Airman Education Programs

Beware of Hypoxia

By Larry Boshers

Each year, as a member of the FAA Civil Aeromedical Institute's Airman Education Program Team, I attend several air shows and Federal Aviation Administration (FAA) — sponsored Wings Safety Programs. The primary purpose for our participation is to promote aviation safety through education. Normally, we present seminars about Aviation Physiology-related topics and provide pilots with the opportunity to experience spatial disorientation by "flying" the demonstration trainer designed specifically for that purpose. We also provide pamphlets and other handouts with information about how pilots can obtain formal training in post-crash survival and high-altitude physiology.

All too often, pilots tell me they don't need physiological training because they don't fly that high. The statement points out the general feelings of a large majority of the aviation population. I suppose then the burning question is "why do we still have aircraft accidents?"

The answer to this question seems to have already been given. According to an official 1991 FAA report, Civilian Training in High-Altitude Flight Physiology, "Some National Transportation Safety Board staff members have expressed a concern that high-altitude flight physiology training for civilian flight personnel should receive greater emphasis than it currently does." The report goes on to say, "When pilots combine their private flying skills with their business transportation needs and use their aircraft to meet those needs, it is inevitable that in order to meet a schedule, arrive at a destination, or get home after a meeting, the urge to complete a mission will lead the pilot into a physiologically-unsafe altitude or into conditions for which there has been insufficient training or experience."

Now that I have my soap box dusted off, I may as well use it. In my opinion, there exists a huge requirement for initial training in high-altitude physiology for all civilian pilots (including general aviation pilots intending to fly above 10,000 feet daytime and 5,000 feet a night). This training is extremely important due to the large number of general aviation pilots who are unaware of the physiological problems that can affect their safety during flight. The training will familiarize pilots with the physiological aspects of high altitude flight, including discussions on physics of the atmosphere, respiration and circulation, hypoxia, hyperventilation, human factors, self-imposed stresses, trapped and evolved gas problems, aircraft decompressions, and oxygen equipment. The training will give pilots an opportunity to experience their personal signs and symptoms of hypoxia in an altitude chamber. Hypoxia is just one of the physiological problems that can impair pilots if they are not aware of the effects of decreased oxygen pressure at altitude.

Hypoxia, by definition, is the lack of sufficient oxygen in the blood, tissues, and/or cells to maintain normal physiological function. Many different factors can cause this state of oxygen deficiency. Breathing air at reduced barometric pressure, malfunctioning oxygen equipment at altitude, drowning, pneumonia, extremes of environmental temperatures, and carbon monoxide are just a few of the causes of oxygen deficiency in the body that results in hypoxia. The most common causes of hypoxia in aviation are: flying, non-pressurized aircraft above 10,000 ft without supplemental oxygen, rapid decompression during flight, pressurization system malfunction, or oxygen system malfunction.

Hypoxia is actually divided into four types: hypoxic hypoxia, hypemic hypoxia, stagnant hypoxia, and histotoxic hypoxia. No matter what the cause or type of hypoxia you experience, the symptoms and effects on your flying skills are basically the same. One factor that makes hypoxia dangerous is its insidious onset; your signs and symptoms may develop so gradually that they are well established before you recognize them. Hypoxia is painless, and the signs and symptoms vary from person to person. To better understand the effects of hypoxia, I will explain each, as well as whether it is the respiratory or circulatory system that is being affected.

Hypoxic Hypoxia

This is the most common form of hypoxia encountered in aviation and occurs at the lung level. This type of hypoxia is commonly called altitude hypoxia. Pilots may experience hypoxic hypoxia when flying at altitude in an unpressurized aircraft. With increasing altitude, the molecules of oxygen in ambient air get farther apart and exert less pressure per square inch. The percentage of oxygen does not change as we ascend; however, the partial pressure of oxygen in ambient air decreases as we go to altitude. In other words, with increasing altitude, the partial pressure of oxygen gets lower and the lungs cannot effectively transfer oxygen from the ambient air to the blood to be carried to all tissues in the body.

Hypemic Hypoxia

This type of hypoxia is caused by the reduced ability of the blood to carry oxygen. To the pilot, this means that, even though there is an adequate supply of oxygen to breathe, the blood's capacity to carry the oxygen to the cells has been impaired. There are a variety of reasons for this to happen. Anemia, hemorrhage, hemoglobin abnormalities, sulfa drugs, nitrites, and carbon monoxide interfere with the ability of the blood to carry oxygen, reducing the amount of oxygen the blood can carry to the cells. The most common cause for hypemic hypoxia in aviation is when carbon monoxide is inhaled because of aircraft heater malfunctions, engine manifold leaks, or cockpit contamination with exhaust from other aircraft. Hemoglobin bonds with carbon monoxide 200 times more readily than it bonds with oxygen.

Stagnant Hypoxia

This type of hypoxia occurs at the circulatory level. If the blood flow is compromised for any reason, then sufficient oxygen cannot get to the body tissues. To the pilot, this means, that even though there is an adequate supply of oxygen to breathe, it is not getting to the cells of the body tissues to support their metabolism. Decreased blood flow can result from the heart failing to pump effectively, arterial constriction pooling of the blood such as occurs during neurologic shock or from enlarged veins in the lower extremities. Stagnant hypoxia also occurs when the body is exposed to cold temperatures because the blood flow is decreased to the extremities. This may happen following a rapid decompression during flight or while operating an aircraft in cold weather conditions without cabin heating.

