What are the necessary components of air to sustain human or mammalian life?

I know we need a certain partial pressure of oxygen in the air we breathe, and a tiny partial pressure of carbon dioxide. We’re most comfortable within a specific humidity range and near-0% humidity is bad for long-term health.

Is there anything else we or mammals in general need in the air to survive, long term? I was wondering if there might be some trace nutrient we normally don’t worry about because we need such minuscule amounts but we only get it through breathing.

Has there ever been any scientific studies done of animals or people breathing absolutely purified air that contains only O2 and CO2 and H2O?

I’ve never heard we need any CO2.

I think I’ve heard O2 is a bit rough on us without dilution, so maybe it would be an irritant long term. And I think He can replace N2 in deep diving, as it’s less a problem getting it dissolved in blood.

In addition to the things you mention, Ar is a small constituent of typical air, but it’s inert, as are several other constituents. Looks like the biggest non-noble gas is methane at 0.00018%, but it’s hard to see that mattering.

Interesting question, but I’m going to guess that we need O2, possibly something inert to dilute it, and for comfort sake we need some H2O, and that’s all.

You got me interested so I did some looking around about CO2. There is a problem with having too little CO2, but the problem happens when we do too good a job of getting rid of what we create inside ourselves, for example through hyperventilation. I still haven’t found any way we use or need or benefit from inhaled CO2.

Right. CO2 levels in the blood help regulate the ventilation rate, but that CO2 is produced internally. We don’t “need” CO2 in the air we breathe.

Breathing pure oxygen is OK only for relatively short periods. At high concentrations it becomes toxic, so it has to be diluted with another gas like nitrogen, argon, or (in deep-diving breathing mixes) helium.

I have never heard of any other gas requirements for mammals.

Don’t NASA astronauts breathe pure oxygen at low pressure?

There is no need for atmospheric carbon dioxide by mammals, or indeed any vertebrates; it is purely a waste product and the only concern is exceeding the maximum threshold of about 4% concentration at standard temperature and pressure (STP) atmosphere (0.5% OSHA Permissible Exposure Limit for an 8 hour exposure). Diatomic oxygen is required at a minimum partial pressure (ppO2) of about 1.7 psi for effective respiration, and should be at least 2.87 psi long term (NIOSH occupational safety guidelines), with a threshold of ppO2 7.35 psi resulting oxygen toxicity at sea level ambient pressure.

All of the other constituents of air aside are biologically inert filler from a respiration standpoint (aside from trace amounts of ozone, carbon monoxide, and other toxins), although all animals require nitrogenous compounds from nutritional sources. There is no specific hazard from breathing air with a relative humidity up to 95%, although obviously air that is saturated with water is denser and harder to breath, has a higher heat capacity, and interferes with cooling via perspiration, and high levels of relative humidity facilitate the growth of fungi and bacteria which can be hazardous. Very low humidity does cause various problems with mucociliary transport and irritation of the sinuses and mucosal surfaces.

There are aerospace medicine studies on breathing in pure oxygen atmospheres because this was done early in high altitude flight and spaceflight to reduce cabin pressurization requirements. The NASA Technical Reports Server has a wealth of papers on the topic although most are from the 'Sixties in support of the Mercury/Gemini/Apollo spaceflight programs. Spaceflight programs since then have used combined oxygen/nitrogen atmospheres because of the flammability hazard of pure oxygen, although pure oxygen is used in prebreathing in preparation for extravehicular activities because pressure suits operate at a significantly reduced internal pressure.


The ISS is maintained at something close to sea-level atmospheric pressure, with something close to a normal atmospheric blend of gases. This was also true of the space shuttle.

When performing an EVA, an astronaut’s suit is pressurized to only 4.5 psi, which helps with dexterity (easier to flex the suit’s joints). This is equivalent to the pressure on top of Mount Everest, and yes they use pure O2 in this setting.

They used to. The Apollo program used such a system. But as mentioned later systems such as Soyuz, Skylab, the Space Shuttle, and the ISS have used an oxygen-nitrogen mix close to the composition of air.

There’s also the problem with nitrogen narcosis at extreme pressure. Too much oxygen partial pressure can be lethal too, so air under pressure to maintain a safe mix might be configured with extra nitrogen - but too much of that is also a problem. IIRC some extreme deep sea diving (oil rigs, undersea cables) helium is added instead to allow the diver to breath highly compressed air without serious side effects.

