Why is decompression explosive?

Cecil’s take on the issue.

So… if you were wearing insulated coveralls and goggles, had your ass plugged somehow, and were breathing pressurized air through a mouth/nose mask, you’d do ok in vacuum, or would something else do you in?

I don’t see any need to plug your ass, unless that’s your bag. Any gas inside you’ll just fart out. And I’d wan’t airtight ear covering too.

I question this. If you let the air flow freely out of your lungs, there wouldn’t be a problem, but if one atmosphere of pressure remains there, it’ll be a big problem. One atmosphere is 17.6 psi if I remember correctly. The surface area of the lungs is supposed to be about that of a football field, but only because of the folded aveoli which won’t exert the force outward. The effective surface area will be much less than that, and based on volume estimates I found on the internet of around 5,000 cubic centimeters, it has to be over 220 sq in absolute minimum. That’s nearly 4,000 lbs pressing outward! It seems like a lot to me.

I’ve seen experimental space suits that are basically full-body leotards made out of elastic fabric and a pressurized helmet. The elasticity of the fabric provides the necessary pressure to the skin to prevent swelling. The helmet provides fresh air and keeps the respiratory system pressurized.

Would it be correct to say that the oxygen is at surface-normal partial pressure, and that the increase in pressure is all in the added inert gas?

That’s essentially it, although you can have a higher than surface O[sub]2[/sub] partial pressure in dive gases. The body can tolerate elevated oxygen partial pressures (up to about 1.4ata) for limited periods of time. According to conventional dive procedures, which allow continuous dives with intervals calculated between dives to ensure nitrogen outgassing, you are permitted only a certain level of exposure to elevated oxygen partial pressures per 24 hour period, after which you have to stop diving.

Since, unlike decompression illness, the onset and effects of oxgyen toxicity are rapid and immediately life-threatening, there is significant margin in dive tables to prevent any routine incidence of OT. A dive computer designed to be used with nitrox, heliox, or other blended breathing gases will track oxygen exposure, provided that it is given the correct blend data. It is wise with such dives, and indeed all dives even including recreational dives to moderate (<60ft) depths to precomputer the dive profile and check against tables to ensure that you aren’t running up against limits rather than to rely on the dive computer to save your life. Unfortunately, many people don’t seem to fully appreciate the hazards and the majority of recreational dive accidents underwater (as opposed to on the surface or on boat, which in my mind aren’t really dive accidents) occur because divers don’t pay attention to their exposure. OT, when it occurs, is frequently lethal and is the main hazard when diving with blended gases.


I think that’s correct. And that seems to be something that’s been developed since the early days of diving.

In his book about the i925 salvage of the Navy submarine S51 off Block Island in Long Island Sound,Cdr. Edward Ellsberg spoke of how the navy divers lost most of their body fat because of overoxygenation during the salvage.

Sure, but as the pressure drops, the point is reached where your body fluids (including the moisture surrounding your eyeballs) will boil at blood heat - not sure if this would create enough gas/vapour to expel the eyes, but it’s some extra puff anyhow.

To follow up, I just pulled out my DSAT OPP table, and I see that at 1.4ata PPO[sub]2[/sub] gives 100% exposure at 150 minutes. So, theoretically, a diver should be able to withstand exposure for 2.5 hours (over a 24 hour period) for an oxygen partial pressure of 1.4ata, which of course will vary with depth. Since the relationship between oxygen utilized and partial pressure for a given ambient pressure isn’t strictly linear, you can’t just blend your gas to reduce PPO[sub]2[/sub] at dive depth to surface PPO[sub]2[/sub], and besides, you wouldn’t be getting enough oxygen on the way down and up. However, for long and deep dives the breathing gases are often staged; that is, a tank with a blend appropriate at depth is lowered down or carried separately and the diver switches over at depth (and the reverse on the way back up.) Some new rebreather rigs can also automatically control the oxygen partial pressure in the breathing gas to maximize time at depth, but I haven’t gotten into that area of tec diving, so I can’t really speak to it.


