When Does Humidity Become Dangerous to Your Health?

Excessive heat causes most of the weather-related deaths in the United States, more so than hurricanes, tornadoes, and floods, according to the National Weather Service.

Learning to cope with extreme heat is increasingly important, as the Earth recorded a streak of record-breaking temperatures last week. Some U.S. states are facing a multi-week heat wave that won’t cool off anytime soon.

While extreme heat can contribute to heat exhaustion, heat stroke, heat cramps, and death, the temperature only tells part of the story. Humidity can make heat waves even more dangerous.

The human body tries to get rid of excess heat and cool off by sweating, which is an “internal air conditioning system,” according to Vishnu Laalitha Surapaneni, MD, MPH, an assistant professor of medicine at the University of Minnesota Medical School.

However, this process is much slower in humid environments. When we don’t sweat fast enough, we can’t release heat, and our internal body temperature may rise to unsafe levels.

“Our bodies need to be around 98.6 degrees Fahrenheit to function optimally. If the temperature gets too high or too low, that’s when we run into trouble,” Surapaneni told Verywell.

When Does Humidity Become Dangerous to Human Health?

Scientists use “wet-bulb temperature” to measure heat and humidity together. This reading is taken when the bulb of a thermometer is wrapped in a wet cloth. As the water evaporates, it cools the bulb, just like the body cools itself by sweating.

Some scientists suggested that 35 degrees Celsius (95 degrees Fahrenheit) was the maximum wet-bulb temperature that the human body could handle—and being exposed to this temperature for six hours could be deadly even for a healthy individual.

It’s not a common occurrence, but some parts of the world, such as Jacobabad, Pakistan, have crossed this wet-bulb reading for short periods of time.

But the threshold for dangerous wet-bulb temperatures might be lower than 35 degrees C, according to a study published in the Journal of Applied Physiology last year. The researchers also found that one temperature threshold can’t be set because many different factors can influence when someone can no longer cool off effectively.

“One number is not going to tell you everything. There’s too much variability, both in physiology and personal characteristics,” said Daniel J. Vecellio, PhD, a co-author of the study and a postdoctoral research scholar with the Virginia Climate Center at George Mason University.

Vecellio added that people can face heat-related health risks well below any established wet-bulb temperature thresholds. “We already have lethal levels of heat,” he said.

Wet-bulb temperatures can help researchers understand the impact of climate change, but wet-bulb readings usually aren’t available on a basic weather report.

What Should You Look Out for When You Check the Weather?

During a heat wave, Surapaneni recommends checking the heat index, which measures what the temperature “feels like” to our bodies. It takes both relative humidity and air temperature into consideration.

A heat index between 103 and 124 degrees is “dangerous,” according to the National Weather Service. Heat cramps and exhaustion are likely to occur in this range.

If you want to check the heat index where you live, look for the “feels-like” temperature on the weather app.

“Regardless of whether it’s dry heat or humid heat, what we’re trying to do is make sure that our core body temperature stays around an optimal level and that excess heat is not building up in our body,” Surapaneni said.

What Should You Do in a Humid Heat Wave?

Extreme heat poses a greater risk for certain populations, including older adults, children, and those “disproportionately exposed to extreme heat because of their work, recreation, or housing environments,” according to Tania Busch Isaksen, PhD, MPH, a teaching professor in the University of Washington Department of Environmental and Occupational Health Sciences.

In periods of extreme heat, the Centers for Disease Control recommends that people stay in air-conditioned buildings, drink plenty of water, limit or pace outdoor activity, and take cool showers.

If you’re in a humid environment, you may need to engage in “exposure reduction interventions” faster than you would in a dry heat wave, Isaksen said.

Where you live also matters. If humidity and heat are common in your region, you might be acclimatized to the environment you live in. But places that used to have mild summers can now experience unexpected high temperatures because of climate change.

“A population’s health response to heat is place-specific. People living in Seattle don’t experience heat the same way as people living in Atlanta,” she said.

If you have underlying health conditions, you might also need to pay extra attention to your body in a heat wave, according to Surapaneni. She said that medications for anxiety and depression can interfere with the body’s temperature regulation, and diabetes can affect how the body manages heat as well.

“You should talk to your doctor and figure out your personalized heat safety plan,” Surapaneni said.