In the hottest parts of the world, high temperatures and humidity will, for longer stretches, surpass a threshold that even young and healthy people could struggle to survive as the planet warms, study says
The paper is here
Figure 1 shows the locations:
Annual hot-hours under (A) 1.5, (B) 2, (C) 3, and (D) 4 °C of warming relative to preindustrial level
humidity is exactly it – the article mentions wet bulb temperature – your body relies on evaporation for cooling – you can survive insanely high temperatures in a desert (ex. Sahara gets up to 130°F) as long as you can sweat and cool off, but if the humidity is too high (ex. tropics), your sweat doesn’t evaporate, not only do you not cool off but you start heating up faster (article mentions that this can start as low as 88°F)