The following transcript has been edited for clarity.

There’s a temperature threshold beyond which the human body simply can’t survive — one that some parts of the world are increasingly starting to cross. It’s a “wet bulb temperature” of 95 degrees Fahrenheit (35 degrees C).

To understand what that means, it helps to start with how the human body regulates its temperature. Our bodies need to stay right around 98.6 degrees F. If that number gets too high or too low, bad things can happen. And since bodies are always producing heat from normal functions, like digesting, thinking, and pumping blood, we need a place for that heat to go. That’s why our bodies have a built-in cooling system: sweat. 

Sweat works by using a physics hack called evaporative cooling. It takes quite a bit of heat to turn water from a liquid to a gas. As droplets of sweat leave our skin, they pull a lot of heat away from our bodies. When the air is really dry, a little bit of sweat can cool us down a lot. Humid air, on the other hand, already contains a lot of water vapor, which makes it harder for sweat to evaporate. As a result, we can’t cool down as well. 

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This is where the term wet bulb temperature comes in: It’s a measure of heat and humidity, essentially the temperature we experience after sweat cools us off. We can measure the wet bulb temperature by sticking a damp little sleeve on the end of a thermometer and spinning it around. Water evaporates from the sleeve, cooling down the thermometer. If it’s humid, it hardly cools down at all, and if the air is dry, it cools down a lot. That final reading after the thermometer has cooled down is the wet bulb temperature. 

In Death Valley, California, one of the hottest places on Earth, temperatures often get up to 120 degrees F — but the air is so dry that it actually only registers a wet body temperature of 77 degrees F. A humid state like Florida could reach that same wet bulb temperature on a muggy 86 degree day. 

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When the wet bulb temperature gets above 95 degrees F, our bodies lose their ability to cool down, and the consequences can be deadly. Until recently, scientists didn’t think we’d cross that threshold outside of doomsday climate change scenarios.  But a 2020 study looking at detailed weather records around the world found we’ve already crossed the threshold at least 14 times in the last 40 years. So far, these hot, humid events have all been clustered in two regions: Pakistan and the Arabian Peninsula. 

The warm water in the Red Sea and the Persian Gulf makes the air above extremely humid. Inland, on the Arabian Peninsula, the arid continental heat causes temperatures to skyrocket. And when these two systems meet, they can tip the wet bulb temperature above that 95 degree F wet bulb threshold. 

In Pakistan, it’s a little less clear what’s driving these hot, humid extremes. But scientists think it’s caused by warm, humid air flowing inland during the monsoon season. As it passes over the Indus River, the air only gets more humid until it hits cities like Jacobabad, often referred to as one of the hottest cities on earth. To date, Jacobabad has crossed that deadly wet bulb threshold a whopping six times — the most of any single city on record. 

If we plot all these events over time, it’s clear these hot, humid extremes are increasing as the planet warms. Scientists expect these events to occur even more frequently in these regions going forward. Other places like coastal Mexico and a large portion of South Asia might soon be at risk of crossing these thresholds for the first time. 

Extreme heat is deadly at temperatures well below the 95-degree threshold. Healthy young adults can experience serious health effects at a wet bulb temperature of 86 degrees F. And even dry heat can be dangerous when people’s bodies simply can’t pump out sweat fast enough to cool themselves. 

Worldwide, extreme heat likely kills at least 300,000 people each year. But it can be notoriously difficult to track the death counts associated with individual heat waves. Heat often kills indirectly — triggering heart attacks, strokes, or organ failures — making it hard to determine whether those deaths were caused by the heat or an unrelated medical condition. 

Even relatively mild heat waves can be deadly when they occur in places where people are not prepared for those temperature extremes. For example, a 2010 heat wave in Russia, where summer temperatures rarely rise above 74 degrees F, killed an estimated 55,000 people despite only hitting about 100 degrees F.

Heat-related death counts are even harder to calculate in regions without accurate or timely death records. In Pakistan — home to many of the world’s humid heat records — the government doesn’t officially track deaths, said Nausheen Anwar, director of the Karachi Urban Lab, a research program that studies the impacts of extreme heat in Pakistan. Instead, her lab often relies on interviews with doctors, ambulance drivers, or graveyard owners to calculate the impacts of heat waves.

With every degree of global warming, these dangerous heat events are becoming even more likely. Stopping climate change may be our best chance to keep them as rare as possible.