We may have sweltered through temperatures exceeding 40°C (104°F) in many parts of Greece last week, but this mini heat wave was, in fact, the country’s first significant one of the year.
In June, for example—while much of Europe was already baking—the average daytime air temperature in Greece (what meteorologists call the “average maximum temperature”) was 32.2°C (90°F). Yet throughout both June and the overall milder-than-expected July, many people had the feeling that the heat they experienced simply didn’t match what the thermometer was showing.
So why the contradiction?
According to Andreas Flouris, professor of physiology and director of the Environmental Physiology Laboratory (FAME Lab) at the University of Thessaly, the contradiction is real—but it lies not in our perception, but in the tool we’ve long relied on to describe heat.
“The standard thermometer measures the air temperature in the shade,” Flouris tells TO VIMA Science. “It doesn’t account for how much solar radiation the body is exposed to, how easily sweat evaporates, how windy it is, what we’re wearing, or how much heat our muscles generate. In other words, it describes the environment—not the overall exchange of heat between the human body and its surroundings.”
A Heat Stress Index, Not Just a Number
To overcome the limitations of the conventional thermometer, scientists now rely on an index known as the Wet-Bulb Globe Temperature (WBGT), which combines air temperature with wind speed, humidity, and radiant heat.
Outdoors in direct sunlight, WBGT assigns 90% of its weight to factors other than air temperature, with the air temperature itself accounting for only about 10% of the calculation.
These variables explain why two summer days with exactly the same temperature reading can affect the human body in completely different ways.
“In a dry environment with low solar radiation, a greater proportion of sweat evaporates and cools the body,” Flouris explains. “With high humidity, sweat simply runs down the skin without providing the same cooling effect. Add intense sunshine, and the body absorbs even more heat, almost as if it’s inside an oven.”
The 38°C Experiment
To demonstrate this difference, researchers at FAME Lab conducted an experiment whose findings are currently under review for publication in a scientific journal and are presented today by TO VIMA Science.
The team used an algorithm they developed themselves—the FAME Lab Predicted Heat Strain model—which predicts how environmental conditions, clothing, and physical activity affect the human body.
The simulations involved a heat-acclimatized adult measuring 1.70 meters (5 feet 7 inches) tall and weighing 70 kilograms (154 pounds). Throughout the experiment, the air temperature remained constant at 38°C (100.4°F). Only the other environmental conditions changed.
In the lower heat-stress scenario, researchers simulated a relatively dry day with low solar radiation and light wind.
In the higher heat-stress scenario, they simulated a relatively humid day with intense sunshine and moderate wind.
One Hour of Light Running
In the first simulation, the hypothetical participant completed one hour of light running.
In both environments, the body’s core temperature reached the precautionary safety threshold of 38°C (100.4°F) after approximately 16 minutes. That result was expected, since exercising muscles generate substantial amounts of heat.
By the end of the hour, however, the two scenarios had diverged significantly.
Under the lower heat-stress conditions, the runner’s maximum body temperature reached 38.8°C (101.8°F), while total sweat loss amounted to 0.8 liters.
Under the higher heat-stress conditions, body temperature climbed to 39.5°C (103.1°F), and sweat loss increased to 1.11 liters.
“With exactly the same air temperature, the participant in the higher heat-stress scenario finished the run with a body temperature 0.6°C higher and 36% greater dehydration,” Flouris says. “Even more importantly, the participant entered the dangerous hyperthermia zone, where heat stroke becomes highly likely.”
Eight Hours on the Job
The second simulation modeled an eight-hour work shift for a delivery driver, involving driving, walking, and carrying small loads.
In both scenarios, body temperature exceeded 38°C within the first half hour.
The crucial differences emerged during the hours that followed.
Under lower heat stress, the worker’s body temperature stabilized at around 38.2°C (100.8°F), with total sweat loss reaching 6.4 liters over the eight-hour shift.
Under higher heat stress, body temperature rose to 39.6°C (103.3°F), while sweat loss climbed to 10.6 liters.
Without adequate fluid intake, researchers estimated the worker would reach dangerous levels of dehydration after just one hour on the job.
Even Lower Heat Stress Carries Risks
Even the lower heat-stress scenario, however, cannot be considered safe.
Working for eight hours with a body temperature above 38°C is not a safe condition.
An analysis by Flouris and his colleagues found that people who routinely work with body temperatures above 38°C face nearly twice the risk of developing kidney disease or suffering acute kidney injury.
The World Health Organization (WHO) also states that during an eight-hour work shift, body temperature should not exceed 38°C.
In an increasingly hotter world, Flouris concludes, asking the right question has become more important than ever.
Simply asking, “What’s the temperature?” is no longer enough.
“The real question is: Given this humidity, this level of sunshine, and this amount of physical exertion, how much heat strain is the body being asked to handle?”
The answer, he says, could save lives.
