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One fundamental problem of spacesuits is stiffnes due to air pressure; at around 0.3atm, with atmosphere of pure oxygen, they grant enough oxygen for breathing, and more than enough pressure to prevent other problems - blood vessels bursting, tissue bloating, moisture on eyeballs evaporating and so on. Indeed, the Sokol spacesuit allows to drop the internal pressure below the 'safe' level in emergency situations.

It seems the primary problems are concentrated in the head area (or areas accessible through head area, like lungs).

One idea to overcome the problem is skinsuits that apply physical pressure instead of atmospheric. And one problem with the skinsuits is that with applying equivalent of 0.3 atmosphere, they are exceptionally difficult to wear, they cause blood circulation problems as stretched areas press too hard, and so on.

One way around that would be to make them less "tight". Of course that would require isolating the head area, or the suits will bloat like balloons from the internal pressure. But assuming this is doable (say, hard chest/back plate with internal sealing against the body), how low could we go with the pressure equivalent before it becomes the problem?

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  • $\begingroup$ I'm not sure it would be easy to get good data on that without some extremely unethical human research studies... $\endgroup$ – Tristan Mar 10 '17 at 15:14
  • $\begingroup$ If we assume a helmet with a diameter of 30 cm at the shoulders and a pressure differential of 0.1 bar between helmet and body, a force of 70 N is necessary to hold down the helmet on the shoulders. It would not be very comfortable to wear the suit for hours with the constant pull between helmet and suit. But exhalation under the pressure difference would overstress the muscles of the torso used for breathing. Besides that, the pressure dif. may cause a rupture of the very weak tissue of the lungs. The pressure dif. will also squeeze the blood out of the lungs. $\endgroup$ – Uwe Mar 16 '17 at 15:06
  • $\begingroup$ @Uwe: 70N is about 7kg of weight equivalent. EMU weighs significantly more. I'm also assuming a rather strong, flexible chest/back plate dynamically helping against the pressure. Minimal skinsuit for limbs mostly. $\endgroup$ – SF. Mar 16 '17 at 15:24
  • $\begingroup$ @SF: It is not sufficient to apply pressure only by a chest and back plate, the abdominal wall is important for breathing too and needs the same pressure. But what about the sides of the torso, the area between the chest and back plate? This area needs pressure too to avoid swellings. If there is a minimal skinsuit for arms and legs only, blood and lymph will be pressed into them and back flow of blood through the veins is disturbed and thrombosis might happen. $\endgroup$ – Uwe Mar 16 '17 at 19:57
  • $\begingroup$ @Uwe: What pressure differential is necessary to induce swelling? This whole question focus is not "what happens if we expose limbs to perfect vacuum" but "what's the least pressure differential we can get by without harm"; the skinsuit still provides some pressure, question being how much. Since certain pressure differentials (like the ~0.18 bar between head and legs, which we experience while standing up) are known to be harmless, this is about quantifying these: how little can we get by with? $\endgroup$ – SF. Mar 17 '17 at 1:47
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The knowledge existing on barotrauma in divers medicin is applicable for those skinsuits. Any larger pressure difference between different parts of the body may cause small or larger injuries. Those injuries caused by pressure differences are called barotrauma.

The air pressure at the mouth should be equal to the pressure on the skin for every part of the body. Pressure differences may impair blood circulation, lymph flow and air flow into and out of the lungs. The minimum blood pressure is at least 80 mbar, any pressure difference should be much smaller. The difference between chest surface and the air at the mouth should be smaller than about 40 mbar, larger differences would be exhausting and may impair the blood flow between both chambers of the heart and the lungs. To high pressure differences may cause severe and dangerous damages to the lungs tissue and to the heart muscle. The muscles of the chest used for breathing are not very strong, if they are exhausted by larger pressure differences, a dangerous situation may occur when less than necessary oxygen gets into the circulatory system and not enough carbon dioxide is removed from the body.

But this restrictions are also a problem when such a skinsuit is already put on but the helmet not yet, the pressure on the chest would inhibit breathing in. How would you don or doff a skinsuit and the helmet without generating to high pressure differences between chest and mouth during the procedure?

The small pressure differences should be sustained for any phase between full inhalation and full exhalation for any surface part of the torso. For the arms and legs, larger pressure differences are possible than for the torso as longs as blood circulation and lymph distribution is not affected.

But what about cooling of the body? If the sweat could not evaporate and thus cool the body, the body would overheat and the performance of the wearer is substantially reduced, higher temperatures will be dangerous. Either a liquid cooling garment should be worn under the suit or the suit should be permeable for water vapour. But the skin might be impaired if to much water is removed from it. If too much oxygen diffuses from the tissue under the surface of the skin and out of the suit, damage to the tissue may result when the suit is worn for many hours.

To avoid skin-abrasions near the joints of the body caused by the moving parts of such a skinsuit would be very difficult if the suit is worn for many hours and on many days in sequence.

