When the temperature hit 40C in Britain last summer, the empty streets appeared “dystopian” to Dr Laurence Wainwright, a sustainability and psychiatry academic at Oxford University. Yet what he had assumed would serve as “a real wake-up call” on the world’s increasingly dangerous temperatures – which resulted in a European heatwave that killed more than 60,000 people last year – failed to prompt action. During the past month or so – as wildfires have torched more than 500 sq km of Greece, Arizona notched up a record-breaking 31 consecutive days at 43C or more and July became the hottest month ever recorded in the modern era – the extreme heat crisis has only appeared to grow. There’s no doubt about it, Wainwright says: “We’re in serious trouble.”
When the human body is exposed to excessive heat, it attempts to maintain its internal temperature by sweating – which cools you down as beads of perspiration evaporate – and by diverting additional blood flow to the skin, which allows for extra heat loss by convection.
“The challenge with that is that sweating causes you to lose water, which can lead to dehydration,” says Gregory Wellenius, professor of environmental health at the Boston University School of Public Health. And as for the upped blood flow to the skin: “There’s less blood flow going to other areas of the body, other organs – and so your heart has to work a bit harder, and your kidneys have to work a bit harder. Even healthy people can experience consequences from those changes in blood flow and changes in sweating.”
A study published last year in the American Journal of Kidney Disease found that emergency hospital visits for the condition rose between 1.7% and 3.1% on ultra-hot days. The harder organs need to work, the more stress they are under – causing potential deaths when coupled with other typical heatwave side-effects, such as dehydration and hyperthermia (when the body’s regulatory mechanisms fail to contend with its raised internal temperature, and fatigue sets in).
Hot weather can also have a significant effect on air quality, reducing lung function: long-term exposure to ozone pollution, which is exacerbated by heat, is linked to 1 million premature deaths every year. The world has warmed by about 1.3C in the past century, with temperatures set to surge to record levels within the next four years; there is a 98% chance that at least one of the next five years will be the hottest ever – and the same applies to the five-year period itself.
While heatwaves are believed to be most dangerous for elderly people (the highest number of deaths during the 2022 heatwave in Europe occurred among over-80s), “people with cardiovascular disease, chronic obstructive pulmonary disease and types of respiratory disease, diabetes, and pregnant women” may also be at greater risk, Wellenius says, as they have “less compensatory capacity”. But “even the healthiest people can succumb to the impacts of heat”, he adds – from the young and fit to decades-hardened hikers. Studies show that there is a general failure to understand the impact of higher temperatures, says Wellenius. “People appreciate that heatwaves are dangerous for people, but think ‘they’re not dangerous for me’.”
There is no exact temperature that should be a red flag, he adds, as extreme heat means something different in London from what it means in Arizona. But it is not only the body’s reaction to the heat itself that harms. In Arizona, for instance, what has been dubbed its “month in hell” led to people suffering third-degree burns from walking on the pavement; a June paper from Cornell University suggests that wildfires could cause between 4,000 and 9,000 premature deaths in the US each year from smoke particulate inhalation.
“Several studies have demonstrated a synergistic effect between high temperatures and high air-pollution levels, leading to a hyperadditive increase in mortality,” says Katrin Burkart, assistant professor of health metrics sciences at the University of Washington. “This is biologically plausible as high temperatures are likely to make the body more vulnerable to air pollution and vice versa.”
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The warming planet is also increasing the spread – and potency – of infectious diseases. Changes in temperature, humidity and rainfall are proving fertile ground for the transmission of conditions such as dengue fever, malaria, Zika virus, chikungunya and cholera in areas where they were previously unknown. And the hotter things are, the more likely people will turn to water, another hotbed of disease risk, to cool down.
“Higher temperatures can increase pathogen activity and reproduction, leading to higher pathogen loads as well as increased virulence, a measure for how sick pathogens can make us,” says Eline Vanuytrecht, European climate and health observatory expert at the European Environment Agency (EEA). “If the disease is transmitted by a vector like mosquitoes or ticks, higher temperatures can increase vector activity and the efficacy with which they transmit diseases to humans.”
Last year, an EEA report warned that Europeans would be at elevated risk of infectious diseases if global heating was not addressed; a paper published in Nature Climate Change months earlier noted that 58% of infectious diseases had been exacerbated by climate change.
