The limits of human thermoregulation—the human body’s ability to maintain a stable temperature in conditions of extreme heat and humidity—are lower than previously thought, according to researchers Robert Meade and Glen Kenny, both associated with the Human and Environmental Physiology Research Unit at the University of Ottawa.
It’s important to maintain a stable system because our entire physiology is based on temperature control, explained Professor Glen Kenny, whose work is published in PNAS, the Proceedings of the National Academy of Sciences of the United States.
What Happens When Temperatures Rise?
Understanding how much heat the human body can tolerate is essential in the current context of climate change. In fact, several regions of the planet are already experiencing temperatures and humidity levels that exceed the limits in which humans can survive, and more regions will join the list in the coming decades.
According to the UN, by 2024, the 50°C threshold will have been exceeded in at least ten locations in Morocco, the United Arab Emirates, India, China, and the United States. This threshold has previously been surpassed in other countries, including Mexico, Jordan, Iraq, and Pakistan.
In Canada
Although Canada has not yet surpassed the 50°C threshold, summers there are getting hotter. During the June 2021 heat wave in the west, temperatures rose to 49.5°C in the town of Lytton, north of Vancouver. Dozens of deaths were subsequently linked to this extreme heat event that lasted several days.
For this reason, it is important that public health authorities have accurate assessment tools that allow them to intervene quickly with the most vulnerable groups, particularly the elderly and people living with chronic conditions. Physiologically, humans use sweat to cool their bodies, which is no longer possible at high humidity.
Toward a Better Understanding of Heat Stress
To date, the theoretical limit of human survival has been 35°C with a wet bulb, meaning a temperature of 35°C with 100% humidity, or 46°C with 50% humidity. However, new research has shown that this limit is lower and ranges between 26°C and 31°C with a wet bulb.
To arrive at these new figures, the research team exposed 12 participants for 9 hours to different heat and humidity conditions while they rested in a temperature-controlled room.
The volunteers then had to return to the laboratory for another full day of exposure to extreme conditions just above their estimated thermoregulatory limit. In this experiment, the volunteers were subjected to conditions of 42°C with 57% humidity, representing a humidex factor of approximately 62. The researchers were thus able to determine the point at which thermoregulation becomes impossible.
Our results are clear. The participants’ body temperature rose continuously, so many were unable to complete the 9-hour exposure, explained researcher Glen Kenny. The thermoregulatory limits obtained by the Canadian researchers are derived from the progressive thermal shift protocol.
Professor Kenny explained that what distinguishes their work from others conducted according to the protocol’s parameters is that the volunteers had to return to the laboratory and spend another day exposed to conditions just above their estimated thermoregulatory limit.
Our data constitute the first direct validation of the protocol, which has been used to estimate the upper limits of thermoregulation for nearly 50 years. Another aspect to consider is that a person’s thermoregulatory capacity can vary seasonally. Furthermore, the thresholds are different for certain groups, such as the elderly and those living with chronic conditions.
The researchers believe that this reality and the new knowledge about thermoregulation in the human body should be taken into consideration and guide implemented health policies and public safety measures.
By integrating physiological data into climate models, we hope to better predict heat-related health problems, allowing us to better intervene with the most vulnerable people, emphasized Professor Glen Kenny.
