The Pilot’s Atmosphere

A Pilot’s Need to Understand Oxygen

Many of today’s home-built aircraft capable of transporting man to high altitudes in near record time, with the average age of the pilot base at well over 50 years old, a practical knowledge of physiological human principals and atmospheric physics are not only desirable, but necessary in order to sustain safe operating parameters. Therefore, the pilot should have a firm understanding of the relationships between oxygen, altitude and the body.

The Composition of the Atmosphere

The various gases that the atmosphere is made of consists of about 78% nitrogen, about 21% oxygen and about 1.1% carbon dioxide and other gases. These three main gases are very important to the body physiologically. Due to the constant mixing of winds and other meteorological factors, the percentages of each gas in the atmosphere are normally constant to about 70,000 ft. throughout a wide range of temperature and barometric changes.


Nitrogen, present in a high percentage, is responsible for the major portion of the atmosphere’s pressure or weight. Some nitrogen is dissolved in and is carried by the blood, but this gas does not enter into chemical combinations as it is carried throughout the human body. Each time we breathe, the same amount of nitrogen is exhaled as was inhaled.


Oxygen is a colorless, odorless, tasteless gas, but is absolutely essential to all life on earth. Each time man breathes, approximately 21% of that breath is oxygen. In the lungs, a portion of this oxygen is absorbed into the blood-stream where it is carried to all parts of the body. It is used to “burn” or oxidize food material and produce energy transformations in the body.

Man can live for weeks without food and for days without water, but only a few minutes without oxygen. Because man can not store oxygen in his body, as he can food and water, he lives a breath-to-breath existence. He continues to live only as long as he can continually replenish the oxygen consumed by his metabolic process. Air is a relatively heavy substance. It weighs 14.7 pounds per square inch at the earth’s surface sea level. That is the pressure created by one column of air one inch square that is about 100 miles high (the approximate total thickness of the layer of free air or atmosphere covering the earth). Because the air pressure is equal from all sides one does not notice the atmosphere’s weight in pressure.

Thinner as You Go Up

The weight of the atmosphere does not remain the same from to bottom. In one respect the atmosphere can be viewed as an ocean where a person finds that the absolute pressure around him increases the deeper (or closer to the earth’s surface) he goes. The composition of the atmosphere always remains the same, but is more dense at the bottom or at the surface).

The pilot should recognize that atmospheric pressure does not diminish at a uniform (linear) rate with altitude. Although the atmosphere covers the earth to a height of about 100 miles, three-fourths of the molecular destiny of the atmosphere rests just below our tallest mountain, Mt. Everest.

At an altitude of 18,000 feet above sea level the absolute air pressure has decreased by 1/2, to only about 7 psia. (pounds per square inch absolute). Deep interstellar space would be near zero psia. In other words 18,000 ft. MSL is half way through the density of the atmosphere. At 34,000 feet, the pressure has been cut in half again to a mere 3.5 psia. At 65,000 ft. there is only 1 psia. and only 0.15 at 100,000 ft. Beyond that, the atmosphere is largely a vacuum.


The principal purpose of respiration is to supply the cells of the body with oxygen and remove the carbon dioxide, a biological waste product, produced by cellular activities. Three basic processes are involved with respiration phases. The first process (external) is ventilation, or breathing, the movement of air between the atmosphere and the lungs. The second and third processes (internal) involve the exchange of gases within the body through the blood stream. External respiration is the exchange of gases between the blood and lungs. Internal respiration is the exchange of gases between the blood and tissue cells throughout the body.

The respiration cycle begins with inhalation of air into the lungs. Inhalation is produced by the contraction of the diaphragm, the large muscle separating the thoracic and abdominal cavity.

Ordinarily, a person breathes 12 to 16 times a minute, although the rate will be slower when resting and faster when exercising. The average, quiet, resting man inhales about a pint (400 ml) of air for each breath, or six to eight quarts (8 liters) per minute.

Oxygen’s Flight Path

Oxygen used in the body is inhaled through the nose or mouth, passes through the trachea and bronchial tubes, and is directed into the lungs where it transfers to the blood. The blood then carries this oxygen to living cells where energy is obtained by molecular cellular transfer for all body functions. This energy transfer produces carbon dioxide (CO2), a biological waste product. As carbon dioxide is produced, blood then carries it back to the lungs to be released to the atmosphere through the exhaling respiration phase back through the nose or mouth.

Within the lungs, there are millions of tiny air sacs called alveoli which inflate like tiny balloons. The number of alveoli in the lungs is estimated to be around 750 million with a surface area between 700 and 800 square feet, or about the size of a tennis court.

