The confusion is because you said "a heat index above 40", so I thought you meant the actual index, not the index of 40C with whatever humidity. 80f is 26.66c (sorry, idk if you use decimals in Celsius or not). The heat index of 136 that I used to work in would be 43.33c and 40% humidity, so about what your index would be. I guess it could be fatal if you didn't stay hydrated, but probably not in 3 hours. At least not for someone who's used to it. I did get heat exhaustion a few times when the temperatures rose to 46.11 C, thankfully never heat stroke. I hope it doesn't seem like I'm trying to argue with you. I'm just trying to have a friendly conversation. Sometimes it's hard to tell how things read on the other end.