How hot is too hot? What to know about wet bulb temperatures, an increasing danger in extreme heat.

·3 min read

The persistent heat wave in the Pacific Northwest has been blamed for hundreds of deaths in Oregon and Washington and into Canada.

It's a scenario experts warn will soon become all too common: As temperatures continue to rise, so will the death toll – potentially by the tens of thousands.

“By the mid-century, we anticipate a pretty significant extra burden of extreme heat and public health somewhere in the neighborhood of about 10,000 additional deaths,” Vijay Limaye, a climate and health scientist with the Natural Resources Defense Council, told USA TODAY.

Heat is already the deadliest weather-related hazard, killing about 1,300 Americans each year, Limaye said, and wet bulb temperatures – an underreported health outcome for decades – have increasingly become a most fatal culprit. Wet bulb conditions occur when it's too hot and humid for a human's sweat to evaporate, specifically at 95 Fahrenheit and 95% relative humidity.

The heat is on: How to stay hydrated, save on your home's cooling bill and protect your pets

A thermometer shows an official temperature at Death Valley National Park in California on July 11, 2021.
A thermometer shows an official temperature at Death Valley National Park in California on July 11, 2021.

But serious impacts can even occur at 79 degrees wet bulb. When that happens, “your body's natural cooling mechanisms can't work,” Limaye said.

In other words, when wet bulb temperatures are high, there’s so much humidity in the air that sweating becomes ineffective at removing the body’s excess heat, according to Colin Raymond, a researcher at the NASA Jet Propulsion Laboratory in California.

“At some point, perhaps after six or more hours, this will lead to organ failure and death in the absence of access to artificial cooling,” he told the National Oceanic and Atmospheric Administration.

WATCH: Record-breaking heat draws people to visit Death Valley

Extreme humidity has more than doubled in frequency since 1979, Limaye said. He pointed specifically to a study written by Raymond last year: Though climate models predicted these extreme temperatures by the mid-21st century, they’ve already occurred in places like India, Pakistan, the Gulf of Mexico and even in California.

The dangerous weather risks are “expected to significantly worsen the already pretty terrible burden of extreme heat on health,” Limaye said.

“We're getting to the point in which even in dry conditions, we are at risk of basically having potentially uninhabitable parts of the world – just kind of too hot for people to be outside working or moving around,” he added.

The sun shines behind the Space Needle in Seattle on June 28, 2021.
The sun shines behind the Space Needle in Seattle on June 28, 2021.

With no wind and sunny skies, an area with 50% humidity will hit an unlivable wet-bulb temperature at around 109, according to an article by MIT. In dry air, temperatures will become unlivable over 130 degrees – the temperature reached earlier this month in Death Valley, California.

Experts like Limaye are certain the increasing frequency of heat waves in general – and deaths that come along with them – are caused by human-created climate change.

The deadly and record-breaking heat wave in parts of the Western U.S. and Canada that began in late June and has stretched into July would have been "virtually impossible" without the influence of climate change, according to a recent study.

Every heat wave occurring today, in fact, is made more likely and more intense by climate change, the study found.

“If we continue to let climate change worsen year after year, what sort of health situation might we be confronted with, by say, the middle of the century around 2050?” Limaye asked.

Contributing: Doyle Rice, USA TODAY

This article originally appeared on USA TODAY: Heat waves are made worse by wet bulb temperatures. Here's how.

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