Humid heat days are a growing threat to public health

In 2024, the planet faced more hot, sticky days than ever before. The American Meteorological Society reported that humid heat days over land rose by 35.6 compared to the 1991-2020 average. That broke the previous record from 2023 by nearly 10 days.

This is not just about hotter summers. Humidity is climbing to levels that make normal cooling impossible.

When air holds too much moisture, sweat doesn’t evaporate. The body keeps the heat inside, which quickly becomes dangerous.

Our body struggles in humid heat

Kate Willett from the UK Met Office explained the risk clearly. “Your body starts to really struggle to offload heat, and so it can be really dangerous.”

Meteorologists track this with a “wet bulb temperature.” They cover a thermometer with a wet cloth.

Normally, evaporation cools the reading. In muggy conditions, that effect fades, and the wet bulb temperature rises close to the dry one. It represents how oppressive the air feels.

Humid heat around the world

A warmer atmosphere holds more water. That means heavier rains, stronger storms, and more humid days. Willett said 2024 was “exceptionally” humid, second only to 2023.

The numbers tell the story. In the Middle East, South-East Asia, and eastern China, wet bulb temperatures often surpassed 29°C (84°F).

Some places even touched 31°C (88°F). At that point, being outside for long periods is unsafe.

How much heat can humans take?

For years, scientists believed survival ended at 35°C (95°F) wet bulb temperature. But research in 2022 lowered the limit. The real danger may begin around 31°C (88°F).

“30°C and above is where your body is really struggling,” Willett said. That means the danger line is much closer than many assumed.

America’s muggiest summer

The United States faced its own climate crisis as the summer of 2025 began. More than 70 million people endured what scientists have called the muggiest start to a summer in recorded history.

Across much of the eastern half of the country, the air felt thick and heavy for weeks, with heat combining with record-breaking humidity.

Data showed that parts of 27 states, along with Washington, D.C., experienced an unusual number of days with dew points above 65°F.

Meteorologists often use this mark as the point when the air shifts from merely noticeable humidity to levels that feel uncomfortable and oppressive for most people. In simple terms, the air felt more like a tropical zone than a typical American summer.

Humidity rises in U.S. cities

Some cities felt it worse than others. Philadelphia reported 29 separate days when dew points soared beyond 75°F, creating air that many residents described as suffocating. Washington, D.C., had 27 days at this extreme level, while Baltimore experienced 24.

To put that in perspective, even the National Weather Service office in Tampa, a city used to sticky summers, classifies dew points of 75°F and above as “oppressive.”

“This has been a very muggy summer. The humid heat has been way up,” said Bernadette Woods Placky from Climate Central.

Dew points rising fast

Dew point measures moisture in the air. It’s the most accurate way to explain humidity. In cities like Washington, Baltimore, and St. Louis, dew points this summer were about six degrees higher than the 1951–2020 average.

Cameron Lee, a climate scientist at Kent State University, measured dew points of 82°F at his home in Ohio.

“There are parts of the United States that are experiencing not only greater average humidity, especially in the spring and summer, but also more extreme humid days,” Lee said.

The data shows that what used to happen once a year is now happening several times a summer.

Nights stay hotter with humid heat

High humidity keeps heat trapped at night. Normally, the body cools off while we sleep. But this summer, cities from the Ohio Valley to the Atlantic broke records for the hottest overnight lows.

“What really impacts the body is that nighttime temperature,” said Zack Taylor from the National Weather Service. Without cooling, the body doesn’t recover from the day. The strain builds day after day.

Meteorologists link these conditions to both climate change and shifting weather patterns. Persistent high-pressure systems trapped heat, while heavy rains added more moisture. Missing were the cooler air fronts that once offered relief.

“Summers now are not your grandparents’ summers,” noted Marshall Shepherd, a professor at the University of Georgia.

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