https://en.wikipedia.org/wiki/Wet-bulb_temperature
The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed.
Sustained wet-bulb temperature exceeding 35 °C (95 °F) is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature our bodies switch from shedding heat to the environment, to gaining heat from it.[8] Thus 35 °C (95 °F) is the threshold beyond which the body is no longer able to adequately cool itself. A study by NOAA from 2013 concluded that heat stress will reduce labor capacity considerably under current emissions scenarios.[9]
A 2010 study concluded that under a worst-case scenario for global warming with temperatures 12 °C (22 °F) higher than 2007, the wet-bulb temperature limit for humans could be exceeded around much of the world in future centuries.[10] A 2015 study concluded that parts of the globe could become uninhabitable.[11] An example of the threshold at which the human body is no longer able to cool itself and begins to overheat is a humidity level of 50% and a high heat of 46 °C (115 °F), as this would indicate a wet-bulb temperature of 35 °C (95 °F).
The 2015 Indian heat wave saw wet-bulb temperatures in Andhra Pradesh reach 30 °C (86 °F). A similar wet-bulb temperature was reached during the 1995 Chicago heat wave.