Human air pressure tolerance and other stuff

[NoXion]

I was playing about with a random solar system generator, which gives information on planets' gravity, air pressure, temperature and whatnot relative to that of Earth. My question is this: what is the maximum human tolerance for air pressure, measured in Earth atmospheres? Google and Wikipedia didn't help, or maybe I'm just not searching for the right terms.

Also, information on human tolerances for the other conditions listed above would be nice. I'd like to get an idea of how far each way the values have to be before terraformation/genetic alteration becomes worthwhile.

Hope this is in the right forum.

[Simon_Jester]

I think part of the reason there isn't much data is that the only way to find this out is to stick someone into a pressure chamber and crank up the air pressure until they start screaming. It's hard to do the experiment ethically, so I have a horrible suspicion that the only actual data we have would be from Nazi medical research on concentration camp inmates or something.

By contrast, people climb mountains, go up in high-altitude balloons and aircraft, and even travel in hard vacuum on a regular basis, so we have a lot more information on the effects of low pressure exposure.
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I can't help much with respect to gravity or air pressure, but as for temperature:

Humans are better at tolerating uncomfortable cold than uncomfortable heat, because we have evolved and old-technology defenses against the cold. If a place isn't colder than, say, Alaskan winters, people can probably live there. On the other hand, like Alaska, most people won't want to unless you give them a damn good reason. And Alaska does have extended periods above freezing; without that, terrestrial plants and animals wouldn't be able to survive in the long run without a climate-controlled environment. And if all agriculture has to take place in a greenhouse dome, it's probably time to start terraforming.

Nobody's going to go live on Hoth without planning to terraform the place unless they're desperate.

When it comes to uncomfortable heat, you have less leeway, because human beings have only a very limited ability to survive at temperatures greater than our body temperature. The laws of thermodynamics are working against you in a big way in that kind of environment. Mostly, we do it by sweating and minimizing physical activity during the hot period.

In a low-humidity, high-temperature environment (the desert), that's easy as long as you have water and salt available... but water is hard to come by in a desert. In a high-humidity environment there's plenty of water, but sweating doesn't do much good because you're not getting evaporative cooling. In which case life at high temperature is going to be even harder than in the dry heat of the desert.

In either case, the temperature range where things start getting difficult enough to require lifestyle changes is 35 to 40 degrees Celsius. Things are going to get really difficult in the 40 to 50, and I doubt anyone would willingly travel through space to live somewhere where temperatures spiked above 50 Celsius (or even 45) unless, again, they're desperate.
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Keep in mind that even planets that are uninhabitable over most of their surface may have areas that are an exception to the rule. A hell-world with blistering 75-degree* deserts may well be habitablee in the polar regions, and an ice-world could easily have some tolerable tundra environments near the equator, especially if things like ocean currents and geological activity are helping.

*Celsius

[Buritot]

The highest permanent settlements are about 4-5 km above sea level.
The hottest are 40+°C at day and close to 0° at night.
The coldest are Inuit with ranging temperatures from -60°C (winter) to 10°C (summer). There are colder settlements in Siberia, though.
The gravity limits are tricky. It has been proposed Lunar gravity might be too low for long-term settlements, and Martian levels might cut it. As for the upper levels: Take an obese person in 1g as a basis on the body stress of a fit person in 1.5-2g.

Dammit. Now I want medium-term (6 months) experiments with centrifugal pseudo-gravity in space. Who'll join me?

You may find this helpful.

[NoXion]

I think part of the reason there isn't much data is that the only way to find this out is to stick someone into a pressure chamber and crank up the air pressure until they start screaming. It's hard to do the experiment ethically, so I have a horrible suspicion that the only actual data we have would be from Nazi medical research on concentration camp inmates or something.

By contrast, people climb mountains, go up in high-altitude balloons and aircraft, and even travel in hard vacuum on a regular basis, so we have a lot more information on the effects of low pressure exposure.

Would data gathered from the effects of deep-sea diving be of any use?

[Buritot]

Would data gathered from the effects of deep-sea diving be of any use?

Actually, no. Deep-sea diving happens with technical aid, namely submarines. The pressure therein is regulated and not close to the pressure outside the vehicles. Abnoe divers reach depths in excess of 200m, but they don't live there.
Decompression sickness may be an indicator. I dare to remember diving to up 20m without fear of it, so that makes it 3 atm (300 kPa). I'm not sure of it, though, I never dived this deep.

[Psychic_Sandwich]

Decompression sickness may be an indicator. I dare to remember diving to up 20m without fear of it, so that makes it 3 atm (300 kPa). I'm not sure of it, though, I never dived this deep.

