About thin air; Redux

Earlier in this Blog I published a piece about Thin Air. One of the cool things about writing in a public forum is that some really smart people end up reading your work and they help you to get it right when a concept seems off center. My earlier piece suggested O2 is the same wherever it's found, even at 20,000 feet. There is no such thing as "thin air." I know, I know. I've already used this term many times in my Blog and there has even been a best selling book entitled Into Thin Air. How is it possible such a basic and intuitive notion could be as manufactured as Jack Lord's hair piece? (see Into Thin Hair).

A reader named Rick commented; Dear David,
I beg to differ on your concept that "Thin Air" doesn't exist. I went to Webster's New Riverside University Dictionary (1988) which defined "thin" as {3. a. Not dense or concentrated: sparse (thin vegitation) b. More rarified than normal (thin air)}. I believe in Naoh Webster.

What you are really trying to explain is how the partial pressure of oxygen inside a fixed volume of air between low and high altitude affects people. When you inhale, your lungs create a fixed volume (about the size of a party balloon). Whether you inhale deep underwater (SCUBA divers), at sea level or at elevation (Mt. Elbrus), the lungs create this same fixed volume. The differential pressure between the air inside your lungs and the air outside your lungs must be equalized. Therefore the air is literally pushed into you from the outside pressure. What changes between altitudes is the pressure of the gas flowing in and out of you. Since pressure is determined by the number of gas molecules occupying the same physical space (Boyle's Law)there are fewer oxygen molecules occupying the same inhaled volume at higher elevations. The "partial pressure" of the oxygen in the air drops as you ascend, and it is the partial pressure of the oxygen that keeps us alive. Too much of it (deep underwater) and you can get CNSO2 (central nevrous system oxygen) toxicity. You pass out. Too little and you get anoxia. You pass out. As the altitude increases and the partial pressure of oxygen decreases we can do one of three things to survive: grow larger lungs, breathe faster, or "thicken the soup" by adding more oxygen to the mix.

What I was trying to say is that in the absence of differing pressure all oxygen would be the same. Oxygen itself is neither thin nor thick on it's own account. So one is well advised to focus on the illness (low barometric pressure) and not the symptom (thin oxygen volume).

Thanks, Rick. Great contribution, and thank you for taking the time!

This was just a sidebar to the piece. The bigger discussion focused on how that air (thick, thin, or otherwise) gets into your lungs and the importance of barometric pressure in forcing it in. Here is the rest of the article.

It is common understanding that we "draw a breath" in when inhaling. But the fact of the matter is that we draw in almost nothing. When we inhale we create an open space. It is the barometric pressure of our immediate environment which fills that space, forcing oxygen into our lungs. Less barometric pressure means less oxygen.

I always laugh during the pre-flight demonstrations of commercial airlines. What gets me is the part where they say "This airplane's cabin is pressurized for your comfort." This is true in as much as dying is uncomfortable. At the typical cruising altitude of 30,000 feet, having just come from sea level, a non-pressurized cabin would see most of its passengers expiring within fourty minutes.

Barometric pressure is greatest at sea level along the equator, diminishing as we move higher or further away from the equator. Thus any high altitude summit is going to have considerably less barometric pressure than what would be experienced at sea level in that region. As well, a given altitude 2,000 miles from the equator will have less barometric pressure than that same altitude only 1,000 miles from the equator. This becomes particularly important when we consider summits like Denali, 20,320ft high and far enough north to be next to the Arctic Circle. In terms of barometric pressure, Denali's summit is the rough equivalent of 23,000 ft on Mt Everest (much closer to the equator).

Barometric pressure will also fluctuate as weather conditions shift. The result is a change in the amount of oxygen we take in. Next time you hear the weather forecast talk about rising barometric pressure take note and see if you don't "just feel better" that day. Most people do. You are getting more oxygen.

Bottled oxygen is typically used by climbers above 23,000 feet on Everest. The pressure provided by the compression in the tank essentially imitates the barometric pressure of a lower altitude. But once they start using supplemental oxygen they must continue until descending below 23,000 feet. That adds up to a lot of bottled oxygen, which means a fair bit of extra weight to carry up hill. I am often asked why we don't take bottled oxygen along as a precaution on these climbs. It's a practical choice. A person can only pack so much weight. When you are already counting ounces for the gear you must have, there just isn't the spare load capacity for something very heavy that you probably won't need. Teams of Sherpas pack these bottles up hill on Everest. A suitable stockpile of oxygen is accumulated for each climber and ferried up to increasingly higher camps. As you might guess, the support logistics for an Everest attempt are truly monumental.

On Elbrus we will be well below the elevation that might typically require supplemental oxygen.