The Air Up There

May 9, 2006 — -- We humans have a lot in common with fish.

We are, for instance, both carbon-based life forms who share a largely identical DNA, and we both have to be totally immersed in the medium from which we get our oxygen in order to stay alive. Our respective oxygen-gathering equipment is markedly different, of course, in that fish collect oxygen by using a dynamic flow of oxygenated water filtered through their gills while we humans use air pressure to force oxygen from air through the membranes of our lungs.

The point here is that when we climb to higher altitudes in an unpressurized airplane and the atmospheric pressure goes down, our ability to force the oxygen into our bloodstream correspondingly goes down. If we climb too high, not enough oxygen will make it into our bloodstreams, resulting in wooziness, unconsciousness and eventually death.

If you fly at all, you need to have a solid grasp of this altitude-oxygen-atmospheric pressure thing.

These are the bedrock basics for unpressurized flight for a normal, healthy, nonsmoking human with no alcohol in the bloodstream (alcohol impedes oxygen transfer):

Sea level to 10,000 feet cabin altitude -- no problem.

Above 10,000 feet cabin altitude -- most of us will go unconscious above 15,000 to 20,000 feet.

Above 10,000 feet, with the yellow emergency oxygen mask the airlines provide (or any supplemental oxygen supply not provided under pressure) -- no problem.

Above 28,000 to 30,000 feet using only the extra oxygen from the yellow mask -- you won't have enough oxygen transferring to your bloodstream and if the condition is sustained for too long, unconsciousness and death will result.

Above 28,000 to 30,000 feet with extra oxygen under pressure -- normal consciousness and life can be sustained to 50,000 feet.

Above 50,000 feet with any form of oxygen -- sustained human life is not possible without a pressure suit like astronauts wear.

Why the difference between oxygen under pressure and the little yellow masks?

Well, our lungs contain millions of tiny sacs called alveoli, essentially tiny membranes that allow oxygen to come in and dissolve in the hemoglobin of the blood. The bloodstream in turn carries the dissolved oxygen molecules throughout the body. The gas-to-blood transfer, however, requires a lot of air pressure, which is the whole point of a discussion like this popping up in a column about things aerodynamic.

Ever wonder why the first thing paramedics seem to do when someone is having a suspected coronary is slap an oxygen mask on their face? I mean, if the patient is at or near sea level, there's plenty of oxygen about, isn't there?

Yes, but only 21 percent of plain air at sea level atmospheric pressure is helping to push the oxygen into the patient's lungs (only 21 percent of air is oxygen). With that medical mask, 100 percent of what the patient is breathing is oxygen and thus all the atmospheric pressure is available to help oxygenate the blood, and that makes a real difference.

We need a blood oxygen level of between 87 percent and 97 percent to maintain consciousness by maintaining the oxygen-to-hemoglobin flow. At 10,000 feet above sea level, the normal saturation for a human breathing regular air is 87 percent. Go to 18,000 feet without supplemental oxygen and the saturation drops to 80 percent (thanks to the partial pressure of oxygen being just 21 percent of the atmospheric pressure at any altitude). That means we're going to start getting woozy and hypoxic, and unless we've added substantial hemoglobin to our bloodstreams by living at very high altitudes, we'll eventually black out.

There is an altitude range, however, where even breathing 100 percent oxygen (with 100 percent partial oxygen pressure) from a little yellow mask in a jetliner won't provide enough life-sustaining oxygen saturation in the bloodstream. That point is around 28,000 to 30,000 feet. Above that, there isn't enough oxygen pressure even when breathing pure oxygen to shove the O2 molecules across the membranes and into the hemoglobin.

The cure is pressure breathing, which you can't do with yellow supplemental masks. Pressure breathing oxygen masks (the type you'll find in the cockpit) force pure oxygen into your lungs at a higher pressure than the surrounding air and keep your blood oxygen saturation level above 87 percent. But the techniques for pressure breathing require practice in an altitude chamber, and are so foreign to our normal methods of respiration that few passengers would be able to cope with it in an emergency even if we provided such masks in the back. Your pilots, however, are well-trained, and have pressure-breathing oxygen masks beside them at all times.

What if you're at 39,000 feet and the airplane depressurizes and the yellow masks drop?

You have nine to 15 seconds to place the mask over your own face before you lost consciousness. That's why it would be so vital for you to immediately put on your own mask first. Now, in the cockpit immediately after an emergency decompression (depressurization), the pilots will instantly don their pressure breathing masks and begin an emergency descent to lower altitude. Even at 40,000 feet, the time during which a passenger would be exposed to a cabin altitude higher than the yellow emergency masks could deal with is around 90 seconds. In other words, you will probably black out on the way down from 40,000 during the emergency descent without a pressure breathing mask. But, with the yellow supplemental mask in place, you'll regain consciousness with no permanent bad effects as the aircraft descends through 28,000 feet. If you fail to get that mask on before blacking out, you won't wake up until the aircraft has descended to around 10,000 feet. That's why those briefings are so important!