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Heat, technically “heat stress,” kills more people than hurricanes, floods, or any other extreme weather event. But how do we measure it?
As Greece experienced the earliest heat wave in its history and many people lost their lives in heatwaves in India and Mexico in recent weeks, here’s why degrees Celsius are not enough to measure the danger heat for humans.
“Silent Killer”
Heat stress occurs when the body is no longer able to naturally cool the body, causing symptoms ranging from dizziness and headaches to organ failure and death.
It is caused by prolonged exposure to heat and other environmental factors which, combined, prevent the body from regulating its temperature.
“Heat is a silent killer because the symptoms are not so obvious. And when there are underlying health conditions, the consequences can be very serious, even catastrophic.“, says Alejandro Saez Reale, of the World Meteorological Organization (WMO).
Infants, the elderly, people with health problems and outdoor workers are particularly vulnerable.
The WMO estimates that heat kills around half a million people a year, but says the true toll could be 30 times higher than estimated.
More than the maximums
Air temperature is the most used and easiest to understand meteorological data, but these “maxima” above 35 or 40°C which appear on the front pages of the newspapers tell us little about heat. truly absorbed by the human body.
At the same temperature, the feeling of heat is much more bearable under 35°C in the dry air of the desert than in the humid and stifling atmosphere of the jungle, where sweat evaporates poorly.
To assess the real impact of heat on organisms, scientists take into account, in addition to temperature, a series of factors such as humidity, wind speed, clothing, direct sunlight, and even the presence of concrete or greenery in the environment.
Several methods exist to measure heat stress and attempt to summarize all these factors into a single figure or graph.
Felt temperature
One of the oldest is the so-called wet bulb temperature. It makes it possible to realize the danger of an air temperature that may seem moderate but which, combined with humidity, can become unbearable, even fatal.
Six hours of exposure to 35°C with 100% humidity is enough to kill a healthy person, scientists have estimated in 2023. Beyond that, sweat can no longer evaporate and the body overheats. , until death.
The climate service of the European Copernicus Observatory uses the Universal Thermal Climate Index (UTCI), which takes into account temperature and humidity, but also wind, sunshine and thermal radiation, and classifies heat stress levels from moderate to extreme.
Extreme heat stress, according to this index, corresponds to a “felt temperature” of 46°C or more, above which it is necessary to take measures to cool the body.
The heat index, used by the United States Weather Service, provides a “feels like temperature” based on heat and humidity in shade, along with a colored graph, ranging from yellow ( “caution”) to red (“danger”) and bright red (“extreme danger”).
Their Canadian counterparts have developed the Humidex index, which establishes a “felt temperature” value in a table combining heat in the air and humidity, associated with four colors for as many degrees of discomfort. Also with its variation to assess the dangers of extreme cold.
Limited measures
Other “thermal stress” indices exist such as the “Tropical Summer Index”, the “Predicted Heat Strain” or the average radiant temperature.
But everyone has their limits.
WMO heatwave expert John Nairn said the Universal Thermal Climate Index is a “very poor measure” when used in countries where exposure to heat is more chronic than in Germany, where it was first developed.
“The way of approaching the issue is not the same everywhere in the world“, says Mr Nairn, who has advised governments and the WMO on heatwave policy.
These indices also do not take into account the non-health impact of heat, he adds, a heat wave can block trains or overload air conditioners, which is also dangerous.
“If the heat reaches a level such that it prevents the operation of an infrastructure, and it begins to break down, there will be a consequence on passengers who will no longer be protected“, says John Nairn.
Slide: Heatwave, high heat… What are the risks for our body?