Blood is pumped from the heart through arteries to microscopic capillaries, or tiny tubes, through which blood is constantly flowing. The walls of the alveoli have micro-capillaries in which the oxygen is diffused into the blood. Pressures inside and outside, (the natural molecular tensions of body fluids and pressure altitude) play an important factor in the effectiveness of the entire respiration system. Once these differential pressures are reversed or placed below a certain point, life-giving gases may not properly exchange through the lungs or tissues.

The Pilot’s Oxygen Needs & Availability

How Much Oxygen is Needed

The amount of oxygen consumed by the body during the respiratory cycle depends primarily upon the degree of physical or mental activity of the individual. A person walking at a brisk pace will consume about four times as much oxygen than at rest. In the course of an average day, a normal adult male will consume about 35 cu. ft. of oxygen or 2.5 lb. This is approximately equivalent to the weight of solid food consumed daily. An oxygen supply which might be adequate for a person at rest would be inadequate for the same individual while piloting an aircraft under severe weather conditions or under mental stress.

It should be noted that since only 21% of the atmosphere inhaled is oxygen, added to the fact that we only benefit from only 1/5 to 1/8 of the total volume of oxygen inhaled per breath, one can see that the actual volume of air used per day can be 80 to 90 Lbs. This number can be quite a bit higher by the respiration quality factor that individual has, i.e. asthma, physical damage and age.

How Altitude Complicates Breathing

Oxygen becomes more difficult for your body to obtain with altitude because the air becomes less dense, and the total (absolute) air pressure decreases compromising your primary (lungs) and secondary (bloodstream) respiratory systems the ability to transport and exchange oxygen throughout the body, even though the percentage of oxygen (21%) remains constant with respect to the atmosphere. As altitude is increased and the pressure of oxygen is reduced, the amount of oxygen transferred in the lungs alveoli is reduced which results in a decrease in the percentage of oxygen saturation in the blood. This causes a deficiency of oxygen throughout the body, and, for this reason, supplemental oxygen is required if the body is to receive adequate oxygen for proper mental and physical functions.

Oxygen Saturation

In a relatively simplistic term “oxygen saturation” is defined as the percentage of available oxygen-carrying hemoglobin that are carrying oxygen in your tissue and/or bloodstream. Another simplistic, but fair, example would be if a given volume of blood has 100 hemoglobin cells and 95 of them are carrying oxygen, then the oxygen saturation level is 95%.

Physiological Effects of Oxygen Deprivation

The total effect on an oxygen-deprived individual is the result of both altitude and amount of time exposed. Every cell in the body is affected by the lack of oxygen, but the primary effects are on the brain and the body’s nervous system. Above 10,000 ft. deterioration of physical and mental performance is a progressive condition. This degenerative condition becomes more severe with increased altitude or prolonged exposure. A person who is flying at 10,000 ft. for 5 hours can be equally affected as a person who went to 16,000 ft. for only one hour.

Oxygen pressure is about 1/5 that of actual atmospheric pressure. Therefore, at a pressure altitude of 10,000 ft., for a standard day i.e. 70° F @ 29.92 In. Hg., the absolute pressure would be about 10 psia. while the working pressure for oxygen would only be 2.0 psia. It’s no wonder why of all our critical life-support organs our lungs are the largest for their function.


Hypoxia or Hypoximia – As Webster defines it:

hyp-ox-emia \,hip-,äk-‘se-me-a, ,hi-,päk-\ n [NL, fr. hypo- + ox-+ -emia]: deficient oxygenation of the blood – hyp-ox-emic \-mik\ adj

hyp-ox-ia \hip-‘äk-se-a, hi-‘päk-\ n [NL, fr. hypo- + ox-]: a deficiency of oxygen reaching the tissues of the body – hyp-ox-ic \-sik\ adj

The effects of an insufficient supply of oxygen on the body that includes mental any physical degradation is generally called hypoxia.

Indicators of Hypoxia

Some of the most common indications (symptoms) of hypoxia are:

  1. An increased breathing rate
  2. Lightheadedness or dizzy sensation
  3. Tingling or a warm sensation
  4. Cold chills and/or cold extremities
  5. Sweating and increased heart rate
  6. Reduced color vision and visual field
  7. Sleepiness, insomnia and/or nervousness
  8. Blue coloring of skin, fingernails and lips
  9. Behavior change, giddiness, belligerence, cockiness, anxiousness or euphoria

Effects of Hypoxia at Different Altitudes

5,000-10,000 ft.

Subtle hypoxic effects begin at 5,000 ft., particularly noticeable at night. In the average individual, night vision will be blurred and narrowed. Also, dark adaptation will be compromised. At 8,000 ft., night vision is reduced as much as 24% without supplemental oxygen. Some of the effects of hypoxia will be noticed during the daylight at these altitudes without supplemental oxygen during long flights, i.e. 3 to 5 hours.

