It is my understanding that ultra-high atmospheric pressure (say, 2.5 atm) could kill you, regardless of the actual makeup of the atmosphere. My question is this: assuming the chemical makeup of the atmosphere is abundantly healthy for human respiratory systems–let’s say it’s richer in the stuff we need, like nitrogen and oxygen, than the Earth’s atmosphere, but not too rich, if there is such a thing, and there’s nothing in the atmosphere that would hurt us in and of itself–and the atmospheric pressure is two and a half times that of Earth,
“Abundantly healthy” and “richer in the things we need” are mutually exclusive. You could have as much oxygen, but it’d have to be as much absolutely, not as much by volume.
I don’t know much about SCUBA diving, but I’m pretty sure if your regulator wasn’t turned up to nearly the pressure of the water, the hydrostatic force would crush your lungs. I’d be pretty similar to an overpressure, I believe. Those start to get deadly around 4 psi.
Of course you are breathing at whatever the surrounding pressure is. That is why you have a regulator that compensates for ambient pressure. If you were at two atm with an air supply at 1 atm, you would not be able to inflate your lungs.
Pressure in and of itself does not kill. Oxygen can be come toxic if in too high a concentration at greater pressures, and Nitrogen can lead to disorientation (Martini’s law says that each 50 feet of depth = 1 dry martini on an empty stomach) but these problems can be minimized via the use of mixed gasses.
The largest problem with high pressures is decompression so that you don’t get the bends on the way back up.
I find the fascinating diving gases to be the “bottom mix” ones that you can’t breathe at sea-level, due to their lowered O2 content (to avoid oxygen toxicity at the partial pressures encountered at depth).
Like heliox. Typically 90% or more helium with the remainder oxygen. Used in deep sea diving because nitrogen at those pressures would essentially poison you, but helium is safe.
I was just wondering, how high a pressure we could survive if the relative pressures of the oxygen and carbon dioxide were cranked back to maintain appropriate absolute partial pressures, and assuming that pressures were equalised inside and outside the body. Nitrogen poisons you at high pressures? How? If we went with helium and oxygen, could we increase the pressure until we were basically breathing liquid?
An excellent question. My dive instructor said that the mechanism wasn’t really understood, but that at sufficient partial pressure even gases that were inert at sea-level had odd effects. The Wikipedia article on nitrogen narcosis gives “inert gas narcosis” as an alternate term and says:
So not going diving with xenon would seem to be in order. Helium isn’t mentioned there, and this scuba-doc article on partial pressures (pdf) states that “Helium lacks this anesthetic property and is used in place of N2 as the diluent for O2 in deep diving”.
I suspect at the pressures you’d need, and despite humans being mostly incompressible water, we’d be just water and some red paste. Lower pressure liquid breathing is possible however, if still experimental. (There’s also a dramatization of it in The Abyss).
As a sport diver, a very occasional commercial diver, and a volunteer at the local recompression chamber, I can assure you the “2.5 atmospheres” is a long way from “super-high pressure”. It translates to only about 42’ of water depth, and people dive to that depth and breathe compressed air at that pressure all of the time. In fact, divers are recompressed to well above that pressure if they suffer the bends, in order to cure them.
Oxygen toxicity does become a problem at higher pressures, but only above something like (IIRC) fifteen atmospheres pressure. In any case, it’s at a pressure equivalent to diving deeper than a couple of hundred feet, which is about 14 atmospheres.
My understanding, based SCUBA diving training classes, is that the lung aveoli can handle about 2 psi of pressure differential. Once that is exceeded, embolism and likely death will result. This is why divers are trained to maintain an open airway at all times and to never hold one’s breath while diving.
Previous posts (including the OP) fail to understand that absolute pressure is of little consequence in this discussion. Very high pressure has no ill effects (strictly from the pressure itself) as long as all pressures are equalized. High pressure can cause oxygen toxicity or various kinds of narcosis if the air mixture is not handled properly. High pressure can also cause embolisms and/or DCS (the bends) when the pressure is released too quickly or improperly.
I don’t know what the absolute upper limit is to the handling of high pressure when all proper precautions are taken, but there does seem to be some sort of nervous system impact when the pressure is increased enough, regardless of all currently known precautions. There is an interesting fictional treatment of some aspects of this issue near the end of the movie The Abyss.
Helium isn’t mentioned there, and this scuba-doc article on partial pressures (