And the obvious side effect of CO2 under pressure is the bends, from when the pressure is released too soon and bubbles form in the blood stream.

From the first paragraph I think you know this, but for the record it’s nitrogen bubbles that cause the bends.

Air loaded with H2O will be lighter, won’t it? Water in the gas phase is only a little over half as dense as the nitrogen and oxygen it’s displacing. And why would it be harder to breathe? My humidified CPAP makes it easier to breathe.

NASA used 100% pure oxygen until the Apollo 1 fire showed how dangerous it could be. Lots more info at: Wikipedia

When designing the Mercury spacecraft, NASA had considered using a nitrogen/oxygen mixture to reduce the fire risk near launch, but rejected it based on a number of considerations. First, a pure oxygen atmosphere is comfortably breathable by humans at five psi, greatly reducing the pressure load on the spacecraft in the vacuum of space. Second, …

As regards “long term”, note that Gemini 7 was in orbit for close to 2 weeks.

A denser medium is harder to inhale/exhale; more effort is required to get that greater mass moving in/out through the airway. However, as you note, water-saturated air has lower density than dry air, so it’ll be a smidge easier to inhale, albeit with a slightly lower O2 partial pressure. This is all kind of academic: the difference in density of 0%RH air and 100%RH air at room temperature is very small (see plot, this page).

CPAP is quite literally cramming air down your throat, which is why it makes it easier for you to inhale. The extra pressure helps keep your airway open - good for obstructive apnea - but I also have mild central sleep apnea (occasional inadequate breathing effort despite no obstruction), and it actually helps out with that too, making up for my lazy breathing effort.

Is concentration of oxygen actually a problem, or is it partial pressure of oxygen? I can’t see how diluting it (without displacing it) would make any difference to fires or metabolic processes.

Gases are weird, they aren’t liquids or solids. If you fill up a balloon with a liter of nitrogen at STP, then add the same volume of oxygen, the balloon will not inflate much, if at all.

Adding water vapor to a gas doesn’t increase or decrease its density. The only reason that it does in the chart that you cited is because as the temperature goes up, if the humidity stays the same, the amount of water vapor in the air becomes more than the amount of other gases, and it becomes the primary driver of the volume.

It would be like if you took that nitrogen filled balloon, and then started adding oxygen. It will not inflate until the partial pressure of oxygen exceeds that of nitrogen, then it will start inflating.

In the Gemini program, they were using 5PSI of pure oxygen, which is higher than what we have here on Earth. Not truly devastating, but a higher risk of fire.

The problem with Apollo 1 was that they were doing a pressure test, and so had it at 5 PSI over atmospheric, so nearly 20 PSI of pure oxygen, and yeah, that’s a pretty severe fire hazard.

Andy Weir’s novel Artemis is set in a lunar colony. In the course of the book, he mentions that the colony has a pure oxygen atmosphere with only twenty percent normal air pressure; basically, Earth’s atmosphere with all of the non-oxygen removed.

Weir’s usually pretty good with the hard science but I’ve always questioned whether this would be workable.

The helium has the additional benefit of allowing deep sea divers to talk to dolphins.

In some processes it definitely makes a big difference to add other gasses without removing the oxygen (that is, to increase the total pressure too). Where diffusion is involved, having another species in the way slows things way down. And even when there’s advection, the last teeny bit of the way is just diffusion.

The working fluid in a heat pipe is very much inhibited by the addition of a bit of other gas. I think heat transfer coefficients can be on the order of 10^5 at the interface if the working fluid is pure but can drop by orders of magnitude due to a minor share of something else (at least that’s the number I think I remember for this parameter).

If the oxygen is interacting at a surface in some way that consumes it, with no other gas present there can be a bulk flow toward the surface (and in the heat pipe example one of the flux limits derives from the local speed of sound for the gas phase in bulk flow). If there’s another constituent present, it piles up at the surface and creates a concentration gradient for the oxygen to fight through.

I don’t know if it’s a problem in health particularly, but generally speaking, there are ways adding even a small amount of something else greatly reduces gas availability and reaction rate.

A morbid trivia note I’ve heard is that when people are confined in a sealed environment it isn’t a lack of oxygen that kills them. They die from an overdose of carbon dioxide while there is still a breathable amount of oxygen.