DCI is a risk at any depth if you ascend too fast, but decompression dives start at about 20m, depending on how long you stay down. People semi-regularly dive to 150 or more meters on rebreathers without having to worry about DCI so long as they meet their deco stops. In terms of diving on air, 52 meters is the MOD for normal, 21% O2 air for BSAC, with nitrox being shallower, and that's due to oxygen toxicity, not nitrogen ongassing. That gives a pressure of 6 bar before you start suffering O2 toxicity. For a more exact number, the BSAC limit for pO2 is 1.4. That's a conservative number because it errs on the side of safety, but even so, you can still suffer from convulsions at that partial pressure.

I should note that O2 toxicity is potentially a problem a lot shallower than this; it used to be the case that people doing a weeks diving at Scapa Flow would collapse on the Wednesday due to CNS toxicity, even if they hadn't gone deeper than 30m in many cases. This is because, like nitrogen offgassing, O2 leaves the body more slowly than it enters, causing an increase in CNS [O2]. The pressure at which you can suffer from O2 toxicity, therefore, is dependent on how long you're at the increased pressure.

[Sea Skimmer]

Hyperbaric Welders have reached more then 850psi of air pressure, allowing them to work at as much as 2,000 foot depths, but it takes weeks to compress and then decompress to do that. Still if the planet had the exactly right atmosphere no specific reason that I’m aware of would stop you from living full time at that kind of pressure. Work is ongoing to extend this type of welding to over 3,000 foot depths and 1,300psi of required pressure too.

However in terms of lower pressures the limits are much less, at about 8000 feet on earth pressure goes down to about 11psi and it is at this limit that altitude sickness becomes a problem for most people. Only special physical conditioning, or being born and raised at such low pressure will let you fully adapt, though of course in the short term a few special crazy people have climbed Everest without oxygen. At the summit pressure is no more then about 5psi.

[Psychic_Sandwich]

Hyperbaric Welders have reached more then 850psi of air pressure, allowing them to work at as much as 2,000 foot depths, but it takes weeks to compress and then decompress to do that. Still if the planet had the exactly right atmosphere no specific reason that I’m aware of would stop you from living full time at that kind of pressure. Work is ongoing to extend this type of welding to over 3,000 foot depths and 1,300psi of required pressure too.

They do that, I think, by breathing trimix. If you ever read the book 'Sphere' or watched the movie, it's the bit about them 'breathing helium'. They don't actually respire helium, they add helium or some other inert gas (not nitrogen, becuase of deco issues) to the air to replace a proportion of oxygen. This keeps the pO2 below the threshold that would cause toxicity, because they're breathing air that has a much lower percentage of oxygen. If you try this with normal air, you'll convulse and drown before getting far past 50 meters.

So, I suppose maximum pressure depends a lot on the atmosphere of the planet you're talking about.

[Sea Skimmer]

They do that, I think, by breathing trimix.

They use mixed gases, but which exactly depends on the depth. The deepest depths ever achieved (only in deep pressure simulators) were reached using Hydrox which is hydrogen and oxygen. Trimix is used for much shallower depths of more like 100 meters, and has serious narcotic effects. You would not want to live on a planet in which the air leaves you punch drunk all the time. It is unlikely to be very healthy in the long term. I'm not sure Hydrox would be either, but it has no narcotic effects.

So, I suppose maximum pressure depends a lot on the atmosphere of the planet you're talking about.

Like I said, you'd need perfect conditions. It’d be pretty unlikely to find a planet with a perfect Hydrox atmosphere which is also not so hot or cold that you need a sealed suit anyway. The tolerance for impurities in these diving gas systems is very very low, making it even more unlikely that you’d ever get a planet with the right conditions, but this is a thread of absolutes.

[Junghalli]

Low Pressure Atmospheres:

According to this the maximum altitude for human survivability is 6000 meters above sea level. According to this atmosphere pressure reduces by half roughly every 5-6 kilometer in altitude.

Based on that I'd say we could live on a world with down to 1/2 our sea level partial pressure of oxygen.

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High Pressure Atmospheres:

Divers have to breath altered air mixtures if they go to significant depths because the mixture of gasses in our air becomes toxic at high pressure due to oxygen toxicity and nitrogen narcosis (a.k.a. "rapture of the deep"). This would presumably be the serious problems on a high pressure world.

According to this a human can breathe oxygen at pressures below .6 ATA (atmosphere absolute) for two weeks without ill effect. In other words (if I'm reading this right) partial pressures of less than .6 bar oxygen should be tolerable. Earth's sea level atmosphere is around .21 bars partial pressure of oxygen. So assuming an air mix the same as Earth's oxygen toxicity shouldn't be problematic in air less than around 2.857 times as thick as ours.

