I feel like the terms in use around this are quite abstract and hard to reason about. I like that the wikipedia article converts this to something more useful to intuit for most of us:

> Even heat-adapted people cannot carry out normal outdoor activities past a wet-bulb temperature of 32 °C (90 °F), equivalent to a heat index of 55 °C (131 °F). A reading of 35 °C (95 °F) – equivalent to a heat index of 71 °C (160 °F) – is considered the theoretical human survivability limit for up to six hours of exposure.

The US NWS, for example, uses heat index commonly. In the US south I seldom see it go much over 110F, and six hours is a long time to be exposed to the elements even at the current levels of heat.

This isn't just a case of hitting some threshold where we all drop dead, but as temperatures rise it will slowly cause people to migrate to more temperate climates and/or seek out better cooling structures (either actively through air conditioning or passively where that isn't an option).