At sea level, the column of gases piled above every square inch of the earth weighs approximately 15 pounds. At 18,000 feet, the pressure of this column of air is halved.
The pressure of the air (and the oxygen molecules we need to maintain visual, physical and psychological fitness) is one part of the hypoxia equation. The other, more variable part is the efficiency of each individual’s respiration system.
During respiration an air pressure differential pushes oxygen molecules from the alveoli in the lungs across cell walls where it bonds to the hemoglobin (oxygen-carrying molecules) in our blood. When we fly higher the natural reduction in atmospheric pressure drives less oxygen into our blood supply. These facts are immutable; the FAA bases their oxygen usage regulations on these facts. They work for most people.
Pilots are trained to compensate for this decrease in pressure by taking in more oxygen. They do so by taking deeper breaths and by using supplemental oxygen. Airplanes designed to fly at high altitudes are pressurized to compensate for the lower pressures aloft.
Oxygen use requirements are spelled out in FAR 91.211. Crewmembers are required to use supplemental oxygen when flying above 12,500 feet MSL for longer than 30 minutes, and continuous oxygen use is required when flying about 14,000 feet MSL. Yet I had never gotten near those altitudes on my flight to Waco.
I began to wonder if I should conduct my own hypoxia research. Pilots are repeatedly warned to set down an ironclad list of personal safety minimums with regards to personal issues such as degree of fatigue and feelings of anxiety, and operational issues such as fuel minimums. Did my saturation of peripheral oxygen (SpO2) levels vary from the norm? Was I more susceptible to hypoxia than the average pilot? If so, at what altitude did I need to start using supplemental oxygen?
Even When I Was on the Bottle!
I was comfortably stretched out in the back seat of John Deakin’s turbo normalized Beech Bonanza as we headed east toward Ada, Okla. for a weekend at GAMI’s Advanced Pilot Seminar. We were level at 16,500 feet MSL. I had been breathing oxygen since we passed through 13,000 feet.
Deakin passed back a small device and told me to clip it on my finger. In a few seconds, two numbers appeared: 79 showed opposite the “%SpO2” icon and 81 opposite the “♡” icon.
When I told John, he said, “The 79 means you’re seriously hypoxic. It’s a good thing I’m flying.” I had oxygen flow valve set at the proper flow for our altitude—and I was still hypoxic. Oy!
Blood Oxygen Saturation Levels
After getting back home, I bought my own oximeter because by then I knew that if I was going to continue to fly safely I needed to amend my oxygen use minimums.
Normal blood oxygen saturation levels are 96 to 100 percent at sea level, and steadily decrease when altitude increases. A blood oxygen saturation level of 93 percent is considered by most medical people to be the lower limit for normal functioning. (See the table above for some information which may surprise you. —Ed.)
Blood oxygen saturation levels
Sea Level 96 to 100 percent
5,000 feet 93 to 95 percent
7,500 feet 93 to 90 percent
10,000 feet 92 to 88 percent
12,500 feet 87 to 83 percent
14,000 feet 83 to 77 percent
A blood oxygen saturation level of 93 percent
is considered the lower limit for normal functioning.
Dr. Brent Blue, a pilot and the owner of AeroMedix.com and AeroMedixRX.com, says that whenever a pilot’s blood oxygen saturation number is more than 10 points below their level at their home airport, they are compromised.
The first test my new oximeter was done at 6,500 feet MSL. I averaged five readings taken over a 20-minute period and got %SpO2 readings of 88 with heart rates of 75. Repeating the tests later that day produced the same saturation levels. I was below the range of normal functioning even though I was 3,500 feet below the altitude where these saturation levels normally occur.
Last summer I accompanied two friends in a hike into a series of high lakes located west of Lone Pine, Calif. in the eastern Sierra Nevada Mountains. We hiked for over six hours at 10,000 to 11,500 feet MSL. My saturation levels were between 75 and 80 percent. I felt like a zombie hiking out—it seemed that I was doomed to always trudge up still another hill and around another turn. I was seriously hypoxic. Yet at sea level, my saturation levels are excellent.
Symptoms of Hypoxia
The first symptom of hypoxia is a feeling of comfort akin to sitting at home in a favorite chair in front of a warm fire. This catch-22 is caused by the fact that our brains use around 30 percent of the oxygen circulated in our red blood cells and any oxygen deprivation chips away at our brain’s ability to function. The less competent we get, the better we feel.
Linda Pendleton wrote an expansive article on hypoxia in late 1999 following the unexplained crash of a Lear 35 that took the life of professional golfer Payne Stewart and five others. The article was published on the AvWeb web site.
Pendleton, an experienced turbine pilot who has taught at FlightSafety International, demonstrates the effects of hypoxia by taking other experienced pilots on a night flight. They are always surprised at how much their night vision improves at 5,000 feet MSL when they’re given a shot of supplemental oxygen. No pilot is immune to the effects of hypoxia.
How High Can You Go?
My pulse oximeter cost me over $330. They now sell for less than $100 on the Internet. Do yourself a favor and get one, or team up with other pilots and split the cost. Then do your own testing to determine how susceptible you are to the onset of hypoxia.
If you’re like me, the purchase and prudent use of a small supplemental oxygen system will transform your flying. It will help you think better and fly better and feel a lot better at the end of each flight.
“When Humans Fly High”
by Linda Pendleton
Aviation Safety Gear
Vital Sign Monitoring