With 100% humidity you definitely can't survive or function but with 95% at 100F exactly it's just barely feasible. That gives you just enough margin due to evaporative cooling that with sufficient air flow you can maintain a temperature of 97-98F via evaporative cooling. And importantly this only works in the shade. Outdoors it's unlikely to be feasible due to the rise in surface temperatures due to thermal radiation from the sun.

That of course requires a strong fan blowing and regular, heavy hydration to sustain but it's feasible. So it's workable in an industrial environment where you can adjust the environment enough to get by but outdoors in large wild spaces like the Grand Canyon (as per your example) it's unlikely to be survivable for long.

And notably in an indoor environment there is a big difference between an operator running a machine or vehicle and an individual under heavy exertion. The added thermal stress of heavy exertion makes it less survivable as well.

So in the end it only really works in factory settings because:

- There's no sun to add radiative heat.

- There's fans and ventilation to maintain evaporative cooling.

- Workers can take regular or semi-regular breaks in a cooler or lower humidity environment to recover some from the thermal stress and to recover water and electrolytes.

- Those workers can limit their activity to rates of exertion/heat production that don't exceed the limited evaporative cooling they have access to.

As soon as you remove one of those advantages or increase the temperature much above 100F or increase the RH above 95%, survivability becomes way less likely.