I understand the concept of pressure. But pressure doesn’t exist at the level I’m talking about. There are only four fundamental forces and only two that exert an influence outside of their originating atom; gravity and electromagnetism. Obviously gravity is not the force responsible for this phenomena; if gravity was having any noticable effect it would be drawing atoms from the low density to the higher one.

So electromagnetism must be the responsible force. It can’t acting as an attractive force from a vacuum; there’s nothing in the vacuum to cause an attraction. So it must be the repulsive force between the atoms in the higher density region.

And if this is true, why is the phenomena of decompression only so noticable in a vacuum? Why, for example, doesn’t the high density of atoms in a liquid cause them to “explosively decompress” into the lower density of a gas if they come into contact? Why don’t the atoms in a solid explode outwards on contact with a gas? I realize there is some degree of movement in these cases, but why is this movement such a relatively sedate affair unless a vacuum is involved?

It doesn’t take a vacuum at all. What is required is a pressure differential. An automobile tire can explosively decompress, as can the air tank on an air compressore. Or they can both decompress withough an explosion as happens every time I open the petcock on the bottom of my air compressor when I am finnished with it. Or when I let the air out of a tire.

Substances in a liquid or solid state experience cohesion. If I remember correctly, cohesion is caused by residual electromagnetic charges, ie the protons from one atom are attracted to the electrons of another atom, etc. It’s enough to hold the atoms close together unless the atoms have enough heat energy (which simply causes them to shake) in which case they bounce away from the other atoms. Solids are held together in a particular lattice, while the atoms in liquids can move amongst each other but not get too far apart. However, if you lower the pressure exerted on a substance, less heat will be required for the atoms to break free.

Although “the liquids boiling” in space argument seems kind of silly, you’ll be losing more fluid, sure, but the reason the fluids in your eyes don’t just all leak out due to gravity (a grave concern for those born in space, I suppose) is due to cohesion and possibly surface tension. It can overcome gravity so I’m going to need a cite before I believe your eyes will get damaged by exposure to 0 ATM. You might need some eye drops.

As for 1 ATM inside your lungs, I’m thinking there’s some error with your reasoning snailboy. What you are describing can be simulated by diving to about 33ft of water, taking a breath out of a ambient pressure air source like a SCUBA tank with a regulator, and then suddenly ascending to the surface while holding your breath. I don’t think you’ll be able to hold your breath, but if your nose/mouth were sealed shut I seriously doubt you would explode into pieces. It just… feels wrong. Anybody have any definitive cites on this?

What will happen if you take a breath at 33’ (or even 10’ at the bottom of a pool) See my link in poist #10.

Would you explode?
Would you do serious damage to your lungs and risk death?

I wasn’t implying otherwise, although does the damage occur before you are pretty much forced to exhale? I would imagine if we don’t have enough muscle strength to draw in 1 ATM air at 2 ATM ambient we most definitely do not have the muscle strength to hold 1 ATM in 0 ATM (or 2 ATM in 1 ATM). What’s going to stop the air from rushing out? Your lips? The larynx? The soft palate? Is there some valve I’m forgetting about?

That’s some excellent reasoning! In some sense you are even right in your conclusion. Atoms in a liquid do decompress (sometimes explosively!) when put into contact with air. This is why a glass of water evaporates. The tendency of liquids/solids to decompress is measured as vapor partial pressure. (In a closed container where there is a given number of atoms per volume, the atoms which are in the form of a gas smash into the liquid/solid and get stuck at precisely as often as other atoms decompress/evaporate. That number of atoms per volume is translatable to “vapor partial pressure”) The higher the vapor partial pressure, the more unstable the liquid/solid is in terms of wanting to decompress. This substance will evaporate/sublime more quickly.

However, there also exist attractive forces between molecules (I won’t discuss metals and metal/nonmetal ionic salts). Without them, the liquid and solid states would not exist. Attraction is, too, caused my electromagnetism. More specifically, by the uneven distribution of electrons around a molecule. Uneven across the atoms averaged through time (eg, water is a “polar” molecule) but also from moment to moment (since electrons are mobile). Depending on the nature of the substance and the uneveness, the molecules statistically orient S-N-S-N- more often than would be guessed from a purely random model. A pile of magnets, after all, will tend to orient a bit like this. There are other factors as well, including mechanical and electromagnetic ones. In the end, you get cohesion.

