Extreme Heat: How To Measure It And Why This “Silent Killer” Is Dangerous For Health

Heat stress occurs when the body’s natural cooling systems are overwhelmed, leading to 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 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, 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 around the planet will be increasingly exposed to conditions that test the limits of human endurance.

More than a maximum temperature

Temperature is the most widely used and easily understood meteorological data, but the “record highs” that attract attention don’t fully reflect how heat can affect the human body.

For example, the same temperature can feel very different 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 range of factors including temperature, but also humidity, wind speed, clothing, direct sunlight, and even the amount of cement 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 of which attempt to simplify different environmental readings into a single number or graph.

Wind chill

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

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

Just six hours of exposure to 35º C with 100% humidity is 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 takes into account temperature and humidity, but also wind, sunlight, and radiated heat, to classify heat stress levels from moderate to extreme.

Heat stress, according to this index, is a “wind chill” temperature of 46º C or higher, at which point measures are required to avoid health risks.

The Heat Index, used by the U.S. National Weather Service, provides an “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 rating, which combines heat and humidity into a single number to reflect “perceived temperature.”