Highest temperature a human being can survive

Highest temperature a human being can survive

These are, of course, just estimates. Empirical numbers are hard to come by. By "short term" I typically mean a few minutes, because most people can survive even extremes for a few seconds; by "long term", I mean hours and possibly a day or two. Many people would die before reaching these numbers; they're half-decent estimates of what a healthy person could survive.

TL:DR numbers:
Dry air: 120+ °C (248+ °F) short term, 70+ °C (158+ °F) long term (with access to water at cooler temperatures).
Tropical air: 60+ °C (140 °F) short term, 47 °C (117 °F) long term.
Saturated air: 48 °C (118 °F) short term, 35 °C (95 °F) long term.
Water: 46° C (115 °F) short term, 41°C (106 °F) long term.

This is an interesting question, and it depends largely on the humidity and time of exposure. Our cells start to die around 41°C (106°F) to 45°C (113°F) but we can survive much higher air temperatures: a healthy person could make a day trip to Death Valley on one of its hottest days-- 55°C (131°F)-- and, so long as he avoided dehydration, would probably not die. In fact, he'd probably be OK as long as he had access to water, preferably stored at a temperature lower than 55°C, because it would actually be painfully hot to the touch at that point.

In water, the upper limit seems to be about 50 °C (122 °F) for short-term exposure; even a couple of degrees hotter, first- and second-degree skin burns become possible within minutes, and that's clearly not a sustainable situation. As with hypo-thermia, these things depend a lot of the individual. Some people die of hypothermia at 10 °C (50 °F) and others don't. (Naked, even most people in northern climates wouldn't survive a night at 0 °C). That said, almost no one who claims that his hot tub is "120 °F" is telling the truth, because 49 °C water is extremely uncomfortable and, if you didn't have a thermometer right there, you'd think it was 60-80 °C. Most hot tubs run at 38-42 °C (100-108 °F) and 46 °C (115°F) seems to be the highest that anyone has intentionally set for exposure, and people who do ultra-hot tub exposure generally don't stay in for very long. 46 °C water is painful to the touch. 

What about air temperature? Well, people can survive dry air temperatures well over 120 °C (248 °F) in the short term. Such levels are achieved in saunas, although most people find it uncomfortable, and the sport of "competitive sauna" is actually quite dangerous, even though people aren't exposed to the 110° C heat for more than a few minutes. Higher temperatures (probably up to 300 °C) are survivable for short bursts (like, measured in seconds) or with protective clothing but, again, no one would consider that sustainable.

With humid air, the limits are much lower. You can withstand 45°C (113 °F) at 100% humidity in the short term (steam room) although you wouldn't last a night in there, but around 50 °C (122 °F) it becomes acutely fatal (or, at least, dangerous enough that people who expose themselves to such environments are protected) because the water vapor can condense in your lungs. The upper limit on steam room temperatures seems to be about 48 °C, so that's a good working short-term maximum for saturated air. The good news is that you'll virtually never see a dewpoint temperature (i.e. the temperature at which relative humidity would be 100%) over 30°C (86 °F) in the wild. 50°C and 100% humidity just never occur together (above ground) in a natural setting.



More interesting is the question of what climactic temperatures are sustainable on the scale of hours or days. With enough water, you'd survive 24 hours of exposure to 45°C air temperatures-- I still wouldn't recommend it-- but not the 80°C of a sauna. For water, we assume that body temperature will reach the environmental temperature and get an upper bound of about 40-41 °C. You'd be miserable as hell after 24 hours in a 39°C hot tub, but you probably wouldn't die.

For air, we use the "wet bulb temperature"-- not the "heat index", which is optimized for accuracy over temperatures of 30-50 °C but not relevant to the outermost limits of survivability-- which the coolest we can get our skin temperature by sweating. There, the danger point is 35°C (95 °C). That might sound low, but even in the hottest days in the deserts, the wet bulb temperature almost never gets close to that. If it's 55 °C in Death Valley with a typical summer dewpoint of 0 °C, the wet-bulb temperature is only 23 °C. The only places where wet-bulb temperatures over 30°C seem to occur are around the Persian Gulf-- for now.



At 100% relative humidity, the dewpoint and wet-bulb temperatures are the same number. Saturated air, paradoxically, is more dangerous in a certain temperature range than dry air but also more dangerous than water. 37°C water won't kill you (that's human body temperature) because water is a good conductor of heat and at worst you'll be mildly uncomfortable, and 37°C dry air won't kill you either (you can sweat) if you're hydrated, but 37°C saturated air probably will (over many hours or days) kill you, because you get the sweat-stifling humidity and the poor heat conductance of air. All of that said, 37 °C saturated air simply doesn't happen in nature. (That said, it's a real risk if we continue to fuck with the climate. 50°C in the desert, we can handle; 40 °C and 90+ percent humidity will kill us in less than a day.) Using a wet-bulb temperature of 35 °C as a proxy for what we can survive in the long term (hours) we get maximum figures of:

• 35°C (95 °F) in saturated (100% humidity) air.
• 47°C (117 °F) in tropical (> 40% humidity) air.
• 70°C (158 °F) in dry (> 10% humidity) air-- if you have cool water on hand.

All of that said, while one could theoretically withstand 70 °C air at desert humidity indefinitely, you'd need regular access to a cooler supply of water. Water at the ambient temperature would cause second-degree burns in seconds. 

I've left out the very long term (days to weeks) because that's a bit more complicated. (Short answer: about 35 °C or 95 °F). Most deaths in heat waves are from cumulative heat stress (over days and weeks) rather than heat stroke, and that has more to do with long periods of exposure to moderate heat (30-35 °C) than the acute kind. 45 °C will give you heat stroke if you're dumb enough to go running without bringing water, but you'll be fine if you stay in the shade, and most of the desert environments capable of such temperatures cool down at night. On the other hand, if you sleep in a concrete building with no air conditioning and the night-time air temperature is still over 30°C, you're putting your health at risk. Most of these deaths aren't attributed to heatstroke or even directly tied to heat, but come from natural bodily causes like heart attacks. Take the hellish heat wave that hit Europe in 2003: while the French government knows that about 10,000 people died because the hospitals saw a rise in deaths, it's impossible to say which people died because of heat, in the same way that you can't say which people got cancer "because of" a radiation accident.

While it's hard to measure, because (unlike heat stroke) heat-stress deaths and injuries are diffuse, long-term heat danger seems to be more of a function of nighttime temperatures than daytime highs. I'd guess that the upper limit of indefinite survivability is somewhere around 35 °C, noting that almost no natural climactic environment maintains that temperature for more than 24 hours: even the hottest deserts (e.g. Death Valley) in the peak of summer typically dip into the low 30s at night.

As for me, personally, I start to complain about the heat at 25 °C (77 °F).