The number of people exposed to extreme heat stress each year has risen by roughly one billion since the 1970s, according to a study published in Nature Climate Change. The analysis, which used the Universal Thermal Climate Index to capture the combined burden of temperature, humidity, wind, and solar radiation, shows that 70% of the world’s population now experiences at least 90 days of strong heat stress annually, compared with 55% five decades ago. The increase is concentrated in tropical and subtropical regions, with sub-Saharan Africa, South and Southeast Asia, the Arabian Peninsula, and the Mediterranean identified as hotspots of population exposure.

The physiological toll of heat is well documented, but researchers are now quantifying its mental health consequences. Andrea Mechelli, a neuroscientist at King’s College London, analysed 12 years of UK electronic health records and found a 7% rise in community mental health service use and a 6% increase in hospital admissions during heatwaves. In parallel, his Urban Mind project has gathered self-reported data showing that people experience irritability, anxiety, depression, and mental fatigue during prolonged hot spells, yet only 9% attribute these symptoms directly to high temperatures. The biological pathway includes dehydration, sleep disruption, and increased physical strain, while the psychological pathway involves altered routines, reduced social contact, and a sense that even simple tasks require greater effort.

Children are especially at risk: a UNICEF report cited in the study notes that 559 million children are already exposed to frequent heatwaves, and their bodies are less able to regulate temperature. The elderly, outdoor workers, and those in poorly adapted housing also face heightened danger. Meanwhile, behavioural research from the University of Southern California, led by Wendy Wood, indicates that stress and fatigue can lock people into unhealthy routines—irregular sleep, poor diet, physical inactivity, overwork, and excessive caffeine—that further amplify stress levels. Conversely, sleep specialists note that a short afternoon nap of 20–30 minutes can improve alertness, memory consolidation, and mood, offering a low-cost countermeasure to heat-related cognitive decline.

The findings reinforce calls for both emissions reduction and adaptation. Urban planners in cities such as Paris, Copenhagen, and Barcelona are already pursuing ‘renaturalisation’ strategies, including the 3-30-300 rule: every resident should see at least three trees from their home, live in a neighbourhood with 30% tree cover, and be within 300 metres of a green space. Such measures aim to combat the urban heat island effect, which can push night-time temperatures several degrees above those in surrounding areas. The next milestone to watch is how municipal governments integrate these design standards into building codes and public health planning ahead of the next summer season.