The US is experiencing 'wet bulb' heat, which can be deadly even to healthy people

• Wet bulb conditions occur when heat and humidity are too high for sweat to evaporate.
• Such conditions can be fatal for humans if the temperature and humidity both exceed 95.
• Extreme heat and humidity are growing more common due to climate change.

People across the US and Canada are trying to stay cool as an historic heat wave sweeps the northern hemisphere.

Air-conditioning is in high demand, and even squirrels are vying for shade in the Pacific Northwest. For hundreds of people who couldn't stay out of the heat, this month's temperatures have been deadly.

The human body has some natural cooling mechanisms in place, but our tolerance for heat only goes so high. When the heat and humidity reach a certain point, there's nothing the body can do to cool itself down.

The phenomenon is called wet bulb temperatures, and it's becoming dangerously common around the world. The combination of high heat and humidity can cause even the healthiest people to overheat and die.

'Wet bulb' conditions occur when it's too hot and humid for sweat to evaporate

Humans typically stay cool by sweating. As sweat droplets form on the skin, they evaporate in the heat and cool the skin, bringing down the body temperature.

However, sweat can only evaporate if the air is dry enough to take up moisture. Once the relative humidity surpasses 95%, your sweat will simply collect on your skin and make you feel even hotter and stickier.

If you prefer dry heat over a more tropical climate, now you know why humid days can feel so suffocating.

When that humidity is combined with high heat (88 degrees Fahrenheit or higher), it's called wet bulb conditions. Such conditions can be fatal, and they're happening earlier than anticipated.

The coastal Middle East is experiencing the worst wet bulb heat in the world, but it's been happening in the US too. Southeastern states have reported multiple incidences of wet bulb temperatures at or above 88 degrees in recent years. Parts of Arizona and California have reported wet bulb temperatures as high as 95 degrees.

Even healthy people can die of overheating on a humid day

According to a 2020 study, wet bulb conditions are becoming increasingly common due to climate change.

The study, which was published in Scientific Advances, found that the human body cannot withstand wet bulb temperatures of 95 degrees Fahrenheit or above.

That level of extreme heat and humidity wasn't expected until the mid 21st century, according to climate models. But the study found more than 7,000 instances of wet bulb conditions between 1979 and 2017.

"Even if they're in perfect health, even if they're sitting in the shade, even if they're wearing clothes that make it easy in principle to sweat, even if they have an endless supply of water," co-author Radley Horton told VICE News. "If there's enough moisture in the air, it's thermodynamically impossible to prevent the body from overheating."

Understanding wet bulb conditions is an important step to avoiding overheating. As more areas of the world see high temperatures and humidity, weather advisories for dangerous heat could help people know when to shelter indoors. However, those without access to AC will have a harder time cooling off and face a greater risk of heat death.