The wet bulb temperature is when your body can no longer cool with sweat. This is the same for everyone and is a relation between humidity and temperature. This is basically a physical thing and is the same for everyone. It means your body temperature will just go up until it reaches a point where you can no longer survive.

Of course there's still differences on how much that matters. A healthy person can sustain a longer period of time with a slightly elevated temperature. A person with health problems may have issues much earlier. But in the end... with wet bulb temperature noone can survive without external cooling for longer periods of time.

Well, interestingly enough, the factors are both what you would normally consider a "vulnerable" population, but also, folks with a high metabolic rate (athletes) as well.

https://ksi.uconn.edu/emergency-conditions/heat-illnesses/ex...

Low hydration is also a factor, and when you consider than millions of USA-ians may not be able to afford their water bill, well...

https://www.consumerreports.org/personal-finance/millions-of...