A study published this week in The Lancet Planetary Health has reviewed human heat tolerance thresholds and concludes that stifling heat will be more frequent and widespread than anticipated, particularly for people over 60 years old. The research estimates that with 1.5 degrees of warming, 22 per cent of older adults worldwide would already have to endure dangerous heat for at least one month a year.
The researchers combined population projections and climate models to determine that people over 60 will exceed heat tolerance thresholds widely and imminently. With a rise of 1.5 degrees, there will be 290 million older adults worldwide at risk from heat, while young adults would not face a similar level of risk until 4 degrees of warming is reached. "Our findings indicate that unbearable heat will affect far more people, in much larger regions and for longer periods than was previously believed," the authors concluded.
Ageing is associated with a decline in thermoregulatory capacity due to reduced sweat production, attenuated cutaneous vasodilation and decreased cardiac output, among other factors, the researchers explained. Older people also tend to suffer from underlying conditions that can be more easily destabilised by heat. Qinqin Kong, a researcher at Stanford University and author of the study, explained that "exposure to uncompensable heat does not mean immediate death, but prolonged exposure can become dangerous and, eventually, fatal".
Each additional degree of warming exposes approximately 1 billion more people to at least 180 hours of stifling heat per year. According to experiments in climate chambers, the environmental temperature at which older adults can no longer maintain a stable core body temperature may be approximately 4.7 to 7.5 degrees lower than that of young adults, depending on humidity.
The risk is concentrated in the hottest months in specific zones: southern Asia, western Africa, southeast Asia, eastern China and some Persian Gulf regions. The authors highlighted that the densely populated Indo-Gangetic Plain and the Persian Gulf coastal zone are "particularly concerning", where older adults will face relentless stifling heat, both day and night, for several consecutive months with 3 to 4 degrees of warming. Kong warned that "when nights no longer provide relief, the body has fewer opportunities to recover from heat accumulated during the day".
Beyond direct health effects, extreme heat can trigger a cascading effect throughout society, disrupting ways of life and work, food systems, infrastructure and communities. The authors warn that this uncontrollable heat "could increasingly drive internal displacement and cross-border climate migration", situations that would pose new political and humanitarian challenges.
