The problem is the reference point. I grew up in New Orleans and in the summer, it was routinely 90+ degrees, 50+% humidity. If someone said it's 94 degrees, 50% humidity, feels like 103 and I stepped outside, I would have said, "no, feels like 94, this is always what 94 feels like." I have no reference point for what 103 degrees, zero humidity (or whatever the baseline is) feels like. The "feels like" temperature is really not valid unless it describes something people can relate to and it should go both higher and lower than actual. Rather than a valid observation it feels like a PR stunt. HEAT INDEX! CHILL FACTOR! No one ever says, "it's hot but dry today, feels pretty good out there but carry your water bottle!" That is not to say that biological stress is not a function of air temperature and humidity, just that the way it is used is more show than meaningful metric to help people. As for Umair Haque, I see where he is going but as a kid, I played for hours weather that he says would kill me. While I'm dramatically more careful now than I was as a kid, I still occasionally play tennis for over an hour in 90-95 degrees and high humidity and I'm chugging water hard, but no one is dying. Hard court tennis in the summer is often over 100. People are working all over the South in conditions that this chart says would kill people. I don't think his hypothesis matches the observations.