Atmospheric Composition And Pressure

A habitable planet must, of course, have a breathable atmosphere; and this now can be rather completely specified in terms of its component gases and their concentrations or partial pressures (see Figure 6).

As far as we know, the only essential ingredients of a breathable atmosphere are oxygen and minor amounts of water vapor. The inspired partial pressure of oxygen must lie between two extreme limits: a lower limit below which hypoxia is encountered and an upper limit above which oxygen toxicity becomes a problem.

100 80 60 40 20 0

Volume, per cent of nitrogen

Figure 6. Breathable mixtures of oxygen and nitrogen as a function of barometric pressure.

100 80 60 40 20 0

Volume, per cent of nitrogen

Figure 6. Breathable mixtures of oxygen and nitrogen as a function of barometric pressure.

In determining the level of inspired partial pressure of oxygen (or any gas) under any specified condition of composition and total barometric pressure, it is necessary to correct for the fact that as air is inhaled it is also humidified in the nasal passages and throat, so that, by the time it reaches the lungs, it is normally saturated with water vapor at body temperature. For this reason, a correction is made:

p02 (inspired) = (pB - pU20) x /0j, in which p02 is the partial pressure of oxygen in the air entering the lungs, pB is the total barometric (ambient) pressure, /;HaO is the partial pressure of water vapor in the air entering the lungs, Fa is the fraction (by volume) of oxygen in the ambient air. For pressures expressed in millimeters of mercury (mm of Hg), pH.¿0 is assumed to be 47 millimeters of mercury. Thus, under normal sea-level conditions where barometric pressure is 760 millimeters of mercury, p02 — (760 — 47) x 0.2093 = 149 millimeters of mercury.

The lower limit of inspired oxygen partial pressure is approached by the inhabitants of a mining settlement at Aucanquilcha in the Chilean Andes, situated at an altitude of 17,500 feet above sea level. This is said to be the greatest altitude at which men are known to live permanently (Pugh and Ward, 1954). It is considerably higher than the environment of the Tibetans, most of whom reside and work their land at altitudes between 12,000 and 16,000 feet. At 17,500 feet the inspired partial pressure of oxygen is about 72 millimeters of mercury, yet the miners of Aucanquilcha lead very strenuous lives and appear to be completely acclimated to the low level of oxygen pressure. To reach the entrance of the mines where they work, they climb 1500 feet each day to an altitude of 19,000 feet, at which the inspired partial pressure of oxygen is 68 millimeters of mercury. Yet even these conditions probably do not represent the ultimate lower level of oxygen pressure that can be tolerated as a steady-state condition by some men. According to mountain climber N. E. Odell, ". . . our evidence has shown us emphatically that one can live and feel fit for an indefinite period at 23,000 feet [p02 (inspired) = 53 millimeters of mercury] . . .." (See Norton, 1925.)

The upper limit of inspired oxygen partial pressure has been found experimentally to be approximately 400 millimeters of mercury (equivalent to about 56 per cent oxygen in the air at sea-level pressure). This limit is approached in the therapeutic use of oxygen in hospitals, where the accepted ceiling is 40 per cent oxygen, to be on the safe side (Ingalls, 1955). It was discovered in the 1950's that blindness in premature babies (retrolental fibroplasia) often resulted from the use of excessively high oxygen concentrations in their incubators. Since human beings vary a great deal in their tolerances of environmental extremes and even in their ability to adapt, it is not be to be expected that a single set of limits would apply to all people. Rather, it is desired here to set limits such that at least some people could live within them. Following this philosophy, for our purposes we may state that the inspired partial pressure of oxygen must be greater than about 60 millimeters of mercury but less than about 400 millimeters of mercury.

An inspired partial pressure of oxygen of 60 millimeters of mercury with an atmosphere of pure oxygen corresponds to a total barometric pressure of 107 millimeters of mercury, equivalent to 2.07 pounds per square inch absolute (psia). At barometric pressures slightly below this level, gaseous swelling of the body due to the formation of bubbles in the blood has been observed in experiments with animal and human subjects. Carbon dioxide and water vapor are believed to be the main gases involved in the swelling phenomenon (Wilson, 1961).

