Human air pressure tolerance and other stuff (Split)

[agent009]

I realize this is a pretty old thread, but I've stumbled upon it while doing some research on this subject for my sci-fi literary work and I thought it might be interesting for the folks here if I shared my findings (actually, I've done quite a bit of research on what atmospheric conditions would be acceptable for a human being during the past few weeks).


LOW PRESSURE

There is probably a minimum limit of absolute pressure humans require to survive, but it is pretty irrelevant since hypoxia would kill you long before that. Earth atmosphere consists of 78% nitrogen, 21% oxygen, and 1% of other stuff (mostly argon and carbon dioxide), but oxygen is really the only gas humans and other animals require – the rest, we merely tolerate. So, the question of minimum pressure basically comes down to the minimum amount of O₂ available. For any given gas mixture, this limit is defined by the partial pressure of oxygen (ppO₂) – that is, how much pressure there would be if the amount of O₂ available per unit of volume would be occupying the same volume alone. The minimum “safe” ppO₂ level for humans is usually considered 0.16 ATA. This figure corresponds to 16% O₂ fraction under 1.0 ATA pressure (standard sea level pressure on Earth), to 0.76 ATA of total atmospheric pressure for O₂ fraction of 21% (which is roughly how much pressure you have around 7,500 ft / 2,300 m altitude on Earth) or to 0.16 ATA of total pressure for pure oxygen. So, that’s basically how much pressure we really need.

This limit is a little conservative at that - the highest permanent settlement on Earth is La Rinconada, Peru located at the altitude 16,728 ft (5,100 m) with Wenquan, China (16,467 ft, 5,020 m) closely behind -this corresponds to ppO₂ of about 0.115 ATA. But let's keep in mind, that ancestors of Andeans and Tibetans have been living at those altitude for millennia and have developed special adaptations - most “lowlanders” would not be able to survive under such conditions for very long, much less reproduce. While humans have survived at altitude of 6,035 m (~ 19,800 ft - ppO₂ 0.1 ATA) for two years, those were healthy young people and none of them gave a childbirth there (pregnant women and newnates require more O₂ than an average adult).

This seems to be about the limit of ppO₂ level any mammalian species can adapt to - Himalayan yaks are the highest living mammals on Earth and they can live up to about the same altitude. Their record has only been beaten by the Himalayan jumping spiders who live at altitudes of up to 22,000 ft (~ 6,700 m - ppO₂ 0.095 ATA). Absolute altitude record in the animal kingdom belongs to Rüppell's vultures who have been reported flying at altitudes of up to 37,900 ft (~ 11,500 m - ppO₂ 0.053 ATA), but they do not permanently live (and certainly do not reproduce) under such conditions. Humans and most other animals will not survive for more than a few days at ppO₂ levels below 0.08 ATA (the so-called "Dead Zone" – 26,000 ft, 6,000 m) and won't last more than a few minutes without an oxygen mask below 0.06 ATA (~ 35,000 ft - 10,700 m).

So, if a planet's atmosphere consisted of pure O₂, those ppO₂ limits would basically be the lowest limits of absolute atmospheric pressure. However, while humans and other animals could breathe 100% O₂ if atmospheric pressure was sufficiently low, they would still need something to eat. This means plants (unless you have some source of food of non-Terrestrial origin) - and plants cannot survive on pure oxygen. One thing plants need is carbon dioxide, but how much of it do they need? Not that much really - studies show, that flowering plants need at least 3×10¯⁴ CO₂/O₂ ratio, but at least 10¯⁴ ATA ppCO₂, in order to thrive. So, if you have 0.16 ATA ppO₂, you'd need (at least) another 0.00005 ATA ppCO₂ for the plants. And then, there is also nitrogen – though plants cannot absorb it directly from the atmosphere, there are nitrogen-fixing bacteria which use atmospheric N₂ to produce ammonia which plants can use. That is known as “nitrogen cycle”. How much atmospheric N₂ is needed to sustain it is not clear, but I would speculate that 0.01 ATA ppN₂ should be sufficient (after all, it takes only 0.0004 ATA ppCO₂ to sustain the “carbon cycle” on Earth right now, and that's 25 times less!).

