Laura’s family farm in New South Wales, Australia, after an extended drought. Credit: Laura Shuttleworth.
Extreme heat already kills hundreds of thousands of people each year, yet its effects on the human body remain poorly understood. Crick PhD researcher Laura Shuttleworth is one of a growing number of scientists looking for answers.
Laura’s family farm in New South Wales, Australia, after an extended drought. Credit: Laura Shuttleworth.
I was 12 when the worst heatwaves hit our home in rural New South Wales, Australia. It was the summer holidays, a few weeks after we’d celebrated Christmas. I sat at the kitchen table, next to the sheets draped as curtains over the front windows of our tin-roofed house, passing the long, hot hours with a 1,000-piece puzzle.
With frustration and discomfort, I stopped every 20 minutes, dashing into the nearest bathroom to run cold water through my thick, dark hair that was holding the unbearable heat so close to my scalp. The air conditioner was running full-blast, night and day; but to no avail, as 47ºC winds blew dust hundreds of kilometres across the countryside. Kangaroos came to lie in the dam by the house, panting and submerging themselves. Small birds died and the outside world of beautiful rolling hills and eucalyptus, normally teeming with life, turned to a desert wasteland of cracked, desiccated soil, and silence. Heat – global warming’s insidious assassin – was knocking at our door.
Extreme heat is a severely underappreciated health risk. It creeps in by day and clings too close to our homes and cities at night, leaving us stifled and putting our most vulnerable people – children, pregnant women, outdoor workers, older people, and those with underlying conditions – at risk of health complications, heat stroke and even death. In fact, extreme heat is the deadliest of all natural disasters, accounting for around 500,000 deaths worldwide each year – a number close to that of malaria, and that is likely underestimated due to under-reporting and difficulties in correctly diagnosing heat-related illness.
Tragically, these numbers are predicted to rise significantly. Global warming is bringing about more extreme weather events, including heatwaves that are becoming hotter, more common, and lasting much longer than they used to. Each day, ever more people are exposed to the dangers of extreme heat – but much of the world is underprepared.
Our ability to withstand high temperatures is so critical to our survival that it is intricately built into our biology. We have receptors in our skin that sense the most minute temperature changes, while a part of our brain constantly monitors our blood temperature and sends signals to the rest of our body to thermoregulate when it strays from normal. If our body temperature does rise, stress or danger signals are deployed to warn our cells that something is wrong; to prepare themselves to withstand the ongoing insult and to repair damaged tissue in the aftermath.
These mechanisms have allowed humans to live almost anywhere in the world, but we do have limits: severe hyperthermia occurs when the body’s core temperature rises above 40-41ºC and, if left untreated, can result in multi-organ failure and death. But even before reaching this lethal threshold, extreme heat puts immense pressure on our bodies as we battle to keep our core temperature from rising. People with underlying health conditions are at particular risk as heat places extra strain on organs such as the heart, kidneys, intestine, liver and brain. As our planet warms, it is critical that we understand the short and long-term health impacts of exposure to extreme heat.
After graduating with a degree in biochemistry and medicinal chemistry from the Australian National University in Canberra, I moved to the Francis Crick Institute in London to join the lab of immunologist Carola Vinuesa. My PhD research has taken me on a journey to better understand the health impacts of heat stress and in particular its effects on the autoimmune condition inflammatory bowel disease (IBD).
IBD comes about when the body’s immune system mistakenly attacks and damages the intestine leading to severe abdominal pain, persistent diarrhoea, fatigue, and weight loss. Many people with IBD or other gastrointestinal issues report experiencing worsened symptoms during the summer heat, with one study finding that patients with IBD were 4.6% more likely to experience a disease flare during a heatwave. It is not yet known how extreme heat might exacerbate IBD but over the last four years, I have set up a model system in the lab that is allowing me to probe the effect of heat extremes on the immune system and intestine, and we are now getting closer to some answers.
