Wet-bulb-pandemics

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 evaporative cooling (sweat in humans and horses, saliva and water in dogs and other mammals); this helps to prevent potentially fatal hyperthermia due to heat stress. The effectiveness of evaporative cooling depends upon humidity; wet-bulb temperature, or more complex calculated quantities such as wet-bulb globe temperature (WBGT) which also takes account of 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.^[11] [12]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 (82 °F).^[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.^[14][15]

A 2015 study concluded that depending on the extent of future global warming, parts of the world could become uninhabitable due to deadly wet-bulb temperatures.^[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.^[13]