So, the minimum “safe” estimate for a self-sustaining human colony would be:

0.16 ATA ppO₂ + 0.01 ppN₂ + 0.00005 ppCO₂ = 0.17005 ATA total atmospheric pressure

0.1 ATA of pure O₂ is about the absolute minimum required for prolonged survival of a bunch of humans, but they won’t be able to reproduce under such conditions and would have to rely on extra-planetary food supply. The actual limit is somewhere between those two figures, but it can only be determined by trial and error.

Another concern in low-pressure environment would be the boiling point of water. It is well known that water boils under lower temperature at lower pressures and, since all your body liquids are mostly water, you would not normally want them to boil under ambient conditions. At atmospheric pressure of 0.1 ATA, water would boil at about 44.5 °C (111.5 °F) which is a little too low IMHO, but should still be ok as long as you do not go into hot places (fever could become a major concern, however). At 0.17 ATA it is at 55.5 °C (132 °F) – this should be marginally ok for human survival, but you’ll have to forget about boiled eggs and coffee.


HIGH PRESSURE

Now we’ve answered the question about the minimum pressure human population would require, but the matter of maximum is a little trickier, as there are a lot more factors that come into play. Divers went as deep as 2,000 fsw / 600 msw (which translates into about 61.5 ATA of pressure) and it is believed that any more pressure than this would be seriously harmful for human health. But then, there is also a matter of what we breathe.

First of all, there is again a matter of ppO₂. Yes, we do need oxygen to survive, but it also happens to be a highly reactive chemical substance and there is only so much of it the tissues of our body can take. So, how much of O₂ is too much? Most experts seem to believe that prolonged exposure to ppO₂ levels above 0.5 ATA (i.e. more than 50% oxygen fraction under “normal” pressure) poses serious danger to our health. Exposure to ppO₂ levels above 0.6 ATA for more than 24 hours is said to cause serious damage to lung tissues, while, according to diving practice, exposures to levels of 1.2, 1.4 and 1.6 ATA may be safely tolerated for up to 210, 180 and 45 minutes respectively. This means, if our atmosphere was pure oxygen under “normal” pressure, we would’ve been in a serious trouble! But fortunately, Earth atmosphere only consists of 21% oxygen, so the resulting ppO₂ at sea level is about 0.21 ATA – this is well within human margin of tolerance. In fact, the pressure of standard Earth air could be increased 2 times and it would still be safe to breathe.

But you’d be in trouble should it ever get above 2.5 ATA – this would raise ppO₂ levels to 0.53 ATA where it becomes directly poisonous to your lungs. That’s what happens to a diver submerge below 50 fsw while breathing standard air – but that’s ok, since he or she doesn’t stay there for very long. Divers went on air as deep as 400 fsw (13.1 ATA, 2.75 ATA ppO₂), though this can be pretty hazardous for one’s health. The normal safe limit for recreational SCUBA diving is usually considered to be about 200 fsw / 80 msw, which translates into about 7 ATA of total pressure and 1.48 ATA ppO₂ on normal air.

So, we can tolerate ppO₂ levels of about 2.75 ATA for a short time, but it’s like with Rüppell's vultures I’ve mentioned earlier – they can occasionally fly as high as at 37,900 ft, yet they would not actually be able to nest at such altitudes, even if there were mountains on Earth this high. The maximum safe ppO₂ level which human body can tolerate indefinitely is usually estimated at 0.48 ATA and it seems that we are not able to adapt to O₂ levels any higher than that (not in the short term, at least). This means that if you want humans to thrive under pressure above 2.3 ATA, you’d have to reduce oxygen fraction in the air below 21% – in theory, you could increase overall pressure to up to 61 ATA as long as you keep ppO₂ within safe values. But this is a bit trickier then you might think as simply increasing N₂ fraction won’t work.

