The threshold for survival in high temperature conditions is lower than previously thought
Photo: AP.
As climate change heats up the Earth, hot days have become a regular feature in weather reports around the world. In July, the record for the hottest day in the world was broken twice. Around 70 percent of the world’s workforce is currently at high risk from extreme heat.
“The problem is that conditions may seem hot today, but we don’t actually know how it will affect people,” explains Ollie Jay. “By modeling these conditions and exposing people to them under close medical supervision, we can better understand the physiology of the human response.”
Jay's team uses a state-of-the-art climate chamber to study conditions under which heat is life-threatening, and how and what practical, scientifically proven ways to stay cool exist.
The chamber is a 4-by-5-metre room that can raise or lower the temperature by 1 degree Celsius every minute, as well as adjust wind speed and simulate sunlight with infrared lamps.
The subjects can eat, sleep and exercise inside the chamber; researchers pass them food and other items through a hatch. Sensors attached to them send information to a nearby control room, which processes data on variables such as heart rate, breathing, sweating and body temperature.
According to Jay, the model used a temperature of 35 degrees Celsius as the limit of human survival. Once that threshold was reached, the body's internal temperature would rise uncontrollably. But back then, the scientists were treating the human body as a naked object that didn't sweat or move.
In a study conducted back in 2021, researchers came up with a more precise estimate: the limit of survival is about 31 degrees Celsius. They calculated it by tracking the body temperatures of young, healthy people in different combinations of temperature and humidity while cycling.
Jay's team is testing a mathematical model of how the body copes with extreme heat. Most studies of heat response focus on young, healthy people in the shade. But Jay's model has been able to estimate survival limits in shade and sunlight based on age, and during rest and exercise. Among their results, they estimated survival limits to range from 26 to 34 degrees Celsius for young people and from 21 to 34 degrees Celsius for older people.
In future trials, the researchers plan to test the body’s response to heat in shaded and sunny conditions, at all ages, and during exercise.
Another goal of the lab’s work — finding effective cooling strategies — is to mimic environmental conditions where heat can affect workers’ health. In one trial, Jay’s team tested cooling strategies that could help garment workers in Bangladesh, where they typically work long hours in hot climates without access to air conditioning.
“We recreated those indoor conditions and the work that people were doing — women sewing and men ironing,” the researcher said. “The participants were wearing clothes that factory workers would typically wear.”
The researchers found that in humid conditions, using fans reduced the workload on the heart up to an air temperature of at least 100.4 degrees Fahrenheit (38 degrees Celsius). But in dry heat, using a fan increased the workload on the heart. Moisturizing your skin is beneficial in both dry and humid heat.