I know that airliners have pressure sensors to check the cabin pressure, and thus check if we can breathe (and get enough O2) inside it so we don't get hypoxia. But that got me thinking:

  • Is there any oxygen sensor to check if there is enough O2 inside the cabin?
  • Is there any situation (emergency or not) in which some other gas could pressurize the cabin and lower the O2 percentage inside it?
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    $\begingroup$ you'll need other sensors to make sure the air is breathable. CO2 for instance becomes a major problem long before it's replaced all the oxygen in air. $\endgroup$ – Hobbes Dec 1 '17 at 19:43
  • $\begingroup$ You cannot inject air without the normal ratio of oxygen in the cabin (as air comes from the outside), then, if we except fire and decompression, the only possibility to have non breathable air is by not eliminating other pollutants, then @Hobbes is right, it's not oxygen that must be measured. $\endgroup$ – mins Dec 1 '17 at 21:18
  • $\begingroup$ There’s a recent incident on AvHerald potentially caused by air quality issues $\endgroup$ – Notts90 supports Monica Dec 1 '17 at 22:16
  • $\begingroup$ Why did I just think of canaries in a coal mine? $\endgroup$ – PerlDuck Feb 15 at 8:15

The air conditioning system on board airliners is an open system: it scoops up air from the atmosphere, compresses it, expels it through the cabin vents, and then outflows it through discharge valves. In modern airliners there is a certain amount of re-circulation: the expelled air was of such good quality that it seemed a waste to just dump it in the atmosphere.

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In the A320 family of airliners:

  • Outside air is compressed in the engine compressors, part of it is tapped of as bleed air: already compressed and warmed, it is used for refreshing the cabin air (marked green).
  • Temperature and pressure regulators then provide automatic mixture control for the amount of outflow: the outflow valve is regulated, decreasing the opening results in increase of cabin pressure and decrease of flow.
  • The recirculation fans regulate how much air is re-used, after being filtered through HEPA filters which remove pathogens and other nasty stuff.

The whole cabin air volume is refreshed every 2 - 3 minutes, as also referenced in this answer: with such a frequent refresh rate, it would be pretty safe to assume that there will always be roughly the percentage of oxygen in the cabin as there is in the outside atmosphere (see below for the actual percentage!) . People have climbed to the summit of Mount Everest without the help of oxygen masks, and that summit is about at cruising altitude of an airliner. AFAIK, the aircon system does not have oxygen sensors, but it does have temperature and pressure sensors which are of more immediate concern.

Low concentration toxic gases are much more dangerous than a drop in oxygen concentration. In fact, the human body makes use of this: when holding your breath, the urge of breathing fresh air again after a while comes from the increase in CO$_2$ in the body, not from any decrease in oxygen. So CO$_2$ detectors on board of aircraft would make sense, as would detectors for the very lethal carbon monoxide. These are available, even in the form of a key hanger for GA pilots (not including any links due to them being adverts).

There have been reports of dizziness and fainting on board of airliners, and investigations into these. A sample of various flights did not reveal a particularly high carbon dioxide content:


With additional measurement reports pending, I found in-flight cabin air quality measurements of oxygen to be relatively stable, ranging from 11.2% to 12.5%.

Typical outdoor CO2 levels are between 350-400 ppm (0.035% - 0.04%) or up to 500 ppm by some sources.

Carbon dioxide levels measured in-flight in the aircraft cabin ranged between 0.04% or 400 ppm and 0.1% or 1000 ppm to date in our studies and were measured at close to 0.5% or 5,000 ppm in earlier studies.

As indicated at CO2 HEALTH EFFECTS, occupants are unlikely to be affected or to notice CO2 levels under 2% or 20,000 ppm - a far higher number than in-flight aircraft cabin carbon dioxide levels.

Notice that the oxygen contents on board is a bit over half of that of the outside air - the cause of dizzy passengers? Reports of dizziness do keep appearing, and nothing beats direct detection of noxious gases by a sensor. Problem is, there are so many of them. Oxygen, carbon monoxide and carbon dioxide sensors would be good candidates for inclusion into the aircon systems.

On your second part of the question

Is there any situation (emergency or not) in which some other gas could pressurize the cabin and lower the O2 percentage inside it?