The actual “safe” limit of diving on normal air is usually considered to be around 130 fsw / 40 msw – that’s about 4.9 ATA overall pressure with ~ 1 ATA ppO₂ (that’s like breathing pure oxygen at sea level – not healthy in the long run, but can be safely tolerated for quite a few hours). But it’s not ppO₂ which defines this limit as another factor comes into play by thas point. This is sometimes known as the “martini law” – you get the effect of one glass of martini on an empty stomach for every 33 ft /10 m (1 ATA pressure increase) below 60 fsw / 18.5 msw (2.8 ATA total pressure) while diving on air. So, diving at 130 fsw would produce the effect of two glasses of martini on an empty stomach which is about how much an average human can handle before his or her judgement gets seriously impaired.

This phenomenon is often referred to as “nitrogen narcosis”, but “inert gas narcosis” is a more correct term – while air is 78% N₂ and that’s where the effect mainly comes from, argon, CO₂ and, (arguably) O₂ also contribute to it. Basically, most inert (non-reactive) gases have narcotic effect on humans and other mammals above certain levels of partial pressure. This effect is comparable to alcohol, but it dissipates rather quickly once the concentration of gas falls back below tolerance limits and is believed to be non-addictive. At extreme levels, it can cause loss of consciousness (that’s how inhaled anesthetics, e.g. N₂O, work) and even death. Reactive gases (e.g. fluorine or chlorine) probably have such effect too, but their chemical properties would kill any living creature long before any eventual narcotic effect might show up.

Different gases have different narcotic potency – N₂ actually turns out to be one of the least narcotic substances in the universe. Argon is about twice as narcotic and N₂O (nitrous oxide, aka “laughing gas”, which they use for general anesthesia in hospitals) has about 22 times narcotic potency of diatomic nitrogen. Cyclopropane (C₃H₆) is estimated to have about 11 times the effect of N₂O on humans (which would make it about 260 times as narcotic as N₂) and experiments on rats have shown that narcotic potency of CO₂ is somewhere between those two values (my personal estimate is about 90 × N₂). The only three gases that are confirmed to have less narcotic effect then N₂ are helium (He), neon (Ne) and diatomic hydrogen (H₂).

The status of O₂ is not clear – as with other chemically reactive gases it becomes poisonous long before it might produce any independent narcotic effect on humans or other animals, so it is hard to determine its potency. However, we can tolerate much higher levels of oxygen then of fluorine of chlorine, so it might contribute to overall narcotic potency of a gas mixture. According to Meyer-Overton rule (lipid solubility), narcotic effect of O₂ should be somewhere between that of N₂ and Ar, but this principle does not necessarily hold true for biologically active substances (by the same rule CO₂ should be slightly less narcotic then N₂O, but empirical evidence clearly suggests otherwise). In case of O₂, much of it is being metabolized by the body, so only a fraction of would be left to produce any narcotic effect it might have. Most divers who experimented with different N₂/O₂ mixtures seem to agree that increasing oxygen fraction reduces overall narcotic effect of the mixture at the same depth as long as ppO₂ stays within tolerable limits. This would mean that even if O₂ has any narcotic effect, its potency must be considerably below that of N₂. Different experts estimate narcotic factor of O₂ anywhere between 0 and 1 × N₂ – personally, I am using a value of 0.5 in my calculations, which is purely arbitrary.

So, in order to dilute oxygen under ambient pressure above 2.5 ATA, you need something less narcotic then nitrogen, which leaves us with He, Ne and H₂ (water vapor is likely not narcotic too, but you cannot have very much of it in the air under conditions compatible with human survival). On Earth, they mostly use helium for deep diving gases – it is chemically non-reactive (unlike hydrogen), has very low narcotic potency (no narcosis effects have been reported up to 2,000 fsw, so it’s narcotic factor could be considered 0 for all practical purposes), and more readily available then neon. It does have a couple of other not very nice properties, however. For once, it causes voice distortion – sound travels much faster in He then in N₂, so it makes you talk in a very funny high-pitched voice.

