https://www.theguardian.com/environment/2017/aug/02/climate-change-to-cause-humid-heatwaves-that-will-kill-even-healthy-people

> Even outside of these hotspots, three-quarters of the 1.7bn population – particularly those farming in the Ganges and Indus valleys – will be exposed to a level of humid heat classed as posing “extreme danger” towards the end of the century.

> The new analysis assesses the impact of climate change on the deadly combination of heat and humidity, measured as the “wet bulb” temperature (WBT). Once this reaches 35C, the human body cannot cool itself by sweating and even fit people sitting in the shade will die within six hours.

Wet Bulb Temperature is the temperature you can cool the air to using water evaporation only, saturating the air to 100%. At 100% humidity it is equivalent to the actual temperature. The theoretical measure can in practice be approximated with a thermometer with its bulb wrapped in a wet cloth.

https://en.wikipedia.org/wiki/Wet-bulb_temperature