Cohesion keeps molecules from flying away on the surface of the material, but also prevents the material from being ripped apart internally by gas (ie, boiling). Getting ripped apart I’d consider explosive decompression. It will happen to water at the temperature of the body if it’s not surrounded by air pushing down on it with 1 atm. In fact, that is one of the scenarios for what’ll happen to your blood in space. However, experience with animals and humans does not seem to have born this out.

For a liquid to explosively decompress, you have to consider the effective outward force of the decompressing molecules (ie surrounding pressure - vapor partial pressure at some specific temperature). However, you have to remember the very strong cohesion that keeps molecules together. Nevertheless, depending on the substance, there will always be some supercritical temperature at which point the liquid will get ripped apart even if structural flaws (microbubbles) do not already in it exit.

You would almost certainly burst some surface capillaries (similar to being punched in the eye) and the protective layer of watery mucus would mostly evaporate if the eyelids were open. More importantly, in a near vacuum your eyes would freeze via evaporative cooling and convection unless there was sufficient radiation to maintain non-freezing temperature in the eye. For a short duration of exposure I suspect it would be survivable with only minor and temporary damage. The eyes wouldn’t burst from a 1ata pressure difference any more than they explode when someone strikes the eye (which certainly generates more than 15psi difference).

You wouldn’t “explode into pieces”, but as Rick notes, there is a significant chance of barotrauma. This can happen with as little a depth change in water of 3 feet, or a pressure delta of roughly 1psi. This is why constant breathing is strongly emphasized in diver training; as long as the diaphram is relaxed and the trachea is open the excess pressure can exhaust from the lungs, but if compressed air is inhaled and breath held during an ascent (intentional or otherwise) great badness can result. I have personally witnessed this, and a diver coming out of the water vomiting blood is not a pretty sight. This is why the correct ascent rate is also limited to 60 ft/min (20m/min), and slower ascents (along with ascent staging/safety stops) are strongly recommended by DAN and GUE.

Back to our exploding man in the OP, again with the expection of sinus cavities and the aforementioned barotrauma, I wouldn’t expect any tissues to rupture explosively. Brief exposure to near vacuum, especially if the loss of pressure wasn’t instantaneous, should be survivable for at least a brief period of time with minimal permanent effects.

BTW, just in case it hasn’t been made clear, atoms and molecules are held together by electrostatic attraction between atoms based upon sharing of electrons (ionic, covalent, and hydrogen bonds). At these levels and distances, gravity is so incredibly weak that it doesn’t even figure into calcuation. Gravity is only so noticable because it acts upon things at the macro level in which we live, i.e. causing an apple to fall upon the head of a certain scholar causing him to suddenly realize the obvious, or making the planets orbit the Sun neatly instead of flying off into space. Gravity has nothing to do with keeping your body parts all attached, though. That’s strictly chemistry.


I had an interesting demonstration of the effect of a vacuum on the body when my son was a baby. He had a rubber clown’s head mounted on a spring with a suction cup at the bottom. You stuck it to the floor and he could bat it around and have fun.

On night I stuck the suction cup on my forehead and was waving the clown around for his amusement. When finished I reached up and pulled the suction cup straight off, instead of gently peeling it off from the edge.

I wound up with a large red blotch of well burst capillaries that took a couple of weeks to disappear.

Yeah, and quite the dabbler in the Cocoa Beach drug world. Go to Amazon and find " Maryjane Tonight At Angels Twelve". He knew the dealers AND the cops chasing them. My father knew Martin Caidin during the heyday of NASA and Gemini/Apollo shots in Cocoa Beach/Cape Canaveral.

He was a pilot, but that didn’t preclude bending the facts to fit his fiction. Not hardly a unique thing to do…