Humans can't sweat fast enough to maintain homeostasis.
https://www.weather.gov/safety/heat-index
Hasn't happened yet for sufficient periods in any substantially populated impoverished area, but when it does, well, the horror will be incalculable.
The summer almost feels milder here. Anecdotally of course.
However the wet-bulb temperature[1] is, as the name implies, the temperature a thermometer covered in water-soaked cloth. If the water evaporates, it will cool the thermometer down, and as such will read less than a regular (dry-bulb) thermometer.
For example, if the dry-bulb temperature is 35C and the relative humidity is 75%, then the wet-bulb temperature would be about 31C[3]. This means the body can still evaporate sweat to cool down, albeit not by much.
If you got both a wet-bulb and a dry-bulb thermometer side by side, you can use it to calculate the relative humidity[2].
[1]: https://en.wikipedia.org/wiki/Wet-bulb_temperature
[2]: https://en.wikipedia.org/wiki/Hygrometer#Psychrometer_(wet-a...
[3]: https://www.psychrometric-calculator.com/HumidAirWeb.aspx
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...
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.