Dangerous humid heat days have more than doubled globally due to climate change, new analysis finds

Human-caused climate change is now the primary driver of dangerous humid heat worldwide, increasing health risks for hundreds of millions of people.

PRINCETON, N.J., June 24, 2026 /PRNewswire/ -- Dangerous humid heat days have more than doubled globally since the 1970s, increasing from an average of 10 days per year to 23 days per year, according to a new analysis from Climate Central. The analysis shows that human-caused climate change is now the primary driver of dangerous humid heat, contributing to nearly two-thirds of all dangerous humid heat days worldwide and increasing the risk of heat-related illness and death for hundreds of millions of people.

Extreme heat has claimed more than a quarter-million lives globally since 2000, making it one of the deadliest weather-related hazards. While high temperatures pose risks to everyone, older adults, children, pregnant people, individuals with underlying health conditions, and people without access to cooling face disproportionately greater dangers. Humidity only piles on to the risks, making even seemingly mild days far more dangerous than they appear. As global temperatures continue to rise due to the burning of fossil fuels, dangerous humid heat is becoming more frequent and widespread.

Analysis Highlights:

• Comparing the 1970s to the past decade (2016-2025), dangerous humid heat days have more than doubled globally, from an average of 10 to 23 days per year.
• Climate change is causing more dangerous humid heat days in 69% (665) of 961 global cities analyzed, by an average of 46 more days per year during the last decade.
• Globally, climate change is now responsible for six times as many dangerous humid heat days each year, compared to the 1970s.
• In 2025, the global average reached 23 dangerous humid heat days, with 19 of those days (83%) added by climate change.
• Climate change is causing more dangerous humid heat days in 65% (161) of 247 U.S. cities analyzed — an average of 19 such days per year during the past decade.

Humidity plays a critical role in determining how dangerous heat is. High humidity reduces the body's ability to cool itself through sweating, causing heat to build up internally and increasing the risk of dehydration, cardiovascular and respiratory problems, heat exhaustion, heat stroke, and other serious health impacts.

Climate Central developed a groundbreaking new approach to quantify the influence of climate change on dangerous humid heat conditions worldwide. The analysis is based on wet-bulb temperature, a measure that combines heat and humidity to assess physiological stress on the human body.

The analysis defines wet-bulb temperatures of 25°C (77°F) or higher as "dangerous" humid heat conditions, under which many people face an elevated risk of heat-related illness. Researchers analyzed global conditions from 1970 through 2025 to determine how often dangerous humid heat occurred and the extent to which human-caused climate change contributed to those conditions.

This analysis builds on new peer-reviewed research published in May 2026.

Lisa Patel, MD, MESc, FAAP, Clinical Associate Professor of Pediatrics at Stanford Children's Health, and Executive Director at the Medical Society Consortium on Climate and Health, said:

"As a pediatrician, these numbers are a wake-up call. Dangerous humid heat has more than doubled since the 1970s. We're already seeing the consequences play out in real time. Fans are fainting at World Cup matches in cities like Houston, and that's not a coincidence. This kind of data is exactly the tool clinicians and public health officials need to anticipate where heat-related illness will strike and who is most at risk before people end up in the ER."

Kaitlyn Trudeau, Applied Climate Scientist at Climate Central, said:

"These findings show how profoundly climate change is reshaping our world. Dangerous humid heat has gone from being an uncommon event to a defining feature of daily life in some regions, pushing conditions closer to the limits of what the human body can safely endure."

SOURCE Climate Central