To keep people safe in a hotter world, we must design strategies around how the body responds to heat and work with physiologists to make them effective.
By Andrew Mackenzie, Associate Director, Strategy and External Relations at The Physiological Society
This summer, heatwaves have swept across the UK, Europe, North America, and Asia, shattering temperature records and putting millions at risk. From scorching cities in southern Spain to stifling conditions in northern England, extreme heat is testing health systems, disrupting work, and costing lives. These events are not rare anomalies, they are the new normal in a warming world. Yet our ability to protect people still relies too heavily on crude temperature thresholds and one-size-fits-all advice.
To be effective, heat resilience strategies must start with a clear understanding of the human body, its limits, and the factors that determine who is most at risk and how to keep them safe. This is the essence of a ‘physiology-first’ approach: starting with the science of how the body responds to heat, then building strategies around protecting people’s health, performance, and wellbeing. Instead of asking “How hot will it get?”, a physiology-first approach asks “How will people’s bodies cope – and what can we do to help them?”
Extreme heat is the deadliest form of extreme weather. It claims tens of thousands of lives each year, reduces productivity, disrupts economies, and deepens inequalities. In Europe, the record-breaking summers of 2022 and 2023 together claimed an estimated 110,000 lives. Around the world, heat stress is already costing billions in lost working hours, and that figure will climb sharply as the planet warms.
At the Global Climate and Health Summit in London this July, organised by The Physiological Society, heat was one of the top priorities for action. Policymakers, researchers, and practitioners from around the world came together to share solutions that protect health in a changing climate. A key theme was the need to integrate physiological science into national and global adaptation strategies. By putting physiology first, the Summit made clear that heat resilience cannot be built on weather data alone. It must be grounded in how the body actually responds to extreme heat.
This builds on work The Physiological Society has been leading internationally, including the Roadmap for Global Heat Resilience published in 2024. Developed with a global group of scientists and supported by over 20 partner organisations, the Roadmap set out a global framework for protecting people from heat, with environmental and physiological vulnerabilities at its core. It provided practical actions for governments, cities, and employers to design interventions that work with the body’s limits. The Summit took this approach further, moving from a global framework to a platform for action, where strategies could be tested, refined, and scaled through collaboration across countries and sectors.
Heat affects people in profoundly different ways. Two people can be standing side by side in the same temperature, yet their bodies may be experiencing very different levels of strain. That difference comes down to two interlinked factors: environmental vulnerability and physiological vulnerability. Environmental vulnerability is about the conditions in which people live and work: housing quality, urban heat islands, access to cooling, workplace design, and green space. Physiological vulnerability is about how well an individual’s body can regulate temperature and cope with heat stress. Age, health status, medications, pregnancy, physical fitness, acclimatisation, clothing, and hydration all play a part.
Physiology tells us that humans operate within a narrow safe core body temperature range of around 36–37°C. Physiological resilience is tested when environmental heat pushes the body beyond its ability to cool itself through sweating, increased blood flow to the skin and behavioural adjustments. For some, such as young healthy adults with good hydration and fitness, resilience is high. For others, particularly older adults, people with cardiovascular or kidney disease, outdoor workers in heavy clothing, or pregnant women, resilience is lower and risk rises quickly. This is the essence of physiological risk: it is not just about how hot it is, but how the body responds. While this article focuses on adaptation, physiology also makes clear that there are hard limits: by which we mean thresholds beyond which no amount of adaptation can protect health. Crossing these limits would make parts of the world effectively uninhabitable, underscoring that adaptation must go hand in hand with urgent action to cut greenhouse gas emissions and limit the worst impacts of climate change.
Most current heat warnings are based on temperature thresholds. These are important, but they are crude tools. They cannot tell us who will be most affected, when the peak strain on the body will occur, or what measures will be most effective for each group. A physiology-first approach integrates weather data with the factors that drive physiological risk. This could include temperature, humidity, solar radiation, wind, clothing, activity levels, and individual health status. From there, we can model when and for whom conditions will tip from manageable to dangerous.
