Extreme 'Wet-Bulb' Heat Waves Are Suffocating the Eastern United States
New Study Challenges Previous Understanding of Human Heat Limit
A new study has challenged the previous understanding of the critical wet-bulb temperature threshold at which the human body can no longer cool itself. Experts have long believed that this threshold was at 35 degrees Celsius (95°F), but recent research suggests that it is actually much lower, around 31°C (87°F) at 100% humidity.
This updated threshold has significant implications for human survivability in a warmer world. When the wet-bulb temperature approaches 98.6 degrees Fahrenheit (37°C), it becomes extremely difficult to maintain a safe internal temperature. Under these conditions, sweat evaporation, the body's natural cooling mechanism, becomes ineffective, leading to dangerous overheating.
This lower threshold challenges earlier assumptions from a 2010 study that suggested humans would be fatally compromised at 35°C wet-bulb, a condition rarely if ever sustained on Earth. However, the 31°C threshold has already been observed in some parts of the world today, correlating with observed deadly heat wave mortality patterns.
Climate change is making it more common for parts of the world to reach or exceed the 31°C wet-bulb temperature threshold. Rising global temperatures and increasing humidity due to changing climate patterns are increasing the risk of heat-related mortality and public health crises.
Meteorologists use the wet-bulb globe temperature (WBGT) to measure heat stress in direct sunlight, and the wet-bulb temperature to understand the combined physiological impact of heat and humidity. These metrics are critical for understanding human survivability in a changing climate. Climate models suggest that certain regions of the world could see wet-bulb temperatures regularly topping 35°C within the next 30 to 50 years.
In the U.S., the oppressive heat dome that has affected the eastern states has prompted heat warnings for nearly 170 million Americans. On July 29, the National Weather Service (NWS) reported WBGT values in the high 80s to low 90s Fahrenheit across much of the eastern U.S., particularly in the Southeast and Midwest. Dangerous wet-bulb temperature events are expected to become more frequent due to human-caused global warming.
There is an urgent need to adapt infrastructure, public health systems, and extreme heat response measures to the changing climate. Kat Fisher, a PhD candidate in the human thermoregulatory lab at Penn State University, has stated that humans cannot maintain a stable core temperature if exposed to conditions above a wet-bulb temperature of around 87 degrees Fahrenheit for hours at a time. Severe humidity is making high temperatures feel even hotter, and this combination is a growing health threat.
| Aspect | Previous Threshold | Updated Threshold | Context/Implications | |-----------------------------|--------------------|-------------------|--------------------------------------| | Wet-bulb temperature limit | 35°C (95°F) | 31°C (87°F) | Based on recent human physiology experiments; 31°C observed in current world heat waves | | Human survivability | Theorized fatal at 35°C WB | Fatal risk begins at 31°C WB | More realistic critical limit, especially under resting conditions | | Impact of climate change | Rising frequency of high WB temperatures | Increased risk of surpassing 31°C WB threshold | More deadly heatwaves and public health crises |
References: [1] Penn State HEAT Project lab experiments, 2025 [3] Greenly Earth blog on wet-bulb and human heat risk, 2025
- Gizmodo recently reported on a new study that challenges the previous understanding of the critical wet-bulb temperature threshold for human survivability, suggesting it is actually 31°C (87°F) at 100% humidity.
- The updated threshold implies that climate change could lead to more deadly heatwaves and public health crises as certain regions of the world could see wet-bulb temperatures regularly topping 35°C within the next 30 to 50 years.
- Science, particularly environmental science and climate-change research, has raised concerns about the impact of technology on health-and-wellness and the environment, with weather forecasting being a critical area of focus as the world becomes hotter and more humid.
- Earthier, a renowned platform for climate-change news and research, highlighted the severe threat posed by ever-rising wet-bulb temperatures to human health, urging policymakers to prioritize extreme heat response measures and infrastructure adaptations in light of this new scientific evidence.
- Beyond the immediate health threats, this updated wet-bulb temperature threshold could also have far-reaching implications for the future of technology and human life as we know it, as our ability to survive and thrive in a hotter world may be more limited than previously thought.
