Hypoxia in Aviation

Carrie Steffen — Sat, 28 Feb 2026 18:14:03 GMT

Hypoxia in Aviation
Written by: John Mulvey MD, Senior AME

Humans are aerobic creatures. We require oxygen to convert our fuel to energy for all the tasks our body has to accomplish every minute of the day. Of course, this oxygen comes from the air we breathe, which contains 21% oxygen from Sea Level to Space; but as we ascend the atmospheric pressure drops. There is less atmosphere on top of us at higher altitude, so the column of air above us weighs less, and creates less pressure around us.

I won't quibble about parts of the body that can briefly operate without oxygen. When you make a demand of your muscles to lift the heavy bench press, they cannot get enough oxygen from the bloodstream for that instant demand; so, for several seconds the muscle works anaerobically, until the circulatory system catches up. But this is very short lived and it's what makes the muscle sore later.

Figure one shows how the atmospheric pressure changes with altitude. If you look at the slope of the curve it is steepest between sea level and 5,000 feet. The pressure continues to drop but not as rapidly as this initial ascent. Converting meters to feet, 2,000 meters is 6560 feet.

Note how the slope of this line is steepest from 0 to 2000 meters. This is the area that MANY of us fly in. Projecting to 4000 meters, this would be 13,120 feet. At this altitude you only have 60 percent of the air pressure you had at sea level. By 2000 meters you have dropped to 80% of the pressure available. This translates to 80% and 60% of the air available. Now let's look at how the body transports oxygen in the body:

The Red Blood Cell is a truck that carries Oxygen. It's basically a bag of hemoglobin, which is an amazing molecule. It is engineered to hold on to oxygen very avidly when oxygen is plentiful, but to release it when it's scarce. This makes beautiful sense, as you WANT the hemoglobin to hold on tight to Oxygen in the lung, where it's supposed to pick it up; but to kick Oxygen out when there is little oxygen around, such as in the tissues where it's SUPPOSED to let it go. The curve below shows how this works, the Oxyhemglobin dissociation curve: (just pay attention to the black line; the colored lines reference what happens with changes in the system).

PaO2 is the partial pressure of Oxygen, across the X axis at the bottom, and Oxyhemoglobin Saturation show what percent of your Red Blood cells are leaving the lung carrying a load of oxygen. Barometric pressure at sea level on a standard day is 760 mmHg. If 21% of this is Oxygen; the math tells you that the Partial Pressure of Oxygen at sea level would be 760x0.21= 160 mmHg. Yes, 160 is not on this graph; it only goes up to 100 but you see how flat the line is at that point. The partial pressure of oxygen in the Alveoli of your lungs is about 100. As partial pressure of Oxygen decreases (moving right to left), the curve is pretty flat until you get to about 70 or so, at which point the curve inflects downward rapidly. This is beauty in biology; as the partial pressure of oxygen in most healthy lungs is around 100. In the tissue capillaries where the blood is feeding oxygen to the cells, the Partial pressure of Oxygen is about 70 and the hemoglobin is much more willing to release the oxygen to the tissues. Marvelous, isn't it? So, with oxygen being at 21% all the way to upper limit of the atmosphere, what's the problem with altitude?

As the first chart demonstrates, the barometric pressure drops with altitude. By 6500 feet we only have 80% of what we had at sea level, meaning that 760 mmHg at sea level is now 608 mmHg. If 21% of that is Oxygen; the PaO2 is now down to 128 from the 160 it was at sea level. Extending this to the alveolus in your lung where it's normally 100 or so; it's now about 70. Look where that puts you on the Oxygen Hemoglobin dissociation curve! You're already at the “knee” where the Hemoglobin is starting to get disinterested in picking up oxygen!

Most of us carry pulse oximeters in the airplane. (Wait, you don't? Well, you should. They are cheap and very easy to use.) This device measures the Oxyhemoglobin Saturation; how many “trucks” are leaving your lung carrying Oxygen. Normal is around 95-100%. As you climb without supplemental oxygen the Saturation does not really change much at first. That's thanks to the flat portion of the dissociation curve on the right of the graph. As PaO2 drops most of the trucks can still pick up oxygen. At 5000 feet you may still have a saturation of 90-94%, which is just a little low, but you are also entering the steeper part of the curve and as you continue flight you will often see that saturation start to drop over time. (At least that has been my experience on several flights.)

“But I feel fine! “Do you? The effects of low oxygen are subtle and come on gradually. I routinely fly a little over 2 hours to Boston to visit my daughter’s family. I was always a little tired when I got on the ground after a flight between 7500 and 8500 feet. I no longer am tired when I get on the ground if I use supplemental oxygen. My oximeter showed that without the Oxygen I was hovering around 85-90% saturated; with supplemental I maintain a normal saturation.

The most remarkable effect of hypoxia is on your night vision. The Brain is VERY demanding of oxygen and tolerates drops in saturation very poorly; hence my fatigue at the end of a Boston run. Your Optic Nerve and the Retina is an extension of your brain, and it also tolerates hypoxia very poorly, and is best demonstrated with a night flight. A couple of decades ago I was flying right seat in a friend's Bonanza. It was about 2030 hrs and we had been airborne for about 2 hours; so, we were dark adapted and stabilized at that altitude without oxygen. The pilot got clearance to go to 10,000 feet. He decided to break out the oxygen at the start of the climb, so he turned the aircraft over to me while he set it up. I distinctly remember squinting at the instruments across the airplane from me and how hard they were to see. Then he turned on my Oxygen, and it was like he had snapped on a light. One breath and I could see SO much better.

Oxygen systems are not that expensive considering the utility they offer. Any flight beyond just local sightseeing I am wearing oxygen on a boom cannula. It is set to start supplying oxygen above 5,000 feet. Not only am I far less tired after a few hours of flying at altitude, but I also have opened up the upper altitudes my Mooney can reach if I must climb over weather. I don't often fly at night, but I know my vision is SO much better in the waning light of twilight.

There are also possible deleterious effects of hypoxia outside the duration of flight. Research in Obstructive sleep apnea, where patients experience short bursts of hypoxia several times a night, demonstrates increased inflammation in their blood vessels, more hypertension, and accelerated progression of coronary artery disease that reverse when the sleep apnea is corrected. Research in mountaineers demonstrates prolonged brain dysfunctions up to a year from their last climb.

So, my fellow aviators, I would encourage all of you to make sure you have enough Os to keep your brain and vascular tree happy. You can just fly low or get an oxygen system, so you don't have to worry about it. Now go out and fly!

Flying Physicians Association Safety First !

Hypoxia in Aviation