Airliner Pressurization

My good friend mentioned a recent incident where he, and some other passengers in an airliner, began "gulping for air" during their flight. He asserted that the airline was recirculating the air as a cost cutting measure, and wasn't providing enough oxygen for everyone to be able to breathe normally. We discussed airliner pressurization systems at some length, and I referenced some articles on Boeing 737 and 767 airliner pressurization, but still he wondered why they didn't provide oxygen masks for everyone, since his experience in the USAF taught him that supplemental oxygen was required above 10,000 feet.

This oxygen question has always been a subject of interest to me, especially since it so directly affects private pilots. When a pilot friend of mine showed me his miniature Nonin pulse oximeter, I realized I "had to have one", and aside from being helpful on long trips over the mountains, it's been invaluable with monitoring K's O2 levels over the past few years.

He's right that oxygen is the key concern, but of course there's oxygen in the air, and if you get enough in the air you're breathing, you're fine without a bottle and mask. The percent oxygen is about 21 in Earth's atmosphere, which works out to a partial pressure of about 3 PSI at sea level. Partial Pressure of oxygen is what the lungs use, and a PP of 3 results in a blood oxygen saturation of about 97% in most people. The experts say most people need about 87% to function normally. Up here in the mile-high city, I generally see my O2 saturation at about 95-97%.

At 10,000 ft, the atmospheric pressure is only 60% of sea level pressure. That results in an oxygen PP of about 1.8, and a blood saturation of about 87%, which is enough for most people to function normally, according to the experts. The USAF and others recommend going on supplemental oxygen at 10,000 feet, in an unpressurized plane, because it's the conservative thing to do. USAF pilots also have to worry about someone shooting a hole in their pressurized aircraft, so they always use bottled oxygen as a backup. Just for the record, the FAA doesn't require supplemental oxygen for private pilots in unpressurized aircraft until 14,000 feet, and oxygen doesn't have to be provided for passengers until 15,000 feet.

About the "gulping for air" symptom. When I lived in San Diego, I attended a course in "Physiological Training for Civilian Pilots" at Miramar Naval Air Station. One of the things we did, was sit in an "altitude chamber" while they ran the pressure down until it was the equivalent of 24,000 feet, and let us experience that high altitude without supplemental oxygen. There was a safety officer in the chamber with us, and his job was to force us back onto oxygen if we didn't do it when instructed. At that very high (simulated) altitude, I did notice some subtle symptomatic clues that hypoxia might be setting in, but they would have been easy to overlook, if I hadn't been watching for them. There was definitely no "gulping for air" symptom in any of the dozen people we had in the chamber. Then many years later, I was flying between Denver and Telluride several years ago, at 14,000 feet in an unpressurized Piper Turbo Arrow, and not using oxygen, although I had a bottle aboard just in case. I was carefully monitoring my blood oxygen with my portable oximeter. I saw my oxygen saturation level go as low as 77%, which is pretty low, but I never got short of breath or felt faint. I easily brought my oxygen level back up into the 90s, by inhaling deeply a few times, even though I didn't feel any need for it. There was no sense of a need to breathe differently, and definitely no "gulping" for air. All the articles I've read about pilots who fly higher than they should without supplemental oxygen, describe them as being unaware of their low oxygen situation. They just start acting stupid, then pass out after awhile. The Payne Stewart Learjet crash is a good example of this phenomena.

Modern day airliners are designed (and required by regulation) to be pressurized to an equivalent altitude of 8,000 feet or below, at their highest cruising altitude, without over stressing the pressure vessel that is the cabin. That leaves the partial pressure of oxygen at more than enough to sustain normal functioning, on all commercial flights, with no oxygen bottles involved. Airliners are also required to have an audible warning system in the cockpit, to warn when the cabin altitude exceeds 10,000 feet. Pilots are trained to act immediately, by descending the aircraft when that alarm goes off, although in practice it rarely does, partly because the systems don't fail very often, but mostly because the pilots are monitoring cabin pressure, and catch any anomalies before the alarm goes off.

I just couldn't accept the assertion that the airline was too cheap to fill the oxygen bottles, partly because I know there are no oxygen bottles involved, but mostly because the pressurization system is a mandatory item -- at the very least, it's required to keep the pilots awake.

I wondered if his shortness of breath might've been related to his asthma symptoms, combined with the likelihood that the cabin altitude was near 8,000 feet. I asked if he had difficulty breathing when he's in the mountains, and he said he does. Maybe the nearby passengers saw him having difficulty, and got nervous enough, that they started experiencing symptoms too. That kind of thing has happened before...

Or maybe the plane had a pressurization failure, and the cabin altitude alarm didn't go off, and the pilots didn't catch it. That's a lot of things to go wrong at the same time, but it's happened before too... in which case, they would have been dangerously close to having the pilots pass out along with the passengers. It's weird though, that more than one person experienced that gulping for air symptom. That's so unusual for that situation.