Vanuytrecht says that heatwaves can also “alter human behaviour and risk perception”. Studies have shown a significant impact on mental, as well as physical, health in the face of extreme heat, with several recording increased risk of suicide, and a surge in emergency department visits for mental health conditions on very hot days. Gun violence levels have also been found to rise in the US, with almost 8,000 shootings attributed to “unseasonable heat” last year.
In researching the impact of record temperatures on mental health, Wainwright admits he was “quite shocked at some of the things that we were finding”, including “increasing overall rates of mortality in mental health conditions, or worsening of symptoms, triggering a relapse into a certain phase of a condition, like in bipolar switching into mania, which is a dangerous phase”. He says that for the general population, there is no major cause for concern as extreme heat’s effect will be “minor to moderate”.
His research also found that some medicines may work less well when the body is hot and dehydrated, aggravating existing conditions; lithium – which is used as a mood stabiliser for conditions such as bipolar disorder – “can have a slightly reduced efficacy”. As to why this is: “There’s a lot we still don’t understand,” says Wainwright, “but we’re seeing that heat does have, in some cases, quite considerable impacts.”
Academics remain concerned that, with unprecedented temperatures expected to continue to occur, vulnerable groups may be left to bear the brunt. “Global warming, like other disasters, is just exacerbating inequality,” says Jason Kwai Lee, associate professor at the Yong Loo Lin school of medicine at the National University of Singapore. Unlike the white-collar workers who can take refuge in air-conditioned offices, labourers – who spend long stretches outdoors in a country where the average temperature ranges from 25C to 31C – may already be in poorer health, and be left with little other choice but to keep going.
Research has shown elevated levels of “summer heat stress” can induce kidney injuries among workers; Lee says that this can be inflamed by a refusal to drink water for fear it will hamper productivity, and thus pay. “They will try to work as long as possible, and therefore they think: ‘If I don’t pee, I could maximise my time.’ And one way to minimise peeing is not to drink,” he says. Doing this for weeks – or even months or years – means that “eventually, you could pay the price. We’re just seeing the tip of the iceberg.”
As other countries and continents tackle the comparatively new issue of extreme heat, Lee is conscious that lessons from those where it has long been a problem have not yet been learned. “We almost forget about it,” he says of intensely hot periods. “We complain, we suffer. And then this whole thing repeats again next summer.” Singapore doesn’t experience heatwaves per se – rather a consistently hot, humid, climate – but “does that mean we are not paying the price? No. In Singapore, [and] in the tropics, we are the boiling frogs for the longest time. Our leeway is getting very tight; it’s time for us to [realise] we are being cooked, and today’s solutions are not good enough.”
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The solution to this crisis varies widely, depending on who you ask. Most agree that governments taking action to reduce carbon emissions is a priority (albeit perhaps not a big one in the UK, where PM Rishi Sunak recently flew to Aberdeen to announce the granting of 100 new North Sea oil and gas licences). “Individuals can protect themselves by staying hydrated, wearing appropriate clothes, avoiding excessive physical activity,” Burkart says. “On the community level, it is relevant to educate the general public as well as to identify vulnerable groups, eg the elderly living in inner cities, or people living in low-income housing, and provide adequate heat-health education and intervention strategies.”
This could include providing access to cooling centres during extreme heat, she says, or financial support for air conditioning to keep people cooler in their own homes. City planning also plays a part in heat mitigation, as “urban green spaces, adequate building materials and increased albedo [the proportion of radiation or light reflected by a surface] can be another tool to decrease extreme heat exposure.”
Wellenius wants to see more “nature-based solutions, like more street trees, and parks, broadly increasing the tree canopy, or maybe using different building materials so that our cities retain less heat, and we minimise the urban heat island effect [an area warmer than its rural equivalents as a result of higher population levels]”.
He also thinks that we need to change how we talk about extreme heat. Instead of telling people simply how hot it is, it would be more effective “to tell them how dangerous it is tomorrow, or today” as this can vary depending on the timing in the season. A project in Greece is trialling this categorisation system; another, in Spain, is looking at whether naming heatwaves, as we do with hurricanes, will prompt people to take more precautions.
In the short term, education remains the most powerful tool in the extreme heat defender’s arsenal, Wellenius adds – and no one is immune. “Heatwaves are dangerous for people’s health, including yours, whoever you are.”
