Experts are raising concerns about heat stress in the hottest year ever recorded, with scorching conditions leading to fatalities from India to Mexico and Greece experiencing its earliest heatwave.
This condition claims more lives than hurricanes, floods, or other climate-related extremes. But what exactly is heat stress, and how is it quantified? Heat stress happens when the body’s natural cooling mechanisms are overwhelmed, leading to symptoms such as dizziness and headaches, and potentially progressing to organ failure and death. It is triggered by extended exposure to heat and other environmental factors that hinder the body’s internal thermostat and its capacity to regulate temperature.
Heat kills about half a million people annually
“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,” explained Alejandro Saez Reale of the World Meteorological Organization (WMO). Infants, the elderly, those with health issues, and outdoor workers are particularly at risk. Urban residents surrounded by heat-absorbing materials like concrete and brick also face a higher danger.
The WMO estimates that heat kills about half a million people annually but suggests the actual toll might be up to 30 times higher than currently recorded.
As climate change causes heat waves to become longer, more intense, and more frequent, people worldwide will increasingly face conditions that push the limits of human endurance.
Temperature might be the most common and easily understood weather measure, but headline-making “maximum highs” do not fully convey how heat impacts the human body. For instance, the same temperature can feel vastly different depending on the location: 35 degrees Celsius (95 Fahrenheit) feels quite different in the arid heat of a desert compared to the humid environment of a jungle. To create a fuller picture, scientists consider various factors, including temperature, humidity, wind speed, clothing, direct sunlight, and even the amount of concrete or greenery in the area.
What is heat stress?
All these elements significantly affect how the body perceives and, most importantly, reacts to extreme heat. There are numerous methods to measure heat stress, some dating back decades, but all aim to condense various environmental readings into a single figure or graph. One of the oldest methods is wet-bulb temperature, a useful measure in cases where the thermometer reading might not seem extreme, but, combined with humidity, becomes unbearable and even deadly.
In 2023, scientists noted that just six hours exposed to 35 degrees Celsius with 100 percent humidity is sufficient to kill a healthy individual. Beyond this limit, sweat cannot evaporate from the skin, causing the body to overheat and shut down. Copernicus, the EU’s climate monitor, uses the Universal Thermal Climate Index (UTCI), which takes into account temperature, humidity, wind, sunshine, and radiated heat to rank heat stress levels from moderate to extreme.
Extreme heat stress, as defined by this index, is a “feels like” temperature of 46 Celsius and above, necessitating actions to prevent health risks. The Heat Index, employed by the US National Weather Service, provides an “apparent temperature” based on heat and humidity in the shade, along with a colour-coded graph indicating the likelihood of illness from exposure. Canada has created the Humidex rating, which combines heat and humidity into a single number to reflect the “perceived temperature” and outlines the associated risk in a four-step “guide to summer comfort” chart.
Other examples of “thermal stress” indices include the Tropical Summer Index, Predicted Heat Strain, and the mean radiant temperature. These indices are not without their limitations, and heatwave expert John Nairn noted that some measures work better in certain climates than others.
“It’s not the same all around the world, about the way you approach it,” Nairn told AFP. For example, the UTCI is highly effective in gauging heat stress in Germany, where it was initially developed, but it is “a very poor measure” in countries of the global south, he said. “It saturates and over-measures far too much. And it would over-alert for those communities who are chronically exposed to heat,” said Nairn, who has advised governments and the WMO on heatwave policy. These regions might achieve more accurate heat stress readings using wet-bulb temperature, he added.
These indices also do not account for the impacts of heat beyond health, he noted, even though a heatwave could halt trains or overload air-conditioning systems. “If your heat challenge is such that it gets to a level where your infrastructure is not going to operate, and it starts failing, that will have a return on humans no longer being protected,” Nairn said.