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One design for underwater human inhabited environments is to have equal pressure between the surrounding water and the submerged habitat, thus allowing a section of the floor to be open to the water and removing the necessity for strong plating to protect it. This requires the air pressure within the environment to be greater than atmospheric pressure so that it can withstand the pressure of both the atmosphere and the pressure from the overlying water.

So how much pressure can the human body survive in without need for special suits and breathing apparatuses (as this would place a limit on how deep you could build such an environment without special accommodation)?

Edit: So far i've found an article about a man trapped in a sunken boat who survived for days in an air pocket trapped in the bathroom 100 feet down. He couldn't resurface without the use of a diving bell and gradual decompression; but this case provides a lower bound for how high the highest pressure can be.

Calculations: 14.7 psi (atm pressure) + 1200 inches*0.037 psi/in^3 = 60 psi for the underwater pocket (roughly 4 times atmospheric pressure). http://news.nationalgeographic.com/news/2013/12/131204-nigerian-air-bubble-survival-shipwreck-viral-video-science/

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    $\begingroup$ Without breathing apparatuses, the pressure isn't the limiting factor for survival. The lack of oxygen is... $\endgroup$ – John Dvorak Sep 12 '14 at 19:17
  • $\begingroup$ Plants might refresh the air or might not; but supposing you had a means to freshen the air, and using whatever composition of the gas would maximize the highest pressure at which humans could breathe. Or if that is too difficult, just plug in condensed atmospheric air. $\endgroup$ – Dargatz Sep 12 '14 at 19:24
  • $\begingroup$ oh... athmospheric pressure. I though you would be completely submersed, somehow, sorry. In this scenario, you would actually fare pretty well, AFAICT. $\endgroup$ – John Dvorak Sep 12 '14 at 19:27
  • $\begingroup$ en.wikipedia.org/wiki/Deep_diving $\endgroup$ – John Dvorak Sep 12 '14 at 19:28
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    $\begingroup$ It's common for saturation divers to live under pressure for weeks if not months at a time beyond a pressure of 1000 feet of seawater, but typically not more than 1500 feet. I believe the limitations are more economical rather than physiological. 1000 FSW requires special mixed gas controls to prevent the narcotic effects of nitrogen that occuer under pressure. That's why helium is added. But then helium steals the diver's heat, so special warming systems are required. Decompressing after these long periods takes days. $\endgroup$ – docscience Nov 17 '14 at 2:19
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The deepest recorded simulated dive is still the on-shore French Hydra 10 experiment at 701 msw (meters salt water); at a pressure of 71 atm. Since breathing a mixture containing helium is required at such depths (to avoid the extreme narcosis produced by nitrogen at such depths), the new problem helium causes is

High-pressure nervous syndrome (HPNS). HPNS, brought on by breathing helium under extreme pressure causes tremors, myoclonic jerking, somnolence, EEG changes, visual disturbance, nausea, dizziness, and decreased mental performance. Symptoms of HPNS are exacerbated by rapid compression, a feature common to ultra-deep "bounce" dives.

Actually that Wikipedia description is a bit misleading, the problem is caused by any gas mixture, not just helium, but other gasses like nitrogen or hydrogen have a narcotic effect that counteracts HPNS. HPNS sets in at about 120m:

The high pressure neurological syndrome (HPNS) begins to show signs at about 1.3 MPa (120 m) and its effects intensify at greater depths. HPNS starts with tremor at the distal extremities, nausea, or moderate psychomotor and cognitive disturbances. More severe consequences are proximal tremor, vomit, hyperreflexia, sleepiness, and psychomotor or cognitive compromise. Fasciculations and myoclonia may occur during severe HPNS. Extreme cases may show psychosis bouts, and focalized or generalized convulsive seizures. Electrophysiological studies during HPNS display an EEG characterized by reduction of high frequency activity (alpha and beta waves) and increased slow activity, modification of evoked potentials of various modalities (auditory, visual, somatosensory), reduced nerve conduction velocity and changes in latency. Studies using experimental animals have shown that these signs and symptoms are progressive and directly dependent on the pressure. HPNS features at neuronal and network levels are depression of synaptic transmission and paradoxical hyperexcitability.

The problem with extreme helim pressure was partially alleviated in the Hydra experiments by mixing/replacing some of it with hydrogen, in the so-called Hydreliox mixture.

For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.

The French did this based on prior experiments which found that the narcotic effect of hydrogen reduces HPNS (or even completely eliminates it at lower depths):

A H2-He-O2 mixture with 54 to 56% hydrogen was studied with 6 subjects (professional divers) during 2 dives to 450 m. The 38-h compression was the same as that used with other types of breathing mixtures (He-O2 and He-N2-O2). The results obtained during compression and during the stay at 450 m in H2-He-O2 show that the EEG changes (increase of theta activities in the anterior regions of the skull, decrease of alpha activities) are similar to those found with other respiratory mixtures. On the other hand, the other symptoms of high pressure neurologic syndrome (HPNS) were clearly improved for the same depths. Thus, neurologic symptoms (tremor, dysmetria, myoclonia, drowsiness) are nonexistent, and the performances during psychometric tests remain similar to those of the surface. Hydrogen, with its narcotic potency, suppresses some symptoms of HPNS and seems to open new perspectives for deep diving.

Of course, having hydrogen (in such large proportion) and oxygen in a gas mixture poses dangers of fire, explosion etc. This is avoided by decreasing the oxygen concentration; alas this cannot be a one-step process (for diving):

The major problem with hydrogen-oxygen mixtures is the potential for explosion. Although the concentration of oxygen needed for combustion of oxygen-hydrogen mixes varies a bit with pressure, a general rule of thumb is that hydrogen-oxygen mixes above 5 % O2 are at-risk.. So, to avoid nasty fires and explosions, hydrogen is only considered as a breathing gas component at pressures where a less-than 5% oxygen concentration in the breathing gas mix gives a partial pressure of oxygen great enough to sustain life. Perhaps the most common example of a hydrogen-fire related disaster is the destruction of the Hindenburg dirigible. [...]