10,000 ft.

At 10,000 ft. the oxygen pressure in the atmosphere is about 10 psia. Accounting for the dilution effect of water vapor and carbon dioxide in the alveoli, this is not enough to deliver a normal (or less than needed) supply of oxygen into the lungs. This mild deficiency is ordinarily of no great consequence. However, flying at an altitude of about 10,000 ft. (not taking density altitude into account) for 3 to 5 hours will more likely than not cause fatigue in which the pilot may have compromised performance once he enters his destination. Since the beginning of powered flight, pilots have reported experiencing difficulty in concentrating, reasoning, judging, solving problems and making precise adjustments of aircraft controls under prolonged flight conditions at altitudes as low as 8,000 ft. MSL.Commercial aviation pilots are required to be on supplemental oxygen for flights lasting 30 minutes or more at 10,000 ft.

15,000 ft.

At 15,000 ft. drowsiness, headaches, weariness, fatigue and a false sense of well-being will most likely be experienced in 1 to 2 hours without oxygen. Most important and less evident to the individual is the psychological impairment which could cause judgment errors, poor coordination and difficulty in performing simple, let alone, important piloting tasks.

20,000 ft.

At 20,000 ft. the absolute pressure altitude drops to 6.75 psia. and the oxygen pressure drops to 1.38 psia. This is less than half that at sea level. Oxygen saturation of the blood drops to 62 to 64% at this pressure altitude. Unconscious collapse and/or convultions will result within 10 to 15 minutes of exposure. Death is not uncommon as a result of complications acquired from long or quickly changing exposures to low partial pressures (high altitudes) without supplemental oxygen or pressurized cabins.

30,000 + ft.

At a pressure altitude of 34,000 ft. the lungs are compromised so much in the ability to transfer gases to the blood and air that the oxygen saturation level will drop to only 30%. Total unconsciousness will result in 3 to 4 minutes. At this point a person breathing 100% oxygen would not benefit from the supply because pressures in water vapor and tissues will be the same as the absolute pressure of oxygen (0.76 psia) where nearly nothing is transferred. One would need to be using a full pressure-demand-type oxygen mask.

Different Individuals, Different Effects

It is true that susceptibility to hypoxia varies from person to person, and there are some who can tolerate altitudes well above 10,000 ft. without suffering from the effects. It is equally true that there are persons who develop hypoxic effects below 10,000 ft. As a general rule, individuals who do not exercise regularly or who are not in good physical condition will suffer from the effects of hypoxia more readily. It is also true that even with tip-top shape athletes the effects of hypoxia are still the same as a person who is in good physical condition, but they simply have the ability to tolerate the effects much better.

Individuals who have recently over-indulged in alcohol, who are moderate to heavy smokers, or who take certain drugs will be considerably more susceptible to the effects of hypoxia. Susceptibility to the effects of hypoxia can also vary in the same person from day to day or from morning to evening.

High altitude acclimation can be an improving factor at moderate altitudes, i.e. 10,000 to 15,000 ft., however, once again, at high altitudes the laws of physics prevail and even the most acclimated will still suffer the effects of hypoxia from the exposure.

While not all of the known symptoms listed occur in each individual, any given person will develop the same symptoms in the same order for each time he becomes hypoxic. For this reason, a person, having once experienced hypoxia is usually better prepared to recognize the onset of hypoxic symptoms the next time around. One can participate in a controlled hypoxic awareness program through an altitude chamber that is offered by many commercial and/or university flight medical training programs.

Hypoxia is a Lot Like Alcohol

Because hypoxia affects the central nervous system, the general effects of hypoxia are almost identical to alcoholic intoxication. A typical individual suffering from hypoxia, induced by exposure between 15,000 and 20,000 ft. will be comparable to an individual who has consumed five to six ounces of whiskey. The most hazardous feature of hypoxia, as it is encountered with aviation, is its gradual and rather insidious onset. Its production of a false sense of well-being called euphoria is particularly dangerous. Since hypoxia obscures the victim’s ability and desire to be critical of himself, he generally does not recognize the symptoms even when they are very obvious to others. The hypoxic individual commonly believes things are progressively getting better as he nears total collapse. There are some false indicators of a hypoxic condition which should be considered. The “blueness” color test of the finger nails has been suggested by some as a guide to determine the degree of hypoxia, but this approach is usually invalid because any hypoxic individual should consider himself an unreliable observer that has all the appearances to himself of operating effectively. Almost all of the symptoms of hypoxia are useless for self-diagnosis, but have proven to be a life-saver from the standpoint of an unaffected observer.


So remember, don that oxygen system before the effects of hypoxia can manifest themselves. This will help you to arrive at your destination safely!