@edgeofeurope @alicetragedy It depends on local conditions.
When we lived along the Florida coast, 35C (95F) was hot but workable if we were hydrated, shaded/hatted, and taking adequate breaks. However, if we were indoors in a building that wasn't designed for airflow and that didn't have air conditioning, conditions degraded quickly. My employer routinely found alternative air conditioned workspaces or sent people home in outdoor temperatures well below 35 when the inside of the building became too hot because the AC was off.
Here in the Canadian Maritimes, 30C (86F) is sticky and unpleasant and worse than 35C in Florida. It feels like a reasonable local threshold since cooling here is still a patchwork of open windows and AC systems, and people here don't cool their buildings to frigid temperatures in the summer like they do in Florida.
Maybe base a threshold on the "feels like" heat index?