so it is temperatures that can kill the sick and frail

this is misinformation!

Humidity worsens the effects of heat because humans cool their bodies by sweating; water expelled through the skin removes excess body heat, and when it evaporates, it carries that heat away. The process works nicely in deserts, but less well in humid regions, where the air is already too laden with moisture to take on much more. Evaporation of sweat slows. In the most extreme instances, it could stop. In that case, unless one can retreat to an air-conditioned room, the body's core heats beyond its narrow survivable range, and organs begin to fail. Even a strong, physically fit person resting in the shade with no clothes and unlimited access to drinking water would die within hours.

Meteorologists measure the heat/humidity effect on the so-called "wet bulb" Centigrade scale; in the United States, these readings are often translated into "heat index" or "real-feel" Fahrenheit readings. Prior studies suggest that even the strongest, best-adapted people cannot carry out normal outdoor activities when the wet bulb hits 32 C, equivalent to a heat index of 132 F. Most others would crumble well before that. A reading of 35—the peak briefly reached in the Persian Gulf cities—is considered the theoretical survivability limit. That translates roughly to a heat index of 160 F. (The heat index actually ends at 127 F, so these readings are literally off the charts.) "It's hard to exaggerate the effects of anything that gets into the 30s," said Raymond.

https://phys.org/news/2020-05-potentially-fatal-combinations-humidity-emerging.html

You cannot sweat at 100% humidity. (Or rather, sweat doesn't evaporate, meaning it can't cool you.)

A wet-bulb temperature of 94 F is fatal, even for the most strong and adapted. Remember that you are warm-blooded and need to shed heat at all times in order to not cook yourself to death.

Right, ok, yeah that makes sense. Definitely don't want your 98 year old grandmother to work in 110+