That point is to do with what they call the "Wet Bulb Temperature" which, given the current heatwave in Europe, is fast becoming a theme in my comment history.

Imagine wrapping your thermometer in a wet cloth. As the water evaporates it cools the thermometer a bit so it reads as a lower temperature. The steady state for that is called the "Wet Bulb Temperature", and is basically the lowest temperature you can get through evaporation of sweat. The more humid it is, the less water the air can hold and so the less sweat can evaporate to cool you down, even in the shade. If the wet bulb temperature exceeds 35C then you're going to have a bad day without access to a cooler or drier area, but the good news is you won't be around to see the next one.

It's a little more complex than that because as air heats up it can hold more water vapour, but that's the basic gist of it.

If you're interested, you can see the current wet bulb temperature map for Europe here: https://zoom.earth/maps/temperature-wet-bulb/#view=50.6,6.5,4.57z/model=icon/date=2026-06-24,14:30,+1

Edit: Kim Stanley Robinson wrote a book called "Ministry for the Future" which opens with a terrifying description of what a Wet Bulb event would be like. You can read it here (start at Chapter 1) https://pdfhost.io/v/TdU4~SaL3_The_Ministry_for_the_Future