It seems just being out in X temperature, Y percent humidity is enough to kill you, sooner or later. Your body can’t cool itself, even if you’re just sitting around, and eventually your body temperature gets too high.
What air temperature would be enough to kill lots of people (in an hour or two, say) if the humidity were 90%? Would dropping to 80% make a big difference?
There are way too many variables to give an exact answer, but the following is probably close enough.
At 90% humidity, 92 degrees or so puts you in the extreme danger category for heat stroke if you stay in that environment and do physical activities.
Dropping to 80% humidity isn’t that big of a difference. At that level 94 degrees has approximately the same heat index as above.
Too many variables person to person, you mean? Or you mean this 90% humidity is worse than that 90% humidity?
What you need to know is the wet bulb temperature, the temp a thermometer shows if it’s dampened with water so evaporation cools it down. When the wet bulb temperature reaches 35°C (95°F), people die with hours. But recent research suggests even 31.5°C (88°F) could be deadly.
There’s a wet bulb temp calculator here, which suggests 36.5°C (96°F) at 90% humidity would definitely be deadly, with a wet bulb temp of 35°C, and 33°C (91.4°F) would be potentially deadly, with a wet bulb temp of 31.6°C. These seem alarmingly low!
Person to person. Activity to activity. Shade to full sun.
Do we infer 90 degrees with 90% humidity isn’t that common? Anywhere?
The question assumes no activity and no full sun. Just hot, humid air that you can’t escape.
It’s common in the tropics. People adjust by doing things early in the morning or late at night. Or not doing much work during the heat of the day.
In my experience humans are very adaptable. Where I worked on the Arabian Gulf, summer days were consistently above 125 degrees and very high humidity. I eventually learned to slow down and could operate a clipboard pretty much all day.
But there were (other) imported laborers who would be in asbestos suits sandblasting the sides of oil storage tanks in the direct sun. All day.
The wet-bulb temperature is the lowest temperature that can be reached at the current humidity by evaporating water - so if the combined humidity and temperature is high enough that the wet-bulb temperature is above 98.6 degrees or so, you won’t be able to cool off by sweating. That’s bad.
It is even worse than that; a wet bulb temperature at human core temperature is the maximum a health person can tolerate for more than a few hours but sustained wet temperatures even as low as 31 ºC as seen in tropic regions can make even that intolerable, and of course children, people with chronic health conditions, and the elderly who are less able to regulate body temperature may find even lower temperatures at near-saturation level humidity dangerous.
But in their new study, the researchers found that the actual maximum wet-bulb temperature is lower — about 31°C wet-bulb or 87°F at 100% humidity — even for young, healthy subjects. The temperature for older populations, who are more vulnerable to heat, is likely even lower.
Stranger
The NSW Cricket Association uses a heat index of temperature and humidity, plus allowances for windspeed to determine whether local weather conditions are too dangerous for play.
If the index (available on an app) reaches 11 play is to cease immediately and players are to remain under shelter until the index has fallen below 10.
The impact of humidity is exponential. And yes, play has been suspended periodically every season since it’s introduction. Previously the limit was based on reported temperatures at a local weather station (ie in shade) (42C) and yes I have been involved in games where that limit was breached.
(the graph above is of my creation, the app simply shows the index based on Bureau of Metrology data)
Here are my go to cites for these sorts of questions. They’re semi-redundant to other cites above, but more background is always good …