Living organisms can survive only within a certain temperature range. When the ambient temperature is excessive, many animals cool themselves to below ambient temperature by  https://en.wikipedia.org/wiki/Evaporative_cooling - evaporative cooling  (sweat in humans and horses,  https://en.wikipedia.org/wiki/Saliva - saliva  and water in dogs and other mammals); this helps to prevent potentially fatal  https://en.wikipedia.org/wiki/Hyperthermia - hyperthermia  due to heat stress. The effectiveness of evaporative cooling depends upon humidity; wet-bulb temperature, or more complex calculated quantities such as  https://en.wikipedia.org/wiki/Wet-bulb_globe_temperature - wet-bulb globe temperature  (WBGT) which also takes account of  https://en.wikipedia.org/wiki/Solar_radiation - solar radiation , give a useful indication of the degree of heat stress, and are used by several agencies as the basis for heat stress prevention guidelines.

It has been thought that a sustained wet-bulb temperature exceeding 35 °C (95 °F)—given the body's requirement to maintain a core temperature of about 37°C—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. ^{https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-pnas-11 - [11]   https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-12 - [12]} In practice, such ideal conditions for humans to cool themselves will not always exist – hence the high fatality levels in the  https://en.wikipedia.org/wiki/2003_European_heat_wave - 2003 European  and  https://en.wikipedia.org/wiki/2010_Russian_wildfires - 2010 Russian  heat waves, which saw wet-bulb temperatures no greater than 28 °C (82 °F). ^{https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-toohot2020-13 - [13]}  A 2022 study on the effect of heat on young people found that the critical wet-bulb temperature at which heat stress can no longer be compensated, T_wb,crit, in young, healthy adults performing tasks at modest metabolic rates mimicking basic activities of daily life was about 30.55°C in 36–40°C humid environments, but progressively decreased in hotter, dry ambient environments. ^{https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-vecellio-14 - [14] https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-timperley-15 - [15]}

A 2015 study concluded that depending on the extent of future  https://en.wikipedia.org/wiki/Global_warming - global warming , parts of the world could become uninhabitable due to deadly wet-bulb temperatures. ^{https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-16 - [16]}  A 2020 study reported cases where a 35 °C (95 °F) wet-bulb temperature had already occurred, albeit too briefly and in too small a locality to cause fatalities. ^{https://en.wikipedia.org/wiki/Wet-bulb_temperature#cite_note-toohot2020-13 - [13]}