ISS Breathing Mix

[Noldi400]

Hi, all. I have a quick question if there's anyone around who knows the answer:

I know the ISS uses a oxygen-nitrogen mix for breathing air; is that in the usual atmospheric mix and pressure?

And why did they go to an O²/N² mix?  Are there known health problems from breathing a pure oxygen atmosphere long term?

I know health/medical matters are supposed to be my area, but I seem to have been absent the day they taught space medicine in Paramedic school.   

[BazBear]

The ISS uses a ~78% O²/21% N² atmosphere at about 14.7 psi. AFAIK the main reason to use that mixture is to lower the risk of fire, and the fact that the pure O² at about 5 psi made hearing speech for any distance (such as in Skylab) difficult due to the thin atmosphere. I can't find anything about any medical issues due to pure oxygen use being discovered during Skylab. Of course, after the Apollo 1 fire NASA no longer used 16 psi pure O² on the pad, instead slowly bleeding off pressure and increasing the O² ratio until it was at about 5 psi (the partial pressure of O² at seal level). I believe the reason they didn't use a 14.7 psi sea level mix in the upper atmosphere and in space was that the spacecraft weren't designed to handle that kind of pressure differential.

[Allan F]

The sea level mix would have made the use of spacesuits very difficult, because the astronaut would be subject to decompression sickness due to nitrogen dissolved in body fluids. The spacesuits used about the same pressure internally as the LM. It would also have added complexity to the spacesuits and space vehicles, because nitrogen would have to be supplied to re-pressurize after each EVA. The use of pure oxygen didn't offer all these difficulties, and the distances involved internally in the spacecraft did not pose a problem with regard to sound propagation in the low pressure.

[Nowhere Man]

Guys?  It's O₂ and N₂ with subscripts, not superscripts.

Fred

[Noldi400]

Guys?  It's O₂ and N₂ with subscripts, not superscripts.

Fred

Yeah, those square oxygen molecules hurt your throat, doncha know.
[/selffacepalm]

[Noldi400]

The sea level mix would have made the use of spacesuits very difficult, because the astronaut would be subject to decompression sickness due to nitrogen dissolved in body fluids. The spacesuits used about the same pressure internally as the LM. It would also have added complexity to the spacesuits and space vehicles, because nitrogen would have to be supplied to re-pressurize after each EVA. The use of pure oxygen didn't offer all these difficulties, and the distances involved internally in the spacecraft did not pose a problem with regard to sound propagation in the low pressure.

Right answer, wrong question. I'm asking whether there was a specific reason why the low-pressure O₂ atmosphere that was used through Apollo was replaced by a oxy/nitro mix.  Health reasons? Russian influence?

[Allan F]

Then all I got is crew comfort. IIRC the russians spacecrafts have mostly been full of surface mix. Maybe some structual rigidity benefits?

[BazBear]

Guys?  It's O₂ and N₂ with subscripts, not superscripts.

Fred

Whoops!

[BazBear]

Guys?  It's O₂ and N₂ with subscripts, not superscripts.

Fred

Yeah, those square oxygen molecules hurt your throat, doncha know.
[/selffacepalm]

Yes, they would, wouldn't they?

[Donnie B.]

The ISS uses a ~78% O²/21% N² atmosphere at about 14.7 psi.

Are you sure this isn't the other way around?  If not, why would they use such a highly O₂ enriched mix at 1 atmosphere pressure?

(the partial pressure of O² at seal level).

I believe seal level is a bit below sea level, at least during certain parts of the year

[BazBear]

The ISS uses a ~78% O²/21% N² atmosphere at about 14.7 psi.

Are you sure this isn't the other way around?  If not, why would they use such a highly O₂ enriched mix at 1 atmosphere pressure?

(the partial pressure of O² at seal level).

I believe seal level is a bit below sea level, at least during certain parts of the year

Yes, the other way around, doh!*face palm* Guesss I shoouda prooof reaad. 

