Roughly speaking (or precisely, if you can back it up), what is the highest pressure that a naked human being could withstand? The lowest? And, now that I tried looking and thought about it more, how do we know (assuming that we do)?
Well, the record for SCUBA diving is over 1000 feet. I think we can consider them “naked” since parts of the skin is exposed. That would be 450 PSI or 30 atmospheres of pressure.
As for the lowest pressure one could withstand, well, hard vacuum is the lowest possible pressure and it appears from animal studies that one could conceivably survive from 60 to 90 secondsf exposure. Some excellent information is available here:
From that same page, at least a few humans have been exposed to vacuum, albeit, accidentally:
Like all damage mechanisms including heat, poison etc pressure tolerance isn’t an absolute, it’s a combination of ‘dose’ and length of exposure, acuteness of exposure, preparations etc…
To give you some idea, at the most extreme range a person will easily survive a vacuum (ie zero pressure) for a few minutes at least. But they will die of suffocation eventually. And while they will have a good chance of immediately suffering fatal injuries if they are exposed to a vacuum instantaneously with no time to prepare, if they are allowed to prepare they won’t.
So it’s not really possible to answer “what are the pressure tolerances of the human body”. There needs to be some sort time factor in there as well. I suspect that a human could survive even the pressures inside a star for a minisule fraction of a second (although being vapourised by the heat/nuclear reactions is less uncertain).
If by “tolerate” you mean indefinitely, not much. As others have said, you can stand quite a bit for a second or a few.
Hard hat deep sea divers are in the most precarious position. A diver is inflated internally with air pressure essentially equal to the surrounding water pressure. Sort of like a tire and you know what happens to a tire when it deflates. For SCUBA the pressure adjustment is by a regulator and is automatic. Hard had divers, on the other hand, need to be able to regulate their suit pressure. They are mechanics doing mechanical or other work and sometimes have to make their suit “heavy” by reducing the internal pressure a little or “light” by raising the internal pressure a tad.
An excellent book, although out-of-date in some aspects by now, is Men Under The Sea by Edward Ellsberg who was a top navy salvage expert for many years and was the inventor of the underwater cutting torch.
He described a tragic accident in New York harbor with a diver who was working at a depth of only about fifteen or twenty feet. There were two divers working and just before time to end the dive the guy tending their compressed air source left for just a minute or two. When he returned he say a trail of water from the rail to the deckhouse and assumed that the divers had surfaced so disconnected the air hoses. Then we went around the corner to help the divers out of their suits and saw only one diver. He quickly raced to the hoses and reconnected but it was too late. The diver still on the bottom had been partially “squeezed” up into his helmet and was dead.
Thanks for the replies. Now that I’ve thought about it, what I’m really looking for is what we could tolerate indefinitely. For example, if I were living on a space station, and the pressure dropped to x, I would feel uncomfortable, possibly light-headed, and would eventually die if it stayed there and didn’t fix it. If the pressure increased to y, I would feel uncomfortable, possibly making it hard to breathe, and if I stayed there (and didn’t fix it) I would die. In both cases, I’d prefer to stick to the pressure effect, ignoring what part of that pressure is oxygen, at least for now.
If you were breathing from a regulator pressurized at 1 Atm with a proper gas mix, I think you could survive quite a bit longer than 60 seconds in a vacuum. There might be some rupturing and bleeding in the weak spots, but if you survive that, I’d venture a WAG that you could withstand vacuum almost indefinitely(having your body internally inlfated at 1 Atm). I am not a doctor though so it’s just a guess.
Not a chance. You’d have gas at 14.7 PSI inside your lungs, and no external pressure to contain it. Per my link, just holding your breath in a vacuum would get you dead almost immediately:
Pretty messy.
I don’t think increased pressure would make it hard to breathe. The increased pressure would force air into the lungs and if the increase were not large and abrupt but rather a little at a time the internal and external pressures would equalize.
One thing that increased pressure does is greatly increase the oxygen intake and burns fat so if you stay in it a long time use lose fat. Again, Ellsberg in his account of the salvage operations on the sunken US submarine S-4 wrote of how much weight the divers lost in the operation which took several months to complete.
I don’t know where that “use” came from. You might think I was trying to write the gangster “youse” as in “youse guys” but you would be wrong.
Actually I made a stab at “you lose fat” and missed.
First of all, here’s the obligatory reference to The Master’s Canon with regard to a similar question.
The distinction has already been made, but bears reinforcement, between gradual pressure change and “explosive” decompression. Note that you [thread=335681]wouldn’t explode the way Hollywood depicts[/thread]; the immediate trauma would come from expanding gases in sinus and thoracic cavities. Going instantaneously from 14.7psia to hard vacuum would likely cause your eardrums and lung tissues to rupture (called barotrauma, pneumothorax), but you wouldn’t explode into a bloody pulp as in that Connery flick or have your eyes pop out like Arnie. Prolonged exposure to vacuum would cause dissolved gases (mostly nitrogen and a small percentage of oxgyen and other trace gases) to embolize, rupturing capillaries and causing embolisms in arterial channels. You wouldn’t have problems breathing so long as your air supply continues to supply air at 1 atm, but the air will rush in forcefully and you’d feel an incomfortable urge to burp. If you do “breathe vacuum,” another problem is that the surfactant (mucus-like coating that protects and supports the alveoli) will quickly evaporate or solidify, making it much harder to breath. Although the OP doesn’t ask about other effects of vacuum, you’d have some [thread=335681]greater concerns[/thread] than the lack of pressure if this happened in space.
