Dr. Madeleine Thomson on the Health Impacts Beyond Heat Deaths
When researchers tallied the devastating toll of Europe’s record-breaking summer heat in 2025, the headlines focused on a grim number: 24,400 heat-related deaths across 854 cities. But as stark as that figure is, it tells only part of the story.
“Heat-related deaths are only one measure of the health impacts of rising temperatures,” says Dr. Madeleine Thomson, Head of Climate Impacts & Adaptation at Wellcome. “Extreme heat kills but it also causes a wide range of serious health problems. It has been linked to an increased risk of cardiovascular disease, miscarriages and poor mental health.”
These hidden health costs—the heart attacks, the pregnancy complications, the mental health crises—rarely make headlines. They don’t appear on death certificates as “heat-related.” Yet they represent a massive and growing burden on healthcare systems worldwide as temperatures continue to climb.
The Cardiovascular Crisis
When your body overheats, your heart works overtime. Blood vessels dilate to help cool you down, forcing your heart to pump harder and faster to maintain blood pressure. For people with existing heart conditions, this extra strain can trigger heart attacks, strokes, and heart failure.
The numbers are sobering. During Britain’s scorching 2022 summer, an estimated 2,800 extra deaths occurred among people over 65, many from cardiovascular complications exacerbated by heat. Across southern Europe last summer, researchers found that extreme heat was responsible for 68% of temperature-related deaths—with the vast majority occurring among people with pre-existing conditions pushed beyond their limits.
“Extreme heat doesn’t just kill—it also increases the risk of heart disease, pregnancy complications, and poor mental health,” Thomson emphasizes. The challenge is that these connections often remain invisible in health statistics, making it harder to mobilize resources and implement protective measures.
Pregnancy at Risk
For pregnant women, extreme heat poses particular dangers. Pregnancy naturally elevates body temperature and makes temperature regulation more difficult. When external temperatures soar, pregnant women face increased risks of complications including preterm birth, low birth weight, and miscarriage.
The physiological stress of heat exposure can trigger early labor, reduce fetal growth, and increase the risk of stillbirth. These risks are especially acute for women who work outdoors or in environments without adequate cooling, and for those living in areas with limited access to air conditioning.
Children face their own vulnerabilities. Their smaller bodies warm up faster than adults, making them particularly susceptible to heat stress. During heatwaves, pediatric emergency rooms see spikes in visits for dehydration, heat exhaustion, and respiratory problems.
The Mental Health Toll
Perhaps the least visible impact of extreme heat is its effect on mental health. Research has established clear links between high temperatures and increased rates of depression, anxiety, aggressive behavior, and suicide.
Heat disrupts sleep—a critical factor for mental wellbeing. When nighttime temperatures remain elevated, as they increasingly do during “tropical nights” that never drop below 20°C, people struggle to get restorative rest. Sleep deprivation compounds stress, worsens existing mental health conditions, and impairs cognitive function.
Emergency services report increased domestic violence and aggressive incidents during heatwaves. Hospital admissions for psychiatric emergencies rise. The psychological strain of enduring successive days of extreme heat, particularly for those without access to cooling or relief, creates a mounting burden on mental health systems.
A Problem That Extends Beyond Temperature
Thomson’s work spans more than heat alone. As former director of the WHO Collaborating Centre on Early Warning Systems for Malaria and Other Climate Sensitive Diseases at Columbia University, she has spent over 25 years studying how climate change reshapes the landscape of infectious disease.
“We know that virtually all vector-borne diseases have a climate dimension,” she explains. “Temperature drives the rate at which vectors and pathogens develop, while rainfall often supports the creation of sites for the vectors to breed.”
Rising temperatures are enabling diseases like dengue fever and malaria to expand into regions once too cold to support transmission. Highland areas of eastern Africa and Latin America that were previously malaria-free are becoming vulnerable as temperatures rise. Lyme disease, spread by ticks, is pushing northward into Canada and even Arctic regions that were once too cold for ticks to survive.
In Pakistan, increasingly intense monsoons linked to climate change have triggered devastating floods. When floodwaters recede, they leave stagnant pools—ideal breeding grounds for mosquitoes. Malaria cases surged from 500,000 in 2021 to 1.3 million by 2024, with numbers continuing to climb.
“The recent expansion of mosquitoes is very much associated with globalization—things like the movement of shipping containers around the world, air travel, population movement, forest clearing and urbanization,” Thomson notes. “We have to expect more of this type of emergence: new diseases that were historically isolated which can spread very rapidly and have a huge impact on a large community.”
The Justice Dimension
These health impacts don’t fall equally across populations. Vulnerable groups—children, the elderly, pregnant women, people with chronic illnesses—face heightened risks. But socioeconomic factors matter enormously too.
Communities with limited healthcare access, inadequate housing, and poor sanitation are far more susceptible to both heat-related health problems and climate-sensitive infectious diseases. People who can’t afford air conditioning, who work outdoors, or who live in densely built urban areas without green space bear a disproportionate burden.
“Currently, lower-income countries—particularly those in tropical regions—bear the higher burden of climate-sensitive infectious diseases,” Thomson and her colleagues wrote in a recent analysis. “However, as temperatures rise, cooler regions, such as Europe, are also becoming more vulnerable.”
This represents a profound injustice: the countries and communities that have contributed least to greenhouse gas emissions often suffer most acutely from climate change’s health impacts while having the fewest resources to respond.
Building Resilience
At Wellcome, Thomson is leading efforts to develop tools and interventions that can help communities adapt. The organization is funding 24 research teams across 12 countries to create digital tools that integrate climate data with health information.
These innovations include early warning systems that can predict disease outbreaks weeks or months in advance, giving health systems time to prepare. In Vietnam, researchers are developing E-DENGUE, a tool designed to forecast dengue outbreaks up to two months ahead specifically for the Mekong Delta region.
Other promising approaches include releasing mosquitoes infected with Wolbachia, a naturally occurring bacterium that reduces mosquitoes’ ability to transmit viruses like dengue and Zika. Recent advances in vaccine development—including new dengue vaccines and second-generation malaria vaccines—offer additional hope for controlling these diseases.
But Thomson stresses that adaptation alone isn’t enough. “We urgently need to cut emissions and adapt our cities,” she says, speaking about heat impacts specifically but with implications for the full range of climate health threats. “Simple changes, like adding green spaces and waterways, can help cool urban areas and protect public health.”
Looking Ahead
As global temperatures continue rising, the hidden health costs will only mount. Every fraction of a degree brings expanded disease ranges, more intense heatwaves, and greater strain on already stressed healthcare systems.
“Climate change is reshaping the global landscape of infectious diseases, with vector-borne diseases at the forefront of this shift,” Thomson and her colleagues warn. “As temperatures rise and extreme weather becomes more severe, the risk of disease outbreaks increases—both for regions where a disease is already endemic and for those that are experiencing it anew.”
The challenge demands action on multiple fronts: aggressive emissions reductions to limit further warming, adaptation measures to protect vulnerable populations, strengthened health systems, and innovations in disease surveillance and prevention.
What’s becoming increasingly clear is that the true health burden of climate change extends far beyond what we can measure in death certificates and hospital records. From the invisible strain on hearts working overtime in the heat, to the pregnancies ending too early, to the mental health toll of unrelenting extreme weather, to the expansion of once-tropical diseases into new territories—the hidden costs are real, substantial, and growing.
Understanding these interconnected impacts is the first step toward building the resilient, adaptive health systems we’ll need in a warming world.