[Chew]

The ISS uses a ~78% O²/21% N² atmosphere at about 14.7 psi.

Are you sure this isn't the other way around?  If not, why would they use such a highly O₂ enriched mix at 1 atmosphere pressure?

(the partial pressure of O² at seal level).

I believe seal level is a bit below sea level, at least during certain parts of the year

Yes, the other way around, doh!*face palm* Guesss I shoouda prooof reaad. 

The other way around? Now you're saying seals can fly?

[ka9q]

I got interested in this a while back so I did a fair bit of reading. Here's what I learned.

There are three main advantages to air at sea level pressure over reduced pressure O₂:

1. Reduced fire hazards.
2. No health issues
3. The Russians were already using it.

Actually, the shuttle was designed to use sea level air back when the notion of a joint US-Russian mission was still a joke. So air was a natural for the station even if the Russians hadn't joined us. Had we used anything else, incompatibility with the Russian atmosphere would have been a big problem. The different atmospheres was probably the biggest single technical problem that had to be overcome in ASTP.

Although the breathability of a cabin atmosphere depends mainly on the O₂ partial pressure, the fire hazard depends more on the O₂ percentage, regardless of the total pressure. A diluent gas reduces the fire hazard by carrying away heat. I've seen some films from Sealab showing what happens when they try to strike a match. Basically, nothing happens.

Everybody knows about the intensive fire safety campaign conducted on Apollo after the Apollo 1 fire. The hazard was certainly greatest before launch when the cabin was at (or somewhat above) sea level pressure, but it was still present in space at 5 psi cabin pressure. So everything that went into an Apollo cabin had to be carefully vetted for flammability in pure O₂. That would have been completely impractical for the shuttle and especially the ISS, both of which were much bigger than the Apollo cabins and would carry lots of scientific experiments built by many different people.

O₂ is very toxic at high partial pressures; it's a good example of too much of a good thing. Anything over 2-3 atm is likely to cause seizures; this has killed many divers. Although it seemed safe enough for the few weeks of an Apollo mission, nobody knew if 5 psi pure O₂ was safe for much longer periods, and tests seemed to show some mild physiological changes. So for this reason and also to reduce the fire hazard, Skylab used a compromise atmosphere of 74/26 O₂/N₂ that could still be acommodated with the Apollo CSM.

[Dalhousie]

It also reduces the experimental variables on the human organism, one of the main subjects of ISS research.  Not having the enhanced fire risk of O2 enriched air also means a much wider range of items can be used off the shelf.

[ka9q]

That's a very good point, though if the goal were to understand the human organism under the conditions of a long-term interplanetary space flight you'd want to use whatever system was most likely to be used for such a mission.

There are some definite drawbacks to sea-level air:

1. Extra complexity. You need two gas supplies and O₂ partial pressure sensors as well as total pressure sensors. On the ISS, O₂ is made by electrolyzing water (including that recovered from urine) but N₂ has to be sent up. You don't metabolize it, but you do have to replace that lost to leakage and through airlocks. (I suppose that N₂ could also be made from urine (specifically the urea) but I doubt that's being worked on at the moment...)

2. The cabin walls, windows and seals have to be much heavier. (I occasionally have nightmares about being on the ISS when it suddenly depressurizes...)

3. EVAs are a real pain. To avoid the bends you have to go through a tedious pure-O₂ prebreathe and decompression protocol. On Apollo, you could just put on your suit and go.

I'm not sure, but the fire hazard inside a Shuttle/Soyuz/ISS pressure suit might actually be greater than it would be in a reduced-pressure pure O₂ atmosphere.

I know that in technical diving it is common to breathe pure O₂ during your last decompression stop just below the surface to flush out the N₂ a little faster. (You can't breathe it any deeper than a few meters, as it would be toxic.) On the ISS, you could also speed up the process by breathing pure O₂ at 1 atm before you start lowering the pressure. Physiologically this would be tolerable for a few hours, but it would greatly worsen the consequences of a fire within the suit.