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  • $\begingroup$ So it seems oxygen intake is still the limit: the pressure in the helmet must be sufficient for breathing, and the pressure across the whole body must be more or less uniform or we're squeezing blood out of high-pressure areas. Now I'm seriously wondering how an "artificial lung" device, tapping into astronaut's arteries and oxidating blood passing through could help; this would allow severe reduction of pressure in the head area as well; surely above Armstrong Limit (water boiling off eyeballs would be a serious problem)... $\endgroup$ – SF. Mar 13 '17 at 11:38
  • $\begingroup$ OTOH I wonder how serious the pressure differential is. A 1.80m tall person with head above water will have around 0.15bar pressure difference between feet and head when standing in a swimming pool with head above the surface. $\endgroup$ – SF. Mar 13 '17 at 11:40
  • $\begingroup$ The pressure differential of the person in the pool is no problem because the pressure of any fluid inside the body rises in the same way from head to feet as the water pressure outside does. But if you do it in a dry diver suit, you feel the suit is pressed to the feet and lower legs but not to the arms and chest. The air pressure inside the suit is the same from chest to feet. But if you would try to breath through a long snorkel from surface down to your body in 2 m depth, you cant do it and there is a high risk for severe damage to the heart and lungs. $\endgroup$ – Uwe Mar 13 '17 at 14:46
  • $\begingroup$ If the pressure in the helmet is not the same around your chest, breathing is seriously impaired by larger pressure differences. Breathing is not only important for oxygen intake, the removement of carbon dioxide is essential too. $\endgroup$ – Uwe Mar 13 '17 at 14:51
  • $\begingroup$ I am sorry, but I don't think an artificial lung is a solution. The artificial and the natural lung need the same oxygen partial pressure in the gas part to transfer enough oxygen into the blood. But the pressure of the gas should be nearly equal to the blood pressure. A larger pressure difference between gas and blood would either squeeze the blood out of the lung (artificial or natural) or suck into it. The necessary gas pressure determines the blood pressure and this determines the tissue pressure. $\endgroup$ – Uwe Mar 13 '17 at 20:48
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Here's my uneducated opinion. All above is just hypothesis without actual sufficient medical knowledge or links. Hopefully you won't kick me too much for that)))

The very least you need to have proper helmet, adequate air pressure and oxygen level in the lungs and protection from barotrauma of lungs and gastrointestinal tract (I don't think it's good to continuously pump air from mouth to anus constantly risking rupture if you stop this constant flatulence). So let's assume that you have proper spacesuit for head and torso but not for limbs. I suppose this will lead to the following issues.

First, human body is capable of intaking extra liquid - the blood vessels and cappilars can be stretched and the liquid can fill the cells and intercellular space. I think the extra pressure on head and torso will be squeezing them out of blood and other liquid. On the one hand you will be depriving your vital organs of the blood. On the other hand your limbs will swell up, cappilars rupture producing internal hemorrhaging. Probably the blood will even leak through skin.

Then as was noted in other answer above Armstrong limit the skin moisture will boil up. This is bad because this will dry out the exposed skin and upper layers killing the cells so at the very least you'll lose your limbs. However combined with the previous point I suspect that even worse will happen - the liquids from below would be pumped to the skin and you'll dry yourself through your unpressurized limbs.

Most likely the death will take some time but I suspect that even short exposure will already lead to dangerous consequences.

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  • $\begingroup$ note: boiling/evaporation takes a lot of heat. It will very quickly cool the skin, but beyond that it will progress only as heat is delivered from inside the body. Way below Armstrong Limit this will cause frost bites. Just below - you'll be cold, but not freezing. (for example, Mars atmosphere is only slightly below Armstrong limit.) $\endgroup$ – SF. Mar 11 '17 at 23:08
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The Armstrong limit of 0.0618 atm or 6.3 kPa (47 mmHg) is probably a good reference point.

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    $\begingroup$ I seriously doubt it. Blood pressure is at least 8.0 kPa; skin and tissue exert elastic force upon contained liquids too. The Armstrong limit was established basing on faulty assumption of pressure inside and outside human body being equal, and 0.06atm is very little force to overcome. Meanwhile, I think other effects could take their toll. $\endgroup$ – SF. Mar 10 '17 at 23:04
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    $\begingroup$ @SF. Well the Armstrong limit is based on the pressure limit where water boils at room temperature. Simple physics proves it's accurate. So obviously pressure would have to be significantly higher than the Armstrong limit as the pressure would increase if the astronaut for instance bent his knees. And what i meant was that the Armstrong Limit would provide a lowest possible reference point. $\endgroup$ – Jonatan Mar 10 '17 at 23:25
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    $\begingroup$ @Johny: where water boils at human body temperature, not room temperature. Yes, it's accurate, it's also pointless and arbitrary, because water inside live human body will never reach this pressure. $\endgroup$ – SF. Mar 10 '17 at 23:34
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    $\begingroup$ providing the human is still alive and not cooling rapidly. $\endgroup$ – SF. Mar 11 '17 at 9:33
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    $\begingroup$ The Armstrong limit is the vapour pressure of water at body temperature of 37°C. At that pressure, the lungs are filled with nothing else than water vapour and no oxygen intake is possible. If the pressure in the lungs is 0.2 bar or more, the partial pressure of oxygen is sufficient for oxygen intake. But if the pressure of the tissues is near the Armstrong limit, the pressure difference between the he lungs and the remaining body is too high for breathing. If the pressure in the lungs is 0.3 bar, the minimum possible pressure of the body must be at least 0.26 bar. $\endgroup$ – Uwe Mar 14 '17 at 8:46

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