Going the other way to increasing pressure offers different problems. Again, a dramatic increase in pressure is going to cause barotrauma to the eardrums (in the other way), and if your air supply is feeding you pressure at 1 atm you’re going to find it nearly impossible to breathe, as the maximum negative (inhalative) pressure that the lungs can exert is approximately 0.01atm equivilent to supporting a 1m column of water. (If you don’t believe that, try jumping in the pool and breathing through a hose at a depth of 3-4’.) With scuba equipment a two stage regulator takes high pressure (500-3500psi) air and reduces it in stages; for recreational gear, first to about 150-200psi and then to ambient pressure at depth.
As long as the change in pressure isn’t instantaneous (so that you can equalize your internal airspaces), and you have an air supply that delivers ambient pressure, the pressure isn’t going to crush your body. Since your body is mostly water and dense bone, compression is minimal and from a strictly biomechanical viewpoint you can descend many thousands of feet. The limit would probably come when the pressure on unprotected nerves causes them to stop transmitting impulses, rather than biostructural failure of tissue. ([thread=337502]Here’s[/thread] a recent thread on the topic.)
However, from a respiratory standpoint, you are going to have a problem breathing plain air at depth for an extended period of time. The reason for this is that while the percentage of oxygen in air remains the same regardless of pressure, the amount of oxygen increases, and at depth you will continually accrue excess O[sub]2[/sub] until oxygen toxicity occurs. This isn’t normally a problem in recreational diving with air because the partial pressure of O[sub]2[/sub] at recreational diving limits isn’t high enough to be of concern for the amount of time a diver can remain at depth. However, for “enriched air” mixtures (increased O[sub]2[/sub] percentage) or Nitrox (a higher than normal percentage of O[sub]2[/sub] mixed with N[sub]2[/sub] to reduce nitrogen accumulation and allow longer dives at moderate depths) oxygen toxicity is a real concern and has to be calculated and dive time/depth closely monitored. Partial pressures of O[sub]2[/sub] greater than 1.6 atm are considered immediately hazardous.
Surviving at high pressures for extended periods of time, therefore, would require a breathing mix that maintains sufficient oxygen to avoid hypoxia while keeping partial pressures and accumulation within safe limits. In diving, this is done by adding other nonreactive gases (trimix, heliox, neox, other exotics.) At really farkin’ extreme pressures you’d need to go to a [thread=308429]liquid breathing fluid[/thread] (see links within) which transports oxygen directly without the hazards of accumulation of inert gases.
I don’t know what the maximum possible pressure that could be withstood by the human body, and given the distinct ethical issues with experimental testing on human beings at hazardous physiological extremes I doubt an emperical boundary exists. The US Navy has long done saturation diving experiments with SEALAB and other programs (I used to know a UDT guy who was, or at least claimed to be, a test subject in the SEALAB program), so between that and the pioneering work of J.S. Haldane we know a fair amount at human physiology at moderate depths, though the field is still an active study by DAN and other research organizations.
There is also quite a bit of emperical data as a result of the somewhat scientifically questionable and morally reprehensible human high altitude experimentation on POW and civilian prisoners at Dachau by a Dr. Sigmund Rascher. Much of the data is of questionable scienific value, owing to the haphazard methodology of the experimentors; however, even as anecdotal data it represents the best single collection of information on near-vacuum conditions on humans. There are, of course, significant ethical questions about the utilzation of this data, as some regard even the acknowledgement of its value to be a tacit acceptance of the experiments. As a result of both issues, there is only a limited amount of cumulative research based on this information and a dearth of original research regarding long-term exposure to near-vacuum pressures.
The manned space programs probably have the most extensive base of information regarding fractional, high O[sub]2[/sub] percentage atmospheres (which are increasingly considered to be a bad idea following the Apollo 1 fire) but I expect that their emperical data on long-exposure to low (but potentially survivable) fractional pressures is limited to short term experiments with healthy safety margins and a small number of accidents.
Basically, in any real-world situation (save for diving) in which pressure extremes might be of concern, the accompanying effects are going to do you in first.
“Dave, this conversation can serve no purpose anymore. Goodbye.”
Stranger
No way hosea! Your lungs and chest will explode quickly with either oxygen or air supplied at atmospheric pressure. Further the water content of the body will boil at it’s own normal temperature under hard vacuum.
In practice you would have to be in a well constructed pressue vessel and by the time it was evacuated to less that 1 pse abs. you would be dead or if it were to be something like a space capsule rupturing it would be bye bye groman…
“Don’t leave home without trying this out!”
The original statement being responded to was that high pressure would cause difficulty in breathing. It won’t and the above examples are of of differential pressure and not high pressure.
I think my description of the diver being inflated with the same pressure as the surroundings says the same thing.
And this:
Appears to be an attempt to foreclose further discussion. It didn’t work.
Well, yes. Perhaps I wasn’t sufficiently clear, but as long as ambient pressure and the pressure of the air supply are equalized, there is no real difficulty in breathing. (The increased density of the air does require slightly more effort in breathing, but it’s not a significant difference. Indeed, scuba regulators are designed to offer a slight positive flow, and should the mechanism within fail they’ll “freeflow” rather than jam.) My point was just that if the pressure of the air supply were even just a little bit less than ambient (for instance, a diver submerged a few feet and breathing from a tube to the surface) that you wouldn’t be able to inhale owing to the relative weakness of the lungs.
“Look Dave, I can see you’re really upset about this. I honestly think you ought to sit down calmly, take a stress pill, and think things over.”
Sorry, I couldn’t resist, what with the name match. 
Stranger
Actually I intended to add this 
but since it got omitted I will take it like a man and blame my wife. She said dinner was ready so I hit send.
Gen 3:12 (It was) “The woman thou gavest to be with me …”