Nitrogen narcosis becomes noticeable at 10-30 meters below sea level, and usually doesn't become noticeable above 30 meters (ref). There's a rule of thumb for it called the "martini effect" which is that every 50 feet you dive past 100 feet down is about as intoxicating as drinking a martini on an empty stomach (ref). The effect becomes disabling at 300 feet (see previous reference). The 33 FSW rule is that every 33 feet of sea level adds the equivalent of 1 atmosphere of pressure (ref), so at 100 ft/30 meters the pressure would be around 4 bars.

So we would probably be able to handle a world with an atmosphere up to 2.8 times thicker than ours at sea level, although nitrogen narcosis effects might occur on a world with at atmosphere as little as ~1.3 times as thick as ours. These are matters of partial pressure so they can change with different atmosphere compositions, it's pretty easy to work out if you convert the above figures into terms of partial pressure.

[starslayer]

However in terms of lower pressures the limits are much less, at about 8000 feet on earth pressure goes down to about 11psi and it is at this limit that altitude sickness becomes a problem for most people. Only special physical conditioning, or being born and raised at such low pressure will let you fully adapt, though of course in the short term a few special crazy people have climbed Everest without oxygen. At the summit pressure is no more then about 5psi.

For a reasonably fit person, if they ascend slowly enough and don't exert themselves much for a while upon reaching altitude, anything up to 20,000 feet shouldn't present much of a problem for general living and working. Severe physical exertion (i.e., mountain climbing, endurance running, long bouts of manual labor, etc.) will be much harder at altitude without training, true. Above about 20,000 feet, however, it becomes impossible to permanently adapt to the conditions, even for peoples who have lived in the high mountains for generations. Upon entering that kind of zone, you slowly begin to die of asphyxiation until you descend.

From Junghalli's site, a rough estimate for atmospheric pressure at 20,000 feet would be about 7-8 psi. At a 21% O2 mix, the O2 partial pressure would be about 1.5 psi. IIRC, O2 toxicity occurs above a partial pressure of 1.5 atm, or ~20 psi. For an Earth normal gas mix, this translates to a total pressure of anywhere from ~7 psi (.5 atm) to ~100 psi (7 atm). Of course, nitrogen narcosis would also play a role, but Junghalli already covered that.

[The Spartan]

I think Sea Skimmer may have meant to type '8000 meters' not '8000 feet' because even I've been above 8000 feet (Engineer's Pass; 12800 ft/3900 m) and the climbers on Everest are said to be in the death zone once they hit 8000 meters.

[Buritot]

It all becomes quite funny when you add breathable liquids, though. I'd really like to experience that... maybe add a little blood to it. Might as well use LCL

[Broomstick]

However in terms of lower pressures the limits are much less, at about 8000 feet on earth pressure goes down to about 11psi and it is at this limit that altitude sickness becomes a problem for most people. Only special physical conditioning, or being born and raised at such low pressure will let you fully adapt, though of course in the short term a few special crazy people have climbed Everest without oxygen. At the summit pressure is no more then about 5psi.

For a reasonably fit person, if they ascend slowly enough and don't exert themselves much for a while upon reaching altitude, anything up to 20,000 feet shouldn't present much of a problem for general living and working. Severe physical exertion (i.e., mountain climbing, endurance running, long bouts of manual labor, etc.) will be much harder at altitude without training, true. Above about 20,000 feet, however, it becomes impossible to permanently adapt to the conditions, even for peoples who have lived in the high mountains for generations. Upon entering that kind of zone, you slowly begin to die of asphyxiation until you descend.

Don't forget, if you're settling a planet you have to account for pregnant women. Pregnancy is quite a strain on the physical body, and how well a woman's body can oxygenate a fetus is a significant limiting factor. I seem to recall that 14,000 feet (about 5500 meters) is the highest permanent human settlements (disclaimer: I'm doing that purely on recollection so it may be off). Women can certainly climb higher than that - after all, women have reached the summit of Everest - but reliable reproduction is, apparently, not within current human capability above that altitude. So that would probably be the lower limit of atmopheric pressure at Earth atmosphere. If you can tweak the atmosphere for higher oxygen content you can manage a lower pressure as long as you can keep the oxygen partial pressure equivalent to that altitude's.

Keep in mind it's the pregnant women that are key here - there are semi-permanent/permanent residents above the level at which humans can reproduce, but they are sustained by people coming to them from lower altitudes

[starslayer]

Don't forget, if you're settling a planet you have to account for pregnant women. Pregnancy is quite a strain on the physical body, and how well a woman's body can oxygenate a fetus is a significant limiting factor. I seem to recall that 14,000 feet (about 5500 meters) is the highest permanent human settlements (disclaimer: I'm doing that purely on recollection so it may be off). Women can certainly climb higher than that - after all, women have reached the summit of Everest - but reliable reproduction is, apparently, not within current human capability above that altitude. So that would probably be the lower limit of atmopheric pressure at Earth atmosphere. If you can tweak the atmosphere for higher oxygen content you can manage a lower pressure as long as you can keep the oxygen partial pressure equivalent to that altitude's.

