WetBulb 35

The temperature+humidity at which the human body can't survive because it can't regulate its temperature.

A 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 human bodies switch from shedding heat to the environment, to gaining heat from it.[9] In practice, such ideal conditions for humans to cool themselves will not always exist – hence the high fatality levels in the 2003 European and 2010 Russian heat waves, which saw wet-bulb temperatures no greater than 28 °C. https://en.wikipedia.org/wiki/Wet-bulb_temperature#Wet-bulb_temperature_and_health

The wet-bulb depression is the difference between the dry-bulb temperature and the wet-bulb temperature. If there is 100% humidity, dry-bulb and wet-bulb temperatures are identical, making the wet-bulb depression equal to zero in such conditions https://en.wikipedia.org/wiki/Wet-bulb_temperature

• at 80% humidity, 95degF = wetbulb 89F; 101degF = wetbulb 95F

Instead of simply looking at the thermometer to see how hot it is, a wet-bulb temperature is taken by first wrapping the bulb of the thermometer in a wet cloth – hence, “wet bulb”. The cloth acts as a sort of proxy for human skin: if the water evaporates, the thermometer is cooled, and the wet-bulb temperature will be lower than the air temperature. But in high humidity, when the water can’t evaporate as well, this doesn’t happen.

https://www.washingtonpost.com/weather/2021/07/24/wet-bulb-temperature-extreme-heat/