The more nasty effect of breathing helium is known as High Pressure Nervous Syndrome (HPNS). It was initially believed to be related to pressure, but it later turned out that, divers who breathe hydrogen or neon based mixtures at the same pressure experience no such symptoms, so it is clearly an effect of breathing helium. Sometimes it is speculated that HPNS is, in fact, a manifestation of He inert gas narcosis, but the symptoms are quite different from narcotic effects of other gases and do not seem to cumulate. In fact, HPNS symptoms and narcotic effects of N₂ or H₂ appear to cancel each other out, so HPNS could be regarded as “antinarcosis” – this is the reason why trimix (He/N₂/O₂) or hydreliox (He/H₂/O₂) mixtures are often used for very deep diving rather than heliox (He/O₂). But we do not know how healthy this HPNS/narcosis mutual cancelation effect is in the long term, so we should regard HPNS as separate phenomenon caused by certain ppHe levels and assume it is not good for human health. First HPNS symptoms appear about 430 fsw (~ 130 msw) which translates into 14 ATA overall pressure. The sources I’ve found did not mention what exact He/O₂ ratio was in the mixture used, but oxygen fraction could not have been any more than 7% at this depth, which gives us ppHe of at least 13 ATA. It is generally considered that HPNS effects may be tolerated up to 1,000 fsw (which gives us ppHe level of about 30 ATA), but prolonged exposures to such levels may not be safe, so we should consider 13 ATA ppHe as maximum to be on the safe side.

Hydrogen is another low-narcotic gas. Based on Meyer-Overton principle, its narcotic factor is usually estimated at 0.5 – 0.6 × N₂, but it is clearly much lower than that. It has been reported that divers experienced “significant effects of hydrogen narcosis” at depths around 1,000 fsw, but if we assume narcotic factor of 0.5, it would mean that breathable H₂/O₂ mixture at this depth should produce the narcotic effect of normal air at about 16 ATA. Such effect would be much better described as “general anesthesia”, rather than merely “significant”. Based on this, my calculations show that H₂ cannot have narcotic potency any more than 0.3 × N₂, which would make a minimally breathable H₂/O₂ atmosphere (around 97/3 ratio) safe for overall pressure range between 5 and 8 ATA. Note however, that mixing hydrogen and oxygen at ratios greater than 4:96 (either way) is not a very good idea since that would make the air you breathe highly explosive.

Finally, there is also neon which could be used to dilute oxygen at high pressures. According to Meyer-Overton rule, it should be at least twice less narcotic then hydrogen, which would put its narcotic factor at no more than 0.15 × N₂. Otherwise, it is chemically non-reactive, has molar mass pretty close to that of N₂ and thermodynamic properties similar to those of argon and helium. Unfortunately, there have been very little experiments with neon due to its very low abundance on Earth and prohibitively high cost. The only source regarding the use Ne/O₂ mix as breathing gas I could find states that divers went as deep as 1,200 fsw (~ 370 msw, 37.3 ATA) and felt no narcotic effect – this would make it at least 12 times less narcotic than normal air and thus put its narcotic factor at no more than 0.8 × N₂.

Based on the above, I have run a few simulations and could successfully model an atmosphere which should be theoretically safe for humans and able to sustain Earth-like ecosystem at as much as 40 ATA absolute pressures. Such atmosphere would consist mostly of neon and helium mixed at about 70/30 ratio (more Ne then He). O₂ fraction could be anywhere between 0.5% to 1.1%, N₂ content would need to be in range between 0.0025% and 0.5% and CO₂ concentration could vary from 0.00015% (depending on O₂ level) to 0.005%. Pushing N₂ or CO₂ levels above these maximum values would risk boosting the narcotic effect of air beyond safety margin, while O₂ fraction values outside the indicated range could put the population at risk of hypoxia or oxygen poisoning in case of pressure fluctuations. This is about the upper limit of absolute pressure I could achieve while keeping all other parameters within “safe” ranges.

