttime temperatures are detrimental to human health, as they prevent the body from recovering from daytime heat. This not only disrupts sleep, which can negatively affect physical and mental health, cognitive function, and life expectancy, but it also increases the risk of illness and mortality.
A 2020 study by the Chinese University of Hong Kong (CUHK) found that five consecutive “hot nights,” defined as when temperatures rise above 28C (82F), would raise the risk of death by 6.66%.
Air conditioning units line the walls of a street in Singapore. Photo: Rym DeCoster via Flickr.
People without access to air conditioning – a huge proportion of the world’s population – are particularly at risk. While about 90% of households in the US and 60% in China have some sort of air conditioning system, the number is worryingly lower in rapidly warming places like Europe (10%) and India (8%), the most populous country in the world. In sub-Saharan Africa and South Asia, the percentage is even lower.
With climate change, nights are not just getting warmer; they are also heating up faster than days in many parts of the world. According to a Climate Central analysis published earlier this month, between 2014 and 2023, 2.4 billion people experienced an average of at least two additional weeks per year where nighttime temperatures exceeded 25C. Over 1 billion people experienced an average of at least 2 additional weeks per year of nights above 20C and 18C.
Tracking Casualties Is Complicated
So far this year, Thailand reported 61 heat-related casualties, India 143. Meanwhile, at least 30 heat-related fatalities were confirmed across the western US in July alone. A month earlier, the Saudi government announced that scorching heat had killed 1,301 Hajj pilgrims. These official numbers, however, are just a fraction of the true toll.
Despite our good understanding of physiological reactions to heat, there is a reason why we often talk about a “silent killer.”
Unlike more immediate threats, such as severe storms or earthquakes, heat-related conditions can gradually build up. They also affect vulnerable populations disproportionately, and may not be perceived as imminent dangers until symptoms like heat exhaustion or heatstroke manifest suddenly and severely, leading to heat-related illnesses and fatalities without obvious warning signs.
To further complicate matters, heat-related illnesses and deaths can be multifactorial, involving a combination of heat exposure, individual susceptibility, and underlying health conditions.
In individuals with pre-existing conditions such as diabetes, kidney disease, respiratory or cardiovascular conditions, heat-related stress can trigger or worsen symptoms, leading to serious health complications and, in severe cases, mortality. In these cases, determining the exact contribution of heat to a death can be complex, as it less likely that fatalities in which temperatures played an indirect role are classified as heat-related deaths.
Who Is at Risk?
While no one is truly immune to extreme heat, there are factors that can increase an individual’s vulnerability.
Susceptibility to heat, and how effectively the human body regulates its temperature in extreme conditions, is influenced by a combination of physiological aspects such age and health condition, as well as exposure variables, including occupation and socio-economic circumstances.
Generally speaking, studies show that women – particularly pregnant women, children, and the elderly are especially at risk of developing severe heat-related symptoms.
In a statement issued earlier this month, UNICEF said that one in five children – or 466 million – will experience double the number of extremely hot days than their grandparents’ generation. The UN body said in July that rising temperatures across Europe and Central Asia killed an estimated 377 children in 2021.
UNICEF Executive Director Catherine Russel explained that children’s bodies are “far more vulnerable” to extreme heat. “Young bodies heat up faster, and cool down more slowly. Extreme heat is especially risky for babies due to their faster heart rate, so rising temperatures are even more alarming for children,” she said, before calling on governments “to get rising temperatures under control.”
According to the same UNICEF statement, extreme heat has been linked to pregnancy complications, including “gestational chronic diseases and adverse birth outcomes including stillbirth, low birth weight, and preterm birth.” Heat stress on pregnant women can also lead to child malnutrition and leave infants more vulnerable to contracting infectious diseases such as malaria and dengue, which spread in high temperatures.
The number of people exposed to extreme heat is growing exponentially due to climate change in all regions. Between 2000-2004 and 2017-2021, heat-related deaths among individuals aged 65 and older surged by around 85%. Age-related physiological changes, such as a decrease in the body’s ability to regulate its temperature and reduced sweating capacity, make older adults less efficient at dissipating heat. Pre-existing health conditions and medications can further compromise their ability to cope with high temperatures. Social isolation, limited mobility, and inadequate access to cooling resources only exacerbate these vulnerabilities.
Other factors heightening heat vulnerability have to do with race and ethnicity. Pre-existing health conditions that are more prevalent in certain ethnic or racial groups can increase susceptibility to heat-related illnesses. Examples include diabetes or cardiovascular diseases. But besides this, socio-economic circumstances also play a role.
