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KEY FACTS

• Dangerous humid heat days have a daily maximum wet-bulb temperature of 25°C (77°F) or higher. These conditions put many people at risk of heat-related illness.

• Climate change is causing more dangerous humid heat days in 65% (161) of 247 U.S. cities analyzed — an average of 19 more days per year during the past decade.

• The South and Southeast regions experience the most days with dangerous humid heat in the U.S. Some areas of the Gulf Coast currently experience more than 100 such days annually, on average.

• Humid heat risks are expanding across the country. Some places that rarely experienced dangerous humid heat in the past now see more because of climate change, particularly in the Upper Midwest, Ohio Valley, and Northeast.

Extreme heat has claimed more than a quarter-million lives globally since 2000. While anyone can be affected by high temperatures, the risks are disproportionately felt by some populations including older adults, people with underlying health conditions, pregnant people, children, and those who lack access to cooling. 

But what makes heat most dangerous is something that often gets overlooked: humidity. Air temperature only tells one part of the story — the rest is hidden in humidity, which can make seemingly mild days far more dangerous than they appear. 

As the air becomes more humid, it interferes with the primary mechanism the body uses to protect itself against extreme heat: sweating. As sweat evaporates into the air it carries heat away from the body. But as the air approaches saturation, that process breaks down, leaving dangerous levels of heat trapped inside the body.

Sweat can’t evaporate as efficiently in high humidity, making it harder to shed heat and cool down. As heat builds up in the body, the heart works harder and the risk of dehydration and heat-related illness (including heat exhaustion and heat stroke) increases. Heat stroke is a medical emergency that can lead to organ damage or even death without rapid treatment. 

More dangerous humid heat on a warming planet 

As our planet heats up, primarily due to burning fossil fuels, humid heat is becoming more frequent and intense. 

Climate Central has developed a method to determine the influence of climate change on dangerous levels of humid heat across the globe, every day. This method is based on the wet-bulb temperature, a combined measure of heat and humidity. 

In this report, we define dangerous humid heat days as having a daily maximum wet-bulb temperature of 25°C (77°F) or higher. These conditions put many people at risk of heat-related illness. We quantify how climate change has impacted the frequency of dangerous humid heat days since 1970. 

Read the full report here.