In 1944 Arne Zetterstrom discovered a way to breach the transition between compressed air and Hydrox without risking explosion. The technique was to descend to 100 feet and switch to a 4% oxygen / 96 % nitrogen mixture. After breathing this mix for sufficient time to allow the oxygen concentration in the lungs to drop below the "explosion threshold," the diver switched to Hydrox and continued descent. On ascent, the diver again used the Nitrox (4% O2 / 96% N2) as a transition between Hydrox and air. Using this technique, he descended to 363 feet. At that depth, the alteration in voice characteristics, coupled with excitement, made communication impossible and additional dives used a telegraph key. [...]

During the mid 1960's research into the use of hydrogen in breathing gases resumed with animals breathing Hydrox for up to 24 hours at 70 Ata. One interesting aspect of the animal research was the suggestion that hydrogen reduced the HPNS (high pressure nervous syndrome) often observed with helium based gas mixes on deep dives. Ultimately animals would be taken to 3500 feet on hydrox.

In 1983 COMEX, the French deep diving concern (perhaps more famous in the US as the company providing the submersible used in the recovery of artifacts from the Titanic) began a series of dives to investigate the narcotic potential of hydrogen. Divers including H.G. DeLauze, President of COMEX, descended in open sea to approximately 300 feet for five minutes. The divers could not perceive a difference between Hydox and Heliox at that depth. Chamber dives to 300 m (984 ft) demonstrated that hydrogen possessed a narcotic effect different from nitrogen. Hydrogen narcosis (the "hydrogen effect") had a tendency to be more psychotropic, i.e. more like LSD, while nitrogen narcosis had an effect similar to alcohol. This deeper work suggested that Hydrox as a binary gas mix would not be too useful at depths below about 500 feet.

Based on that COMEX developed a protocol of switching from heliox to hydreliox at 250msw.

A more recent (1994) COMEX paper reported that at 500m (on hydreliox), manual dexterity was about 80% of the surface one, while the arithmetic ability decreased to 60%. So not dying and being able to function as on the surface, aren't the same thing.

Also these experiments are time consuming and costly. In Hydra VIII, the whole thing took one month, of which only 10 days or so were spent at maximum depth. And in case "something bad happens" interventions aren't easy in such settings.

A 1984 American experiment with trimix at 650m was aborted after one of the three subjects (despite negative previous neuropsychiatric screening, as well having substantial commercial diving experience and successfully taking part in the previous experiments of the series) developed hallucinations and then full blown mania at 625-650m.

The Atlantis IV dive was designed to test the ability of 5% N2 trimix breathing mixture (5% N2, 0.5 atmosphere oxygen, with the balance helium) to counteract the [high pressure nervous] syndrome without nitrogen narcosis. [...]

The dive began with an initial compression rate of 30 m/hour to 300 m (1000 ft) sea water equivalent pressure. By day 3 of the dive, at about 470 m, subject C had developed significant insomnia. On day 4 (540 m), this subject became stressed and complained of mild auditory illusions of music and visual distortions consisting of a “halo effect” around the chamber’s apparatus. On day 5 (625 m) of the dive, the subject’s irritability, insomnia, and agitation worsened. He was medicated with temazepam, 60 mg, and achieved substantial relief of these symptoms after 15 hours’ sleep. However, with further compression to 650 m, subject C experienced a recrudescence of auditory and visual illusions and developed rapid speech, racing thoughts, shortened attention span, and difficulty in focusing on tasks. Ankle clonus, marked hyperreflexia, and myoclonic jerking were documented, and he complained of nausea and weakness. In contrast, the other two subjects performed normally in almost all respects. On day 9 (650 m), the subject stated he felt “paranoid” and that he felt like “I’m going insane.” He began to carry a mirror to look at himself to “remind myself that I’m not a raving maniac.” He stated that colors were enhanced and vivid and he could see “unusual” mosaic patterns on the metalwork within the chamber. He was unable to comply with instructions to enable cognitive function to be tested because of extreme agitation and distractibility.[...]

Early on day 9 (650 m), the decision was made to halt the dive, but decompression from 650 m is still experimental and could not be accelerated. Because the decompression process was expected to last as long as 30 days, it was necessary to consider medicating the subject more intensively. Since lithium had been shown to exacerbate symptoms of the high pressure nervous syndrome in animals and the effects of phenothiazines at such high pressures were virtually unknown, the decision was made to control the diver’s agitation with benzodiazepines. Accordingly, diazepam was used in doses up to 120 mg/day. This treatment only mildly sedated the subject and provided no real control of his behavior. The subject’s agitation persisted over the next 14 days and was marked by decrements in his cognitive function and bursts of aggressive paranoia, grandiosity, and irritability. By day 24 of the dive (383 m), the subject’s irritable and aggressive behavior was becoming potentially dangerous and led to the decision to medicate with chlorpromazine, up to 300 mg/day. Within 48 hours of the beginning of chlorpromazine treatment there was dramatic improvement in the subject’s behavior. He became relatively calm and better organized. He denied having illusions or hallucinations. [...]

Ten days after exiting the chamber, the subject again exhibited signs of hypomania including hyperactivity, insomnia, and irritability. Lithium carbonate therapy lessened these symptoms and was continued prophylactically for 6 months. It was then discontinued without any recurrence of symptoms.

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