There are only certain diluents that may be mixed with the oxygen in a breathable atmosphere and each has an upper limit of inspired partial pressure that should not be exceeded. Symptoms of narcosis due to nitrogen, argon, krypton, and xenon (all chemically inert gases) have been reported when inspired partial pressures exceed certain levels. Xenon, in fact, has even been used as an anesthetic in surgical operations: an 80-per-cent xenon and 20-per-cent oxygen mixture, at 1 atmosphere pressure, will produce unconsciousness in 3 to 5 minutes (Cullen, 1951). The narcotic effects of carbon dioxide are even mote widely known. Presumably neon and helium, and possibly hydrogen, also have narcotic effects at sufficiently elevated pressure levels. Table 2 shows the approximate upper limits for each of the permitted inert diluents in a breathable atmosphere.

Table 2. Approximate Upper Limits for Permitted Inert Diluents in a Breathable Atmosphere

Approximate maximum permissible inspired partial pressure Diluent (mm of Hg)


Nitrogen 2,330

Argon 1,220 Krypton 350 Xenon 160 Carbon dioxide 7

Hydrogen is a special case in that only noncombustible mixtures of hydrogen and oxygen could be regarded as acceptable and one would never expect to find both free hydrogen and free oxygen simultaneously present in a planetary atmosphere. The figures for helium and neon are extrapolated. None of the gases listed is known to be necessary to make up a breathable atmosphere; hence, presumably, any or all can be completely absent as long as the oxygen partial pressure falls within the proper range. Really prolonged tests on people living in atmospheres containing no inert gases have not yet been carried out, however, so it can not be stated categorically that inert gases are unnecessary. (The longest test carried out on human beings in atmospheres containing no inert gases has been of 17 days' duration.) Human evolution has taken place in an atmosphere containing almost 80 per cent inert ingredients, and it may be that some inert gas fraction is needed for the proper functioning of the respiratory system during critical periods of life. Atelectasis (the collapse of mucous-clogged lung alveoli in certain respiratory illnesses, due to the absorption of the contained gases), for example, would be more apt to occur if there were no inert gases in the air being breathed. Also, since carbon dioxide is needed by plants, a lower limit is required on the partial pressure of this constituent. The normal concentration of carbon dioxide in the Earth's atmosphere is 0.03 per cent, equivalent to a partial pressure of 0.21 millimeters of mercury. A reasonable minimal value for supporting normal plant life has not been determined, but possibly it would be of the order of 0.05 to 0.10 millimeters of mercury. Some nitrogen is also needed to supply nitrogen compounds to plants and animals. The minimum necessary amount is probably small but is not known. A small fraction of the free nitrogen in the Earth's atmosphere is constantly being converted into the oxides of nitrogen by lightning flashes (over 3 billion lightning strokes per year, according to the Lightning Protection Institute of Chicago) producing some 100 million tons of fixed nitrogen annually; without this, plant life on the planet would not be able to obtain adequate supplies of available nitrogen. Nitrogen fixation is also accomplished by bacteria attached to the roots of certain leguminous plants well known to farmers all over the world.

Only traces of other gases could be tolerated in the atmosphere of a habitable planet. Upper limits on the tolerable concentrations of some naturally occurring or elemental gases are shown in Table 3.

All of the gases listed in Table 3 are quite active chemically. Thus, on a planet with an atmosphere containing both free oxygen and water vapor, one would not expect to find any of them present as permanent constituents, except in trace amounts. Methane in low concentration is not generally regarded as a toxic material, but it would oxidize slowly and tend to disappear from an atmosphere containing oxygen.

There are many other toxic gases with which we need not be concerned, since there is no reason to expect them to occur naturally in any important concentration on the surface of any planet. Tables 2 and 3 include all of the common gases that might potentially be found in planetary

Table 3. Tolerable Concentrations of Selected Gases"

Threshold limits (parts per million by volume Gas at 1 atmosphere pressure)

Ammonia, NH3

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