Aside from my own research on IBD, there is increasing evidence that extreme heat jeopardises many aspects of our health.
In Central America, for example, cases of ‘nontraditional’ chronic kidney disease are rapidly rising as agricultural workers face repeated exposure to unbearably hot and humid conditions during their workdays. Heat exposure gradually wears down kidney function as a result of dehydration and reduced renal blood flow. Chronic kidney disease is the leading cause of death in El Salvador and Nicaragua, third in Costa Rica, and fourth in Panama. Cases are also rising in the US and South Asia – areas that are increasingly feeling the impact of global warming, with one study estimating that there are now as many as 26.2 million people worldwide living with chronic kidney disease related to occupational heat exposure. This is a disease predominately afflicting young, and otherwise healthy men – no one is safe from the dangers of extreme heat.
The risks faced by pregnant women and their unborn babies during heatwaves are also becoming more apparent: for example, a study in southern India in 2024 found that the risks of miscarriage and stillbirth doubled for women working outdoors during heatwaves. The extra heat load and physical strain placed on women’s bodies as their baby grows puts greater pressure on their ability to regulate their own body temperature. The risks of adverse pregnancy outcomes, low birth weights, and pre-term births are significantly increased during heatwaves, particularly for women who cannot access cooling.
Extreme temperatures are negatively impacting our psychosocial health too. Hospitalisations associated with mental health conditions increase with hotter weather. Rates of impulsive and violent behaviour, depressive episodes, and suicide attempts are sadly significantly more common during heatwaves. In addition, when heat stressed, we face reduced cognitive ability and reaction times, undermining our ability to work and function normally.
Our bodies are already warning us about the dangers of extreme heat, and we really need to listen.
Heat targets inequitably: its risk is inextricably intertwined with socio-economic privilege and geography (not to mention the severe impact on ecosystems around the world that are forced to adapt rapidly or perish). Though I experienced severe heatwaves during my childhood, I was lucky to have air-conditioning in my home and classrooms. Others are not so fortunate.
The catastrophic heatwaves that laid siege to the UK and western Europe in late spring and summer this year exposed just how vulnerable we are without proper adaptation. In London, bus drivers felt faint and exhausted behind the wheel as their cabs were reported to exceed 40ºC. Trains were cancelled funnelling more commuters into packed tube lines, and schools closed forcing many children to stay back in suffocatingly hot homes. The London Ambulance Service reported two of its busiest days on record – even surpassing the peak of the COVID-19 pandemic – as they treated their highest number of patients in a week and attended the greatest number of life-threatening emergencies.
It is clear that an urgent and global response is needed. Despite this, the World Health Organisation is yet to declare global warming a ‘Public Health Emergency of International Concern’ – the highest level of health alert. Such a declaration would catalyse a coordinated international response by drawing political attention to the health impacts of climate change, supporting the mobilisation of resources and accelerating action to protect people from climate-related health threats, including extreme heat.
Science has a crucial role to play in this response. My research has led me to join a relatively small but growing community of heat specialists around the world. But much more work is needed. Given the profound array of effects that heat has on our bodies, we need experts from across biological fields to come together and share their knowledge. Understanding the health impacts of extreme heat will enable us to develop treatment strategies for heat-related illnesses and inform adaptation measures to support our communities more effectively during heatwaves.
But there are some obvious changes that we can make now that will help protect us from extreme heat well into the future. We must re-wild and green our cities, ensuring that these areas and cooling infrastructure are equitably accessible. We need to build or adapt our homes, schools, workplaces and public transport networks with materials and architectural designs that will shelter us on hot days. And, ultimately, we have to swap fossil fuels for renewable energy sources, invest in promising carbon-sequestering technologies, and restore ecosystems to bring back a balance to our planet and prevent continued warming.
We are as healthy as our Earth is – it feeds us, homes us, and provides the air we breathe. A healthy planet is paramount to ensuring the health and prosperity of humanity and all other species into the future.
A version of this article won the Crick’s inaugural science writing prize.