[NecronLord]

Split from here as the other thread's long dead but this is a very interesting post.

Welcome to the board.

[agent009]

Sorry, there was a typo in my original post but, unfortunately, this forum won't let me edit it. The paragraph before last should read:

Finally, there is also neon which could be used to dilute oxygen at high pressures. According to Meyer-Overton rule, it should be at least twice less narcotic then hydrogen, which would put its narcotic factor at no more than 0.15 × N₂. Otherwise, it is chemically non-reactive, has molar mass pretty close to that of N₂ and thermodynamic properties similar to those of argon and helium. Unfortunately, there have been very little experiments with neon due to its very low abundance on Earth and prohibitively high cost. The only source regarding the use Ne/O₂ mix as breathing gas I could find states that divers went as deep as 1,200 fsw (~ 370 msw, 37.3 ATA) and felt no narcotic effect – this would make it at least 12 times less narcotic than normal air and thus put its narcotic factor at no more than 0.08 × N₂.

[Broomstick]

That was an excellent post and you've clearly done some research.

Now - how would people come to be living in such an unnatural atmosphere as your maximum pressure mixture? I have trouble conceiving of that coming about naturally, so what purpose to such a thing?

[agent009]

Now - how would people come to be living in such an unnatural atmosphere as your maximum pressure mixture? I have trouble conceiving of that coming about naturally, so what purpose to such a thing?

Well, basically the main purpose of my research was to create a more or less realistic model of atmospheric conditions on a given planet and determine whether a human civilization could thrive under any given set of conditions. The post just demonstrates what would be the extreme minimum and maximum values of atmospheric pressure given the right composition of atmosphere end explains how it works, but my model lets me input some key parameters for any imaginary or real world and see what it would be like there (it actually worked pretty accurately for Earth and Venus).

As for my high-pressure example – granted, it might not be very likely to find a world with those exact atmospheric conditions, but I do not personally see anything particularly unnatural about an Earth-like planet with high-pressure helium-neon atmosphere with just a little oxygen and nitrogen mixed into it (much weirder things exist in the universe). So, if such a existed, those conditions would be perfectly compatible with development of Earth-like life on it. Or, if humans found a some world with helium-neon atmosphere under comparable pressures, all they'd had to do in order to colonize it would be adding just a little bit of O₂, N₂ and CO₂

As I said, those extreme examples are probably more of a proof of concept then anything else - but there are plenty of possible variations inbetween those two extremes

[biostem]

Somewhat related - I remember watching a show where they were talking about developing a flexible suit that would sort of apply compressive force in order to maintain something like .7 to .8 of a sea-level air pressure, then you'd only need to feed the required atmosphere into a rigid helmet, to keep people alive - perhaps if a planet somehow had enough oxygen in its atmosphere, but was otherwise too low pressure, such a garment would suffice, (you'd still need a helmet to stop your eyes from getting blown out by your higher internal pressure, however).


I found these 2 articles on it; It's called a "Mechanical counter-pressure suit":

http://www.bostonmagazine.com/news/blog ... va-newman/

http://www.space.com/4074-slimming-futu ... suits.html

[agent009]

"Garment" for extremely low-pressure environments have existed for quite a while - they are known as "pressure suits" or "space suits" As long as you have sufficient partial pressure of oxygen, you do not need it. If ppO₂ is too low, there is simply not enough of it in one m³ of air to sustain you and nothing will be able to extract sufficient quantities of it any better then your own lungs – so you would need oxygen tanks.

"Counter-pressure suit" for high-pressure environments is actually quite a bit more tricky to make then a "pressure suit". It is relatively easy to keep the pressure inside a flexible container above ambient, but keeping it below ambient is quite another thing.