Ethnic and racial minorities often face higher rates of poverty and may live in urban areas with fewer green spaces and more concrete surfaces. Temperatures in these neighborhoods are generally significantly higher.
Sham Shui Po, one of the most densely populated districts in Hong Kong. Photo: Anne Roberts/Flickr.
In the US, an Environmental Protection Agency (EPA) review of multiple studies revealed that low-income neighborhoods and those with larger populations of people of color often experience higher temperatures compared to wealthier, predominantly white neighborhoods within the same city.
More on the topic: How Marginalised Groups Are Disproportionately Affected by Climate Change
Another striking example where this is an issue is Hong Kong. The city’s infamous subdivided flats and “cage homes” are becoming increasingly greater health risk factors as the city faces higher temperatures in the warm seasons. Anti-poverty local NGO the Society for Community Organisation (SoCO) last month warned of extreme temperatures in subdivided flats, which house more than 220,000 people. The NGO surveyed 308 people living in “inadequate housing,” who often do not have access or cannot afford ventilation or air conditioning. Over 90% of them said they “felt ill because their apartments were too hot.”
“The heat makes it very hard for me… I feel tired,” 84-year-old Chun Loi, who lives in a windowless, poorly ventilated, one-room flat in Hong Kong told AFP on a hot summer day this month. “I try to stay in as much as possible with my fans… Otherwise, where can you go? It’s embarrassing to stay in restaurants and malls if I am not eating anything,” Chun said, as the temperature crept past 32C (90F).
As Eva Yeung of Red Cross Hong Kong put it: “Climate change affects everyone. But the impact is not equal because some people, due to their living conditions and physical conditions, are affected more than others.”
Outdoor workers are another highly vulnerable category. The International Labor Organization (ILO) estimates that almost 4% of the world’s total working hours could be lost to climate-induced extreme temperatures by 2030.
A 2022 Oxfam Hong Kong study on working conditions in waste collection centers in the city revealed that temperatures inside these facilities averaged 32.2C (90.0F), surpassing the July average by two degrees. Issues such as poor ventilation, heat, humidity, pests, foul odors, and a lack of rest areas are prevalent in both existing and renovated waste collection centers. Under these circumstances, over 60% of cleaners expressed experiencing discomfort while on duty. More than 70% indicated that they must stay in cooler and shaded areas along the streets for breaks and meals. Upon returning home from work, these workers may also have to endure extreme hot weather.
Cleaners share handmade lemon tea while working in the New Territories. Photo: Kyle Lam/hongkongfp.com.
Street cleaners face a similar threat. According to interviews conducted by Hong Kong-based local group Concern for Grassroots’ Livelihood Alliance in August 2024, 90% of the street cleaners reported experiencing fatigue, thirst, headaches, dizziness, increases in body temperature, and nausea while working, despite their employers providing portable fans.
“I have to drink seven bottles of drinks a day, including water, homemade herbal tea, energy drinks, and more,” Pattie, a 55-year-old cleaner, told Hong Kong Free Press in July. “Sometimes, when it’s too hot, even the taste of water makes me feel nauseous,” she added.
Irreversible Trend?
An October 2023 study warned that heat and humidity levels will reach lethal levels for hours, days, and even weeks in some parts of the world by the end of the century – even below 2C of warming – making it impossible to stay outdoors.
While shocking, this finding is nothing we did not already hear before. Since the 1970s, climate scientists have warned us that the relentless burning of fossil fuels is heating up our planet, and that crossing a specific warming threshold could lead to irreversible impacts.
The world has been largely slow at reacting to these warnings and what we are experiencing now is the result of this inaction.
The past nine years have been the hottest on record. 2023 was the hottest year globally, with global average temperatures at 1.46C above pre-industrial levels. Europe, which is warming twice as fast as any other continent, saw above-average temperatures for 11 months, a record number of “extreme heat stress” days, and 7% more precipitation than average. And now, scientists are almost certain – 95% certain, to be precise – that 2024 will be even warmer.
Monthly global surface air temperature anomalies (°C) relative to 1850–1900 from January 1940 to June 2024. Data source: ERA5. Image:: Copernicus Climate Change Service /ECMWF.
According to the Intergovernmental Panel on Climate Change (IPCC), with further global warming, we can expect an increase in the intensity, frequency and duration of heatwaves. And unless we prepare societies to deal with this, extreme heat will continue to claim millions of lives.
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