mind & bodyanalysisThe Conversation (Australia)
Humidity can make heatwaves more dangerous. It’s time to warn Australians about this threat
Heat is more dangerous when it is humid. The body cools itself mostly by sweating, and sweat only cools when it evaporates; in humid air it evaporates poorly, so the body struggles to shed heat. That raises the risk of heatstroke, strains the heart and lungs and makes sleep poor, so people recover less overnight. In the Northern Territory of Australia, an analysis of more than two decades of emergency-department data found more people attended in intense humid heatwaves than in comparable dry ones. Elsewhere, humid heat has already been deadly: over 1,300 pilgrims died during the 2024 Hajj in Saudi Arabia, and India and Pakistan have been hit hard. Yet Australia's heatwave warnings use temperature alone and ignore humidity.
Sweat removes heat only as it turns to vapour. When the air is already close to saturated, evaporation slows, so sweat drips off instead of cooling the skin and core body temperature rises faster. The more humid it is, the lower the temperature at which this becomes dangerous.
As the world warms, humid heat is becoming a growing health threat in the tropics and beyond. Warning systems that account for humidity, for example using wet-bulb measures, can give people a clearer sense of real danger.
A wet-bulb temperature of about 35°C is often cited as a limit beyond which even healthy people cannot cool themselves for long.
The Conversation
Illustrative photo · U. S. National Weather Service/National Ocean Service, Public domain, via Wikimedia Commons
