Across much of the United States, extreme heat is becoming not just more common, but more dangerous. As temperatures soar past 100°F in many regions and dew points climb into the 70s, wet-bulb temperatures—which combine heat and humidity—are approaching levels at which the human body can no longer cool itself.
A wet-bulb temperature of 72°F or higher may not sound catastrophic, but it marks a critical threshold. At this level, sweating—the body’s primary cooling mechanism—becomes far less effective. Once wet-bulb temperatures exceed 70°F for extended periods, even healthy individuals in the shade with unlimited water face rising risk of heat stress. Vulnerable populations—including the elderly, outdoor workers, and those without access to cooling—face even greater danger.
At the extreme, a wet-bulb temperature of 35°C (95°F) represents the upper physiological limit for human survival. Beyond this point, no human can maintain a safe internal body temperature, regardless of hydration, activity level, or shade. Death from heat stress can occur within hours. While this threshold was once considered impossible outside of the tropics, parts of the U.S. are now beginning to approach it during extreme heat events.
This escalation is not just a result of higher temperatures—it’s also driven by higher humidity. Warmer air holds more moisture, and as the climate warms, that capacity increases exponentially. According to the Clausius-Clapeyron equation, for every 1°C (1.8°F) rise in air temperature, the atmosphere can hold about 7% more water vapor. This feedback loop intensifies heatwaves, making humid heat a uniquely dangerous aspect of climate change in the U.S.
The consequences are serious. Wet-bulb temperatures between 70°F and 87°F can trigger widespread heat exhaustion, dehydration, and heatstroke, especially for those without adequate cooling. Once body temperature rises above 104°F (40°C), the risk of neurological damage and death increases rapidly. In the most extreme conditions, even brief outdoor exposure can be fatal.
This is not just a southern or coastal issue. Interior states, once buffered from tropical-like conditions, are seeing record-high dew points. Urban areas, with their heat-retaining infrastructure and limited vegetation, are especially at risk. And rural regions, often lacking access to air conditioning and medical services, are becoming more vulnerable with each passing summer.
What’s emerging is a new kind of climate emergency—not simply about rising temperatures, but about breaching the fundamental limits of human biology. Wet-bulb temperatures are a more accurate measure of this crisis than air temperature alone, and they’re telling us that large regions of the U.S. are dangerously close to becoming intermittently uninhabitable during extreme events.
To prevent this from becoming the new normal, the U.S. must act on multiple fronts:
- Rapidly reduce greenhouse gas emissions to limit further warming
- Upgrade infrastructure to support widespread access to cooling
- Implement national heat alert systems based on wet-bulb temperature, not just air temperature
- Ensure equitable protection for those most at risk
The science is clear: wet-bulb extremes are a warning light on the dashboard of the American climate system. We ignore them at our peril.
