How to measure extreme heat, dangerous for health

The scorching ones temperatures that are taking lives since India until Mexico and Greece They are part of the hottest year recorded to date, and experts offer a series of recommendations to detect the danger threshold.

“Silent murderer”

Heat stress occurs when the body’s natural cooling systems are overwhelmed, causing dizziness, headaches, then organ failure and death.

It is triggered by prolonged exposure to heat and other environmental factors that together overwhelm the human body’s internal ability to regulate temperature.

“Heat is a silent killer, because the symptoms are not obvious. And when these underlying conditions are present, the consequences can be very bad and even catastrophic,” explains Alejandro Sáez Reale from 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 elevated risk.

The WMO estimates that heat kills around half a million people a year, although the true number is unknown and could be 30 times higher than currently recorded, according to the organization.

As climate change makes heat waves longer, more intense and more frequent, people across the planet will be increasingly exposed to conditions that test the limits of human endurance.

More than a maximum temperature

Temperature is the most used and easily understood meteorological data, but “historic highs” that attract attention do not fully tell how heat can affect the human body.

For example, the same temperature can feel very differently in one place compared to another: 35º C in a desert is not the same as in a jungle.

To build a more complete picture, scientists consider a number of factors including temperature, but also humidity, wind speed, clothing, direct sunlight and even the amount of concrete or vegetation in the area.

All of these factors play an important role in how the body perceives and, most importantly, responds to extreme heat.

There are many ways to measure heat stress, some of which are decades old, but all attempt to simplify different environmental readings into a single number or graph.

“Thermal sensation”

One of the oldest methods is known as wet bulb temperature, a useful measurement in situations where the thermometer reading may not seem so extreme, but when combined with humidity, it becomes unbearable and even lethal.

He “bulb” It is the mercury deposit of a traditional thermometer, wrapped in a damp cloth, whose evaporation serves to measure the humid temperature of the air.

Only six hours of exposure at 35º C with 100% of humidity are enough to kill a healthy person, scientists said in 2023.

Above this limit, sweat cannot evaporate from the skin, and the body overheats and dies.

Copernicus, the EU climate observatory, uses the Universal Thermal Climate Index (UTCI), which considers temperature and humidity, but also wind, sunlight and radiated heat to classify heat stress levels from moderate to moderate. extreme.

Stress caused by extreme heat, according to this index, is a temperature of “thermal sensation” of 46º C or higher, at which point it is necessary to take measures to avoid health risks.

The Heat Index, used by the US National Weather Service, offers a “apparent temperature” based on heat and humidity in the shade, and a color-coded graph indicating the likelihood of illness from exposure.

Canada has developed the Humidex classification, which combines heat and humidity into a single number to reflect the “perceived temperature”.

Limitations

Heat wave expert John Nairn explains that some heat measures work better in some climates than others. “It’s not the same all over the world, about the way you approach it”explains Nairn to AFP.

The UTCI, for example, is excellent for measuring thermal stress in Germany, where it was first developed, but “a very poor measure” in southern countries, he said.

In those countries it is better to use the wet bulb temperature method.

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