In June, Seattle had a major heatwave with temperatures reaching 41C with 30-40% humidity. It was a bit cooler in the mountains but still quite unpleasant during the day. This made me wonder - at which temperature is it no longer safe to go hiking? Now, I do understand that this varies based on cloudiness, weight in your backpack, what clothes you're wearing, how much wind is blowing, etc, but I presume someone like the US military would have a nice chart going into detail on this topic, due to numerous military campaigns in desert areas.
I did try to search "temperature limit for hiking" but all the search results were opinion based rather than linking to solid scientific research.