Wet bulb temperatures are a combination of temperature and humidity, basically a rough measure of how much heat gets removed through optimal evaporation in given conditions.
Unfortunately the metric is both unclear on its face and a bit flawed in that a given wet bulb temperature can mean wildly different things depending on the actual temperature. The lowest wet bulb numbers that have been found to be extremely dangerous are basically a combination of extremely high heat with low or moderate humidity (IIRC those studies also entailed taking Americans used to AC and making them run on a treadmill in an extremely hot, dry room while hooked up to monitoring equipment), while the same wet bulb temperatures being caused by lower temperatures and high humidity are unpleasant but not life threatening unless there are extenuating circumstances (age, illness, certain medications, long covid, etc), and most of those extenuating circumstances already make lower, more normal temperatures dangerous already.
Basically, as long as the actual temperature is still below body temperature you still lose heat to the environment, you just lose a lot of efficiency through evaporation not working as well, it's when the temperature is above body temperature (and the higher it goes the worse it is) that it gets bad even when the wet bulb is still lower, because then you're not passively losing heat to the environment anymore (and in fact are gaining heat from the environment) and you need the efficiency gains of perspiration to survive.
This isn't the event horizon that it gets billed as, it's just further escalation of already deadly conditions that kill tons of people every year for class based reasons, part of the ongoing capitalist social murder of the working class. It means outdoor workers need more ability to stop work and cool off, or outdoor work needs to stop during the hottest part of the day altogether, and it means elderly people or people with other health problems need climate controlled shelter, but all of these things are already true, and they are materially stopped for classist and racist reasons.
Wet bulb temperatures are a combination of temperature and humidity, basically a rough measure of how much heat gets removed through optimal evaporation in given conditions.
Unfortunately the metric is both unclear on its face and a bit flawed in that a given wet bulb temperature can mean wildly different things depending on the actual temperature. The lowest wet bulb numbers that have been found to be extremely dangerous are basically a combination of extremely high heat with low or moderate humidity (IIRC those studies also entailed taking Americans used to AC and making them run on a treadmill in an extremely hot, dry room while hooked up to monitoring equipment), while the same wet bulb temperatures being caused by lower temperatures and high humidity are unpleasant but not life threatening unless there are extenuating circumstances (age, illness, certain medications, long covid, etc), and most of those extenuating circumstances already make lower, more normal temperatures dangerous already.
Basically, as long as the actual temperature is still below body temperature you still lose heat to the environment, you just lose a lot of efficiency through evaporation not working as well, it's when the temperature is above body temperature (and the higher it goes the worse it is) that it gets bad even when the wet bulb is still lower, because then you're not passively losing heat to the environment anymore (and in fact are gaining heat from the environment) and you need the efficiency gains of perspiration to survive.
This isn't the event horizon that it gets billed as, it's just further escalation of already deadly conditions that kill tons of people every year for class based reasons, part of the ongoing capitalist social murder of the working class. It means outdoor workers need more ability to stop work and cool off, or outdoor work needs to stop during the hottest part of the day altogether, and it means elderly people or people with other health problems need climate controlled shelter, but all of these things are already true, and they are materially stopped for classist and racist reasons.