Human heat tolerance is significantly lower than previously thought, creating a dangerous reality where billions could face life-threatening conditions with less than 2°C of additional global warming.

At a Glance

• New research confirms human heat tolerance limits are lower than previously estimated, with uncompensable heat stress occurring at wet-bulb temperatures of 31°C or lower
• Extended periods of dangerous heat could occur with less than 2°C of global warming, affecting regions in sub-Saharan Africa, east China, the Indus River Valley, and the Persian Gulf
• Heat adaptation can reduce performance losses by over 50%, but reliance on air conditioning is neither sustainable nor equitable
• Older adults have significantly lower heat tolerance thresholds (21.9–33.7°C) than young adults (25.8–34.1°C)

The Critical Threshold for Human Heat Tolerance

For decades, scientists believed humans could tolerate wet-bulb temperatures (a measure combining heat and humidity) up to 35°C before thermoregulation became impossible. Recent studies have dramatically revised this threshold downward, revealing that dangerous heat stress begins at much lower temperatures. This revelation has profound implications for public health as climate change accelerates, potentially exposing billions to deadly conditions much sooner than anticipated.

Researchers at the University of Ottawa conducted groundbreaking experiments exposing volunteers to various heat and humidity conditions. Their findings validated concerns that the human body begins to lose its ability to regulate temperature at wet-bulb temperatures significantly below 35°C. This creates a much narrower safety margin between current climate conditions and truly dangerous heat events, especially for vulnerable populations like the elderly and those with chronic health conditions.

Hello everybody,
We're all thinking about Trump winning the US election. It's easy, isn't it, to go into panic mode? It's all terrible; it all feels so hopeless.

What I want to suggest in this talk is that all of this is very far from being true. In fact, Trump winning the… pic.twitter.com/CktpcUHCB0

— Roger Hallam (@RogerHallamCS21) November 21, 2024

Regional Vulnerabilities and Economic Impacts

The consequences of exceeding human heat tolerance aren’t distributed equally across the globe. Research indicates that regions including sub-Saharan Africa, east China, the Indus River Valley, and the Persian Gulf could experience extended periods of uncompensable heat with less than 2°C of additional global warming. These areas are home to billions of people, many in developing nations with limited resources for adaptation measures like widespread air conditioning.

Beyond the immediate health risks, economic consequences of increasing heat are substantial. Studies show declining labor productivity, human capital formation, and income growth during hot periods. While large-scale heat mortality events haven’t been widely documented yet, chronic exposure to extreme heat can cause long-term organ damage and exacerbate existing health conditions. These findings suggest current estimates of climate change damages may be significantly underestimated.

This announcement by Sundar Pichai, the CEO of Google and its parent company Alphabet, introduces the “Willow” quantum computing chip, a groundbreaking advancement in technology that addresses one of quantum computing’s greatest challenges: scaling up qubits while exponentially… pic.twitter.com/mtWoldBKZG

— The Intellectualist (@highbrow_nobrow) December 10, 2024

Individual Vulnerability and Adaptation Capacity

Heat tolerance varies significantly between individuals and populations. A particularly concerning finding shows older adults have heat tolerance limits between 21.9°C and 33.7°C wet-bulb temperature, substantially lower than young adults at 25.8°C to 34.1°C. This age-related vulnerability creates additional challenges for regions with aging populations. Interestingly, some populations show physiological adaptations to heat, as documented in a study of Malaysians who demonstrated less sensitivity to warmth.

While some adaptation is possible, its extent is limited. Research analyzing collegiate track and field performances found that heat adaptation can mitigate performance losses by over 50%, but complete adaptation remains impossible at extreme temperatures. Reliance on mechanical cooling like air conditioning presents its own challenges—it’s energy-intensive, contributes to urban heat islands, and remains inaccessible to many vulnerable populations in developing regions.

Climate Policy Implications

These revised heat tolerance limits strengthen the scientific case for the Paris Agreement’s target of limiting warming to well below 2°C. With dangerous heat conditions possible even below this threshold, policymakers must integrate these findings into climate adaptation strategies and health policies. Cities and communities need to implement heat action plans that prioritize vulnerable populations and create cooling centers for extreme events.

Looking ahead, global warming of approximately 7°C could render significant regions uninhabitable due to heat stress—a worst-case scenario that would trigger unprecedented migration and humanitarian crises. Understanding these lower heat tolerance thresholds emphasizes that climate change isn’t just an environmental concern but a direct threat to human habitability of large portions of Earth. This makes aggressive emissions reduction and adaptation planning not just advisable but essential for preserving human health and wellbeing.

Sources:

https://www.journals.uchicago.edu/doi/10.1086/715509

https://www.pnas.org/doi/10.1073/pnas.2316003120

https://pmc.ncbi.nlm.nih.gov/articles/PMC2906879/

https://www.uottawa.ca/about-us/news-all/new-study-validates-lower-limits-human-heat-tolerance

https://www.nature.com/articles/s41467-023-43121-5

https://medicalxpress.com/news/2025-03-validates-limits-human-tolerance.html

https://pmc.ncbi.nlm.nih.gov/articles/PMC9281079/

https://www.sciencedaily.com/releases/2025/03/250331192148.htm

https://www.sciencedirect.com/science/article/pii/S0013935124007072