In the hottest year on record, with scorching conditions claiming lives from India to Mexico and Greece sweltering in its earliest-ever heatwave, experts are sounding the alarm over heat stress. The condition kills more people than hurricanes, floods or any other climate-related extreme, but what is heat stress exactly, and how is it measured? 'Silent killer' Heat stress occurs when the body's natural cooling systems are overwhelmed, causing symptoms ranging from dizziness and headaches to organ failure and death. It is brought on by prolonged exposure to heat and other environmental factors that work together to undermine the body's internal thermostat and its ability to regulate temperature.
"Heat is a silent killer, because symptoms are not so easily evident. And when these underlying conditions are present, the consequences can be very bad, and even catastrophic," said Alejandro Saez Reale of the World Meteorological Organization (WMO). Infants, the elderly, people with health problems and outdoor workers are particularly vulnerable.
City dwellers surrounded by concrete, brick and other heat-absorbing surfaces also face an elevated risk. The WMO estimates that heat kills around half a million people a year but says the true toll is not known, and could be 30 times higher than is currently recorded. As climate change makes heatwaves longer, stronger and more frequent, people across the planet will be increasingly exposed to conditions that test the limits of human enduranc.