Histoxic Hypoxia

This type of hypoxia happens at the cell level. This means that the cell expecting and needing the oxygen is impaired and cannot use the oxygen to support metabolism. To the pilot, this means that even though there is an adequate supply of oxygen to breathe and that oxygen is being circulated by the blood, the cells are unable to accept or use the oxygen. Alcohol, narcotics, and cyanide are three primary factors that can cause histoxic hypoxia. Cyanide is one of the byproducts during the combustion of plastics.

As I said earlier, no matter what the cause for or type of hypoxia you are experiencing, the signs and symptoms and the effects on your flying skills are basically the same. Hypoxia is easy to succumb to because the human body does not have an effective warning system against the threat. Many incidents and some accidents are "officially" attributed to the pilot's inability to detect hypoxic conditions, with the result that the pilot becomes unsafe because of compromised skills and judgment.

Hypoxia Signs and Symptoms

Because hypoxia is insidious and because the signs and symptoms are varied, the safest and most effective way to reduce your risk of becoming hypoxic is to attend a formal course in aviation physiology. Participating in an altitude chamber flight would give you the opportunity to experience hypoxia first hand; you would be able to experience your own symptoms, and would be able to observe the signs of hypoxia in the other class participants. Table 1 shows some of the more common signs and symptoms of hypoxia.

Pilots must understand that the signs and symptoms of hypoxia are as varied and individual as the person experiencing them. Pilots who are hypoxic will experience (most of the time) similar signs and symptoms. However, the signs and symptoms may appear in a different order and in varying intensities.

The greatest benefit in experiencing hypoxia signs and symptoms personally in the hypobaric (altitude) chamber during a high altitude-training course is that you will know what to look for while you are flying. This is important because your signs and symptoms of hypoxia will remain relatively constant throughout your flying career.

Time of Useful Consciousness (TUC) or Effective Performance Time (EPT)

These interchangeable terms describe the period of time between the interruption of the oxygen supply or exposure to an oxygen-poor environment and the time when a pilot is unable to perform flying duties effectively, such as putting on oxygen equipment or descending to a safe altitude. Table 2 shows the average TUC or EPT for various altitudes. The table is to be used as a guide only; the times are based on healthy individuals at rest in a hypobaric (altitude) chamber. One important fact to keep in mind is that following a rapid decompression to and above 30,000 feet, the average TUC/EPT will be reduced from 1/3 to 1/2 its original value. This is due to phenomenon known as reverse diffusion or fulminating hypoxia. This occurs when oxygen is forced out from the lungs due to the rapid expansion of gas during a rapid decompression. The result is acute and immediate hypoxia.

Factors Influencing Tolerance to Hypoxia

It is impossible to predict exactly when, where, or how hypoxic reactions will occur in an individual pilot. The appearance and severity of the signs and symptoms are aggravated by several factors; rate of ascent, time spent at altitude, physical activity at altitude, fatigue, self-imposed stress, extreme ambient temperature, and individual physiological fitness.

Prevention of Hypoxia

There is nothing magic about preventing hypoxia; fly a well-maintained pressurized airplane or fly at an altitude where oxygen is not required. Obviously this is not always practical, and by following these simple guidelines you will be able to reduce your chances of experiencing hypoxia during flight. If pressurization is not an option and supplemental oxygen is not available, limit your exposure time to less than 1 hour between 10K feet and 14K feet, including not more than 30 minutes between 12K feet and 14K feet. If you have supplemental oxygen, use it above 5K feet during night flights and above 10K feet during daytime flights.

Treatment of Hypoxia

If hypoxia is suspected in yourself or others on board an aircraft, follow these simple steps:

  • Administer supplemental oxygen (don your oxygen mask)
  • Check your equipment for proper operation
  • Ensure the regulator is turned on
  • Check the flow indicator (this will tell you that something is coming to the mask)
  • Ensure that all oxygen equipment connections are secure
  • Monitor your breathing rate and depth (intentionally slow your breathing to prevent hyperventilation, use the flow indicator to help you monitor respiration)
  • If safe, descend to an altitude (below 10K feet) where supplemental oxygen is no longer required

Some Final Thoughts

Hypoxia is a constant and dangerous companion while flying. The insidious nature of hypoxia means that you must constantly be suspicious of how you and your passengers feel. Once hypoxia is recognized, quick and decisive action means recovery is only seconds away. The key, then, to flying safely at altitude is to be able to: identify the flight condition in which you may become hypoxic, recognize your personal hypoxia symptoms, and to recover from hypoxia before you have gone beyond your ability or desire to help yourself.

The most effective way to prevent hypoxia is through education and experience. When pilots are trained in the proper use and care of their pressurization systems and supplemental oxygen equipment, and are aware of their personal hypoxia signs and symptoms, they are safer and better prepared to meet the challenge of flying in an oxygen-poor environment. Pilots can get this experience by participating in a formal aviation physiology course. The FAA Civil Aerospace Medical Institute, located at the Mike Monroney Aeronautical Center in Oklahoma City, OK, offers this training free of charge to all aviators who are at least 18 years old and have a current medical certificate.


  • Get information about how to register to attend high-altitude physiology or the post-crash survival course,
  • or call our general scheduling number; 405-954-4837.
  • Read technical report Civilian training in high-altitude flight physiology.
Last updated: Tuesday, July 21, 2015