[madd0ct0r]

hmm. that neon/helium mixture is interesting.
1) human powered flight (things like helicopters get astoundingly efficient at high pressure, there was a project looking at one for a venus rover.)

2) is such a mix stable? i'm sitting here trying to remember the escape velocities, but can't quite get my head around it this early. Basically, to keep the helium long term, X amount of gravity would be needed, which would cause the atmosphere to be squished quite tight against the planet, giving you a pressure of Y at sea level. It's seems like it might be internally consistent, but need more coffee.

3) other reasons. Obviously, the low pressure mix would be useful for big bag space habitats (less mass = better acceleration). Also got the option of blimpworlds on the gas giants. The ultra high pressure might simplfy deep pressure habitats (like bottom of shallow seas). All the extra mass might be useful in cases where thermal buffering is needed, although I can't think of any cases that couldn't be achieved more simply by a sun shade.

[agent009]

hmm. that neon/helium mixture is interesting.
1) human powered flight (things like helicopters get astoundingly efficient at high pressure, there was a project looking at one for a venus rover.)

Btw, even though 30% of atmosphere in my high-pressure model is helium, pure helium would still have lifting of about 19 kg/m³ under 20°C (that's 17 times more then under ambient conditions on Earth at sea level). Hydrogen would have lifting power of 22.5 kg/m³ (nearly 19 times greater then on Earth). So, balloons/airships would be quite efficient as well. If we also add high gravity, aerostatic lift becomes much more advantageous, since it is independent of g - it is merely a function of densities and density is a function of pressure, temperature and substance properties. Aerodynamic lift, on th other hand, is a function of density and weight, the latter being a function of gravity.

[Sea Skimmer]

Have you looked into what body thermal regulation and humidity is going to be like at 100 atmospheres? I'm thinking this would be a very serious problem.

[agent009]

Have you looked into what body thermal regulation and humidity is going to be like at 100 atmospheres? I'm thinking this would be a very serious problem.

I never claimed we could survive under 100 ATA - I've put my maximum "safe" limit merely at 40 (which is less then some saturation divers have to endure occasionally) and I've mentioned that anything above 70 would likely be incompatible with human survival.

[NoXion]

Thanks for taking an interest in the original thread! Do you mind if I use what you've written here as a reference?

[agent009]

Thanks for taking an interest in the original thread! Do you mind if I use what you've written here as a reference?

By all means

[Zeropoint]

If ppO₂ is too low, there is simply not enough of it in one m³ of air to sustain you and nothing will be able to extract sufficient quantities of it any better then your own lungs – so you would need oxygen tanks.

Well, there is such a thing as a portable oxygen concentrator.

[Cykeisme]

Very enlightening, thanks for writing and posting this, agent009!

[puskas78]

Another concern in low-pressure environment would be the boiling point of water. It is well known that water boils under lower temperature at lower pressures and, since all your body liquids are mostly water, you would not normally want them to boil under ambient conditions. At atmospheric pressure of 0.1 ATA, water would boil at about 44.5 °C (111.5 °F) which is a little too low IMHO, but should still be ok as long as you do not go into hot places (fever could become a major concern, however). At 0.17 ATA it is at 55.5 °C (132 °F) – this should be marginally ok for human survival, but you’ll have to forget about boiled eggs and coffee.

No, just use a pressure cooker
Relevant links:

http://www.faa.gov/data_research/resear ... wheelwell/
"Survival at High Altitude: Wheel-Well Passengers"

http://www.outsideonline.com/outdoor-ad ... ation.html
"Q. What's the highest altitude at which humans can habitate?"
... a small community of gold miners in northern Chile, who resided at 19,500 feet for nearly two years, starting in 1984. These men were never studied for mountain sickness, but John B. West, a professor of physiology and medicine at the University of California at San Diego, met the miners and described them as "a little blue."