As heatwaves become more frequent and intense due to climate change, researchers are urging a fundamental shift in how societies prepare for extreme heat. A new analysis published in Nature argues that current heat-warning systems — which focus almost exclusively on ambient air temperature — fall short. Instead, experts say strategies must prioritize how human bodies actually respond to heat.
Between the summers of 2022 and 2023, an estimated 110,000 people died from heat-related causes in Europe alone. That’s equivalent to a fully packed jumbo jet crashing every day for 16 consecutive weeks. And with global temperatures exceeding 1.5 °C above pre-industrial levels in 2024, the threat is only growing. Without urgent adaptation, Nature reports, heat-related deaths could quadruple by mid-century if warming reaches 2 °C.
Despite these staggering projections, most public health alerts continue to rely on broad temperature thresholds — like “stay hydrated” or “avoid outdoor activity” — without accounting for the complex ways heat affects individuals.
“We need to stop thinking just about hot air, and start focusing on hot people,” the authors argue.
Why the Body Matters More Than the Air
According to the Nature article, heat stress isn’t determined by air temperature alone. Factors like humidity, wind speed, solar radiation, clothing, and even light physical activity can significantly increase risk. For instance, walking in direct sun can expose someone to radiant temperatures 20 °C higher than shade, and simple activities like walking can double internal heat production.
Most people can tolerate internal body temperatures up to about 39 °C. But surpassing that can lead to heat exhaustion or even heat stroke, where the body’s core systems start to shut down. The physiological damage includes blood thickening, organ failure, and in severe cases, death — particularly in vulnerable populations such as the elderly or those with kidney or heart disease.
A Physiology-Based Solution: HeatWatch
To address this gap, researchers — including two of the article’s co-authors, Ollie Jay and Francesca T. — have developed a free, online tool called HeatWatch. The system blends local weather forecasts with personal health data — such as age, medical history, and activity level — to estimate an individual’s “heat-health risk” over a seven-day period.
Users receive personalized cooling advice and risk scores using a six-point scale. This tailored approach could significantly improve public safety by helping governments and health organizations deploy targeted interventions — such as pop-up cooling centers — to those who need them most.
In pilot programs across Sydney, HeatWatch has already been co-designed with at-risk communities, including pregnant women, families with young children, and people over 70. The team is now expanding the initiative into culturally diverse and Indigenous communities, and is in talks to launch pilot studies in Delhi, India.
Rethinking Cooling Strategies
Nature cautions that many so-called “cooling” interventions — such as reflective paints or greening programs — are often measured only by their impact on air or surface temperatures, not on human thermal comfort. For example, unless someone is physically lying on a cooled pavement, the body may not benefit at all.
By contrast, simple, targeted measures that reduce physiological strain can be far more effective. Some examples:
Fans can reduce perceived temperatures by up to 4 °C.
Combining fans with air conditioners set at higher temperatures (26–27 °C) can cut energy use by up to 70%.
Wet-skin cooling mimics sweat evaporation and is especially beneficial for older adults with heart disease.
Reflective roofing, when paired with airflow, can significantly reduce indoor heat strain.
Importantly, physiology-based models can also optimize timing: the hottest part of the day may not coincide with when the body is under the most heat stress.
From Reactive to Proactive
One of the most critical takeaways from the Nature article is the need to shift from reactive responses to proactive planning. Traditional systems often rely on hospitalization or mortality data that come after a crisis. In contrast, physiology-driven approaches allow for early warnings, enabling targeted action before harm occurs.
As cities across the globe face mounting heat risks, especially in low- and middle-income countries with limited infrastructure, these models could offer life-saving guidance even with minimal inputs. Where internet access is scarce, systems like HeatWatch could support community health workers in delivering timely, locally adapted care.
“Rising global temperatures are already costing tens of thousands of lives annually,” the authors write in Nature. “It’s time to start using physiology to translate hot weather warnings into health risk more effectively.”
