Humans have a much lower heat tolerance than expected

Every summer feels hotter than the last. News reports cover deadly heatwaves, rising humidity, and collapsing infrastructure under soaring temperatures. But while climate models predict extreme weather, a deeper question lingers: how much heat can the human body actually tolerate?

A recent study from the University of Ottawa’s Human and Environmental Physiology Research Unit (HEPRU) has provided fresh insights.

The team, led by Dr. Robert D. Meade and Professor Glen Kenny, set out to measure the biological limits of thermoregulation – our body’s ability to control internal temperature under conditions of environmental stress.

This research does more than update old data. It redefines our understanding of human survival in a warming world.

Heat tolerance is lower than expected

Until recently, scientists assumed that humans could tolerate “wet bulb” temperatures – where humidity and heat combine – up to 35 °C. This threshold was widely cited as the maximum point for survival. However, new experiments have challenged that belief.

“The conditions under which humans can effectively regulate their body temperature are actually much lower than earlier models suggested,” said Professor Kenny.

This revelation comes from a series of precise laboratory experiments that exposed volunteers to controlled heat and humidity.

The research team applied a method called thermal-step protocols, where air temperature or relative humidity is increased in steps to determine when the body loses control over its internal temperature.

This point, called the “core temperature inflection,” was assumed to mark the edge of thermoregulation. Yet, until now, no study had tested this assumption directly.

Testing human thermoregulation limits

To validate the threshold, researchers subjected 12 healthy adults to extreme conditions in a climate-controlled chamber. First, each participant underwent a humidity-step protocol.

They were gradually exposed to increasing levels of heat or moisture to determine their personal inflection point – the moment when their core temperature started climbing sharply.

But the experiment didn’t stop there. Each participant returned for two additional 9-hour trials. In one session, they were exposed to a wet bulb temperature just below their individual limit (around 30.9 °C).

In the other, they faced a temperature just above it (approximately 33.7 °C). The goal was to see whether the body could adapt and stabilize, or whether it would succumb to the heat.

When the body fails to cool itself

The outcome was decisive. “The results were clear. The participants’ core temperature streamed upwards unabated, and many participants were unable to finish the 9-hour exposure,” Dr. Meade explained.

These extended trials confirmed that even slightly exceeding the inflection point leads to dangerous, unmanageable rises in body temperature. Heat stroke levels (around 40.2 °C core temperature) could be reached in under 10 hours under “Tabove” conditions.

Even in the “Tbelow” trials, where conditions were slightly milder, the body struggled. It would take more than 24 hours to reach heat stroke, but the trajectory was still upward.

Predicting heat impacts on humans

This research does more than confirm a theory – it validates a method.

Thermal-step protocols have been used for decades to estimate limits for thermoregulation, but they lacked direct validation. Now, with this study, scientists can confidently rely on these models when projecting future human risks.

“Our findings [are] especially timely, given estimated limits for thermoregulation are being increasingly incorporated into large scale climate modeling,” noted Dr. Meade.

By testing human physiology in real-world simulations, this study connects lab science with global climate data. It also provides detailed measurements of physiological strain during prolonged exposure.

Even when core temperature didn’t immediately reach lethal levels, participants showed signs of uncompensated heat stress – rapid heart rates, fatigue, and inability to finish the trial.

Global climate map

As climate models grow more detailed, they are beginning to include human physiological limits. This study adds confidence to those models by showing that the lab-based thermal-step method can predict real-life responses.

It also shifts expectations: survivability thresholds now fall between 26 °C and 31 °C wet bulb temperature, which is much lower than the previously cited 35 °C mark.

“By integrating physiological data with climate models, we hope to better predict and prepare for heat-related health issues,” said Professor Kenny.

In other words, this isn’t just about lab numbers. It’s about how cities will respond to heat emergencies, how healthcare systems can adapt, and how urban planning must change. Policies can now draw on data that reflect the actual stress on human bodies under conditions of rising temperature.

Heat exposure may exceed human capacity

What makes this study urgent is the projection that large regions across the globe will soon experience these dangerous conditions.

Some of these zones include densely populated areas where millions may face extended periods of wet bulb temperatures of above 32 °C. With each degree rise, the margin for survival shrinks.

These findings serve as both a warning and a guide. They remind us that biological systems – unlike machines – can only adapt so far. The limits are real, measurable, and fast approaching.

Human survival under climate stress will depend not only on environmental policy but on understanding and respecting the boundaries of our own physiology.

This research marks a turning point. It shows that the future of climate adaptation must start with the human body. It is not a matter of theory, but of the unmistakable rise of core temperatures, the lab data collected over nine sweltering hours, and the shared vulnerability of every person facing a hotter planet.

The study is published in the journal PNAS.

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