Huh, interesting. I'd always heard of the 20,000 foot ceiling, although you're right, it was probably for non-pregnant people. I did a quick search, and turned up a paper establishing the highest permanent human settlement as at about 5100 m, or about 16,700 feet. So it would seem the answer lies somewhere in the middle. However, the pressure at that altitude is still about 7-8 psi, so the end conclusion remains relatively unchanged.

[Junghalli]

Something to keep in mind: a different air mix will have effects beyond biological ones. The big one that I can think of is that the more oxygen there is in the air the easier things burn. An atmosphere significantly higher in oxygen than ours will be a major fire hazard environment.

I've saved a graph of oxygen content of Earth's atmosphere through time I found here, oxygen levels peaked at .3-.35 bars partial pressure of oxygen (it's varied between around .1-.35 bars over the last 600 million years or so). From what I've read it's thought this is about as high as it could go; significantly more oxygen and fires would be so frequent and massive they'd start to seriously denude the continents of vegetation, which would have slowed down the rate at which carbon was removed from the atmosphere, which would lead to a drop in atmospheric oxygen. So a planet with significant land life probably won't have a significantly higher oxygen partial pressure than .3-.35 bars, and if it does the local terrestrial ecosystem will have to deal with every lightening storm creating an enormous inferno that burns across huge swathes of land.

From Junghalli's site, a rough estimate for atmospheric pressure at 20,000 feet would be about 7-8 psi. At a 21% O2 mix, the O2 partial pressure would be about 1.5 psi. IIRC, O2 toxicity occurs above a partial pressure of 1.5 atm, or ~20 psi. For an Earth normal gas mix, this translates to a total pressure of anywhere from ~7 psi (.5 atm) to ~100 psi (7 atm). Of course, nitrogen narcosis would also play a role, but Junghalli already covered that.

So if I read this correctly humans could take anything from around a .5 bar atmosphere (at which point lack of oxygen becomes a serious problem) to a 4 bar atmosphere (at which point nitrogen narcosis becomes a serious problem), assuming an air mix like ours. Is that right? That puts a number on things nicely.

Don't forget, if you're settling a planet you have to account for pregnant women. Pregnancy is quite a strain on the physical body, and how well a woman's body can oxygenate a fetus is a significant limiting factor. I seem to recall that 14,000 feet (about 5500 meters) is the highest permanent human settlements (disclaimer: I'm doing that purely on recollection so it may be off). Women can certainly climb higher than that - after all, women have reached the summit of Everest - but reliable reproduction is, apparently, not within current human capability above that altitude. So that would probably be the lower limit of atmopheric pressure at Earth atmosphere. If you can tweak the atmosphere for higher oxygen content you can manage a lower pressure as long as you can keep the oxygen partial pressure equivalent to that altitude's.

Keep in mind it's the pregnant women that are key here - there are semi-permanent/permanent residents above the level at which humans can reproduce, but they are sustained by people coming to them from lower altitudes

If this is true it would really be only a big problem if you demand the planet be inhabitable to even a primitive human society. Sequestering pregnant women into specialized pressurized facilities shouldn't be too much of a challenge for a technological society.

Of course, the same can be said for even more marginal worlds. A world where sea level air pressure is comparable to the top of Mt. Everest would still be much easier for humans to live on than any world in the solar system (other than Earth). The colonists would simply have to artificially pressurize their dwellings and use bottled oxygen like mountain climbers on Earth when venturing outside for any significant time. The same would be true for a world with too much nitrogen.

Heh, on a planet like the latter I'm sure there'd be people who'd try to use the native air to get high. It'd be the cheapest drug ever. All you have to do to get is take a walk outside.

[NoXion]

Heh, on a planet like the latter I'm sure there'd be people who'd try to use the native air to get high. It'd be the cheapest drug ever. All you have to do to get is take a walk outside.

Hmm, I wonder what the long-term consequences of that would be? If they're none too severe, I could easily see such an activity becoming something of a cultural fixture.

[starslayer]

So if I read this correctly humans could take anything from around a .5 bar atmosphere (at which point lack of oxygen becomes a serious problem) to a 4 bar atmosphere (at which point nitrogen narcosis becomes a serious problem), assuming an air mix like ours. Is that right? That puts a number on things nicely.

Yes, that's correct. Again, however, if you change the gas mix, these pressures are no longer valid, and we have to consider the partial pressures of the gases involved.

[Uraniun235]

Actually, I think we do have some idea of the minimum pressure needed for survival - I remember reading that above a certain altitude, you'll eventually (not immediately, but eventually) die from lack of oxygen, and that this is one of the many perils of attempting to summit Everest.