https://www.scientificamerican.com/article/heat-and-humidity-are-already-reaching-the-limits-of-human-tolerance/

Over the hundreds of thousands of years of our existence on the planet, modern humans have managed to adapt to a huge range of climates—from the arid heat of the Sahara Desert to the icy chill of the Arctic. But we have our limits. If temperatures and humidity rise high enough, even a robustly healthy person sitting still in the shade with access to water will succumb to the heat.

As heat waves grow hotter and more frequent, research has suggested some places will begin to see events that reach that limit of human tolerance in the coming decades. But now a new study shows they already have. The findings, published on Friday in Science Advances, underscore the need to rapidly curtail emissions of heat-trapping greenhouse gases and develop policies that will help vulnerable populations stay cool.