This allows us to pinpoint which groups are most at risk on any given day, target resources such as cooling spaces, hydration points or adjusted work hours to where they will have the most impact, and test whether interventions are genuinely reducing physiological strain, not just air temperature. By shifting from temperature alerts to heat–health risk assessments, we move from generic advice to tailored action that saves lives.
The UK may historically not be known for extreme heat, but it is one of the countries projected to see the largest rise in cooling demand as the climate warms. In 2022, the country recorded its first 40°C day, resulting in nearly 3,000 excess deaths during just five heat periods, and more than 11 million potential work hours lost to heat. In response, The Physiological Society and the Faculty of Public Health developed Red Alert: Developing a human-centred National Heat Resilience Strategy. This strategy applies the ‘physiology first’ principles to a national context, and it makes environmental and physiological vulnerabilities the foundation of action.
The Red Alert strategy highlighted that while there are significant research gaps, particularly around vulnerable populations, we know enough to act now – and there is no time to delay. The challenge is for research and knowledge to rapidly translate into policy action as well as inform the development of health and safety guidelines and business practices. The Red Alert strategy recommends adapting the built environment to reduce heat strain (using shading, ventilation, reflective materials, and green infrastructure) and then applying physiological testing to ensure these measures actually help people cope. It calls for protecting workers through heat-safe work plans based on job demands, clothing, and environmental conditions, and strengthening public health communication so individuals understand their personal physiological risk and how to act. This is a physiology-first approach in action: not replacing environmental or social measures, but ensuring they are targeted to real human limits.
Making physiology the foundation of heat resilience has three critical benefits. First, equity – it identifies not just where heat will hit hardest geographically, but who will suffer most and why, allowing for targeted protection. Second, effectiveness – it ensures that policies and interventions are judged by whether they actually reduce heat strain in the body. Third, efficiency – it directs resources where they will have the greatest physiological impact, rather than applying blanket measures. This also avoids unintended consequences. For example, physiological testing shows that some cooling measures work well in dry heat but are far less effective in high humidity, where evaporation is limited. Without that insight, policies may look good on paper but fail in practice.
Physiology-first approaches are important right across the world. In many lower-income countries, where costly, energy-intensive cooling is not an option, they may be even more valuable. Starting with physiology means designing low-cost, high-impact interventions, such as from shading and ventilation to hydration and work–rest cycles, that are proven to work with the body’s limits. It also means using physiological evidence to adapt solutions for different climates, jobs, and communities. Global collaboration is essential. Shared physiological data can inform guidelines across climates and cultures, while national strategies, such as those developed through the Roadmap for Global Heat Resilience and reinforced at the Global Climate and Health Summit, ensure those solutions are implemented in ways that protect the most vulnerable.
Climate change guarantees that extreme heat will intensify. The question is not whether we will need heat resilience, but whether we build it on the right foundations. A physiology-first approach offers those foundations. It begins with understanding why some people are more at risk, how much heat the body can tolerate, and which interventions truly protect health and performance. By rooting resilience in the realities of the human body, we ensure strategies are built around people – their health, their environments, and their ability to withstand extreme heat – rather than relying solely on weather forecasts and temperature thresholds.
If we are to protect people from the growing dangers of extreme heat, those designing strategies, from governments and city planners to employers and emergency services, must adopt a physiology-first approach. That means using the science of how the human body responds to heat as the starting point, and working with physiologists to identify who is most at risk, when risks peak, and which interventions truly protect health and performance. Physiologists can provide the evidence to ensure that policies, workplace plans, building designs, and public health messages are grounded in real human limits, not just temperature forecasts. Building heat resilience without this expertise is like trying to navigate without a map. With physiology at the core, we can create solutions that are targeted, effective, and life-saving.