Yes, in air conditioning systems that use bleed air there definitely is, an engine malfunction can for instance inject gases from burning oil into the cabin air, a problem that was reported a few times for the BAE 146. From this report:

The British Aerospace BAe 146 was the aircraft type most commonly involved in fumes/smoke events when taking into account flying activity. The Airbus A380, Boeing 767, Embraer EMB-120 and E-190 were among other aircraft types that also had a higher than average rate of fumes/smoke occurrences over the period.

The most common source of fumes/smoke was aircraft systems issues, primarily relating to failure or malfunction of electrical and auxiliary power unit (APU) systems. Equipment and furnishings also featured highly as a source of fumes and smoke. Within this category, air conditioning and galley equipment were the most common sources of fumes/smoke. External sources of fumes/smoke and cargo/baggage related events were relatively rare.

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    $\begingroup$ "People have climbed to the summit of Mount Everest without the help of oxygen masks, and that summit is about at cruising altitude of an airliner". I would suggest you delete that part. Some very prepared people have survived climbing Everest without oxygen; many have died, and a percentage of people die of altitude sickness even at heights 75% of Everest. Also, airliners fly 25% higher than Everest. And, in any case, all airliners pressurize to about 2000 meters, so the point is moot. $\endgroup$ – Martin Argerami Dec 2 '17 at 13:16
  • $\begingroup$ Also the people who attempt Everest, with or without oxygen, spend weeks at base camp acclimatizing, and the slow yak-trail trek to base camp is its own acclimatization. Base camp is at 17,600 feet. It may not be humanly possible to skill up to being able to breathe at MSL and then 40,000 feet an hour later. $\endgroup$ – Harper - Reinstate Monica Dec 2 '17 at 19:06
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    $\begingroup$ @MartinArgerami The reference is included as an Order Of Magnitude check for assessing if we need to do anything additional before compressing it for cabin air. The proof is in the summit. Nowhere did I suggest that it would be a good idea to expose Aunty Mable to unaltered cruise altitude air. $\endgroup$ – Koyovis Dec 3 '17 at 8:22

While it would not significantly affect the O₂ percentage one thing that must be tabulated on a cargo aircraft is the amount of dry ice loaded. Dry ice is very common in packaging to keep perishable shipments cool during transport. Since dry ice sublimates into gaseous CO₂ as it warms there is a limit to how much can be loaded into an aircraft. If there is too much there's a possibility that the pilots could be overcome by gas trapped inside the plane. It's a rather insidious threat because you can't smell it so you would only realize something was wrong when you started feeling physiological effects.

An O₂ sensor would not help since pilots would be overcome long before it made any detectable difference in O₂ levels. AFAIK there are no sensors for CO₂ either. The amount of dry ice loaded is carefully tabulated and pilots who fly cargo containing dry ice are aware of the possible hazard and how to recognize symptoms of CO₂ poisoning.

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    $\begingroup$ CO₂'s effect on humans is not just that it displaces oxygen: our lungs can only move CO₂ from our blood to the air if the partial pressure of CO₂ in the air is quite low. Fun fact: CO₂ build-up in the bloodstream triggers the breathing reflex (not actual lack of oxygen). High CO₂ is a problem even if there's also plenty of O₂ in the air. See also en.wikipedia.org/wiki/Hypercapnia#Tolerance for the physiological effects of increasing CO₂ concentrations, and how long until you're incapacitated. (Note this is CO₂, not the far more deadly carbon monoxide, in case anyone missed that.) $\endgroup$ – Peter Cordes Dec 2 '17 at 1:28

The system would be kind of superfluous, since the ambient environment around the airplane, and the cabin itself, is filled with air. The air in the atmosphere is a mixture of approximately 78% nitrogen and 19% oxygen by volume, so the known quantity of oxygen, and the partial pressure of oxygen, would be known, or relatively easy to calculate, if you knew what the cabin pressure itself was. As such there is no reason to put in an oxygen sensor within the cabin of an airliner.

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    $\begingroup$ While technically correct, there are (rare) situations in which air could become contaminated. See: aviation.stackexchange.com/questions/14664/… $\endgroup$ – egid Dec 1 '17 at 19:10
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    $\begingroup$ There are a number of ways the air could become contaminated, but those largely don't involve any significant difference in the concentration of oxygen in the air, so an oxygen sensor wouldn't measure them. $\endgroup$ – Zach Lipton Dec 2 '17 at 3:19

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