Wet-bulb temperature is a weird metric, but basically, it’s an effort to incorporate both heat and humidity into one number. In short, it’s the measure of what a thermometer would read with a wet cloth wrapped around it. In a dry environment, water evaporating off that cloth will cool things down, lowering the temperature. But if the air is already saturated with humidity, there will be less evaporation, and therefore less cooling.
Take two examples of conditions that would reach a 35 °C wet bulb temperature. With mostly dry air, temperatures have to top 130 °F (54 °C) to reach that limit. On the other hand, a temperature of 109 °F (43 °C) and a relative humidity of 50% would result in the same wet-bulb temperature.
It’s a useful metric because it can give you an idea of how much your sweat will be able to cool you down. Above a wet-bulb temperature of 35 °C, your body won’t be able to lose enough heat through the evaporation of sweat. But that’s still a theoretical limit—one that hadn’t been tested much in humans until recently.
Early research has found that the limit might turn out to be more varied, but lower, than theory would suggest. One 2021 study found that even in healthy young adults, heat loss couldn’t keep up at lower temperatures than the theoretical limit, especially in humid environments.
Bottom line: researchers are still trying to understand where our limits lie when it comes to heat, though we do know it’ll depend a lot on specific environmental and health factors. There’s also some interesting research showing that our heat tolerance can change over time—as we age, yes, but even with the amount of heat we’re exposed to.
How can we handle the heat better?
One thing that I found fascinating when I started looking into extreme heat a few years ago is the concept of acclimatization: our bodies can adjust to the heat.
If you’re exposed to heat consistently, your body will go through a few changes, Schlader says. You will start making more plasma, basically pushing up your total volume of blood. That means your heart won’t have to work as hard to move blood around (one of the major ways we lose heat is through blood carrying it to our skin). The process of sweating also changes—you’ll be quicker to sweat, your sweat will increase in volume, and it will get more dilute, so you’ll lose fewer electrolytes. The whole thing is somewhat akin to how you can adjust to a higher altitude.
There’s been a lot of fighting online this week over a Washington Post story that talked about this exact concept. People argued not only about whether this effect is real, but also about whether it’s a big distraction from the need to address climate change.