**Advanced Aerospace Medicine**

Definition of atmosphere, altitude, elevation

Gas laws (definition, explanation, and practical implications to
aviation).

Section I THE AEROSPACE ENVIRONMENT

***PHYSICAL CHARACTERISTICS***

**The Atmosphere:**

(Implications in aviation)

Composition of the atmosphere and properties of each gas (nitrogen,
oxygen, carbon dioxide, carbon monoxide, ozone).

Barometric pressure (definition, characteristics, standard atmosphere
concept, relationship between altitude and pressure changes, pressure
altitude, true altitude, absolute altitude)

Physical divisions of the atmosphere and their characteristics
(temperature, density, weather, etc.)

Physiological divisions of the atmosphere (physiological, deficient, and
space equivalent) and their characteristics.

The atmosphere envelops the Earth and has four layers: troposphere,
stratosphere,

mesosphere and ionosphere. The troposphere extends from the Earth's
surface to between 6 and

19 Km and is characterized by weather phenomena, i.e.,
temperature/humidity changes, winds,

storms etc. Also, for every 1,000 ft. above the Earth's surface, there
is a decrease in temperature

of 4° F. Thus, colder temperatures are encountered on mountain tops
compared to sea

level. The stratosphere extends to 45 Km and contains the main portion
of the ozone (O~3~) layer which shields the Earth from high ultraviolet
(UV) light energy emanating from the sun. Short UV frequency light is
absorbed by O~3~ converting it to O~2~ with long UV frequency light
converting it back to O~3~ according to the chemical reaction

O~3~ → O~2~ → O~3~ + heat

↑ ↑

short UV freq. long UV freq.

Temperatures in the stratosphere hold steady at about 67° F. (-55° C.).
The third layer is the mesosphere (into which the ozone layer extends)
reaching from the stratosphere, which contains the main portion of the
ozone layer) to approximately 85 Km. It is a relatively warm region
because of heat given off by the reaction of UV and O~2~. The uppermost
layer is the ionosphere extending beyond 85 Km. It is characterized by
ionized air serving as a protective electromagnetic shield that prevents
excessive amounts of galactic and solar ionizing radiation from reaching
the Earth's surface. Practically all commercial and general aviation
operations are within the troposphere and lower stratosphere.

The atmosphere consists mainly of nitrogen (78%), and oxygen (21%) with
small amounts of

inert gases such as argon, neon, helium et al. These percentages are
relatively constant whether

at sea level or at any altitude.

Barometric pressure is 760 mm Hg at sea level which is the summation of
the partial pressures

exerted by all gases (see previous paragraph) in the atmosphere. As one
ascends from sea level to

the barometric pressure progressively decreases until becoming
essentially zero in the

vacuum of space. It must be emphasized that the percent of oxygen is
constant (21 %) at any

point in the atmosphere, however, calculation of the ambient pO~2~ can
be facilitated by

multiplying the barometric pressure by 20% (rather than 21%) as
indicated in Table I, 3rd

Column. Key altitude landmarks that are listed in Table I will be
further discussed and should be

committed to memory.

TABLE I

ALTITUDE AND BAROMETRIC PRESSURE

[Altitude (Ft.)]{.underline} [Barometric Pressure]{.underline}( [mm
Hg)]{.underline} [Ambient pO~2~ (mm Hg)]{.underline}

SL 760 (1 atm.)\* 152

8,000 564 (3/4 atm.) 112

18,000 380 (1/2 atm.) 76

34,000 187 (1/4 atm.) 37

40,000 141 28

50,000 87 17

63,000 47 (Armstrong's Line) 9

\*Sea level barometric pressure of 760 mm Hg is also expressed as 1
atmosphere (atm.). Consequently a barometric pressure of ½ that at sea
level, 380 mm Hg, would be ½ atm.

With a decrease in the amount of O~2,~ there will be a commensurate
decrease in

PAO2 (alveolar oxygen pressure) and PaO~2~ (arterial oxygen pressure)
with increasing altitude.

Hence, there will be a degree of hypoxemia (O~2~ deficiency in the
blood) and hypoxia

(O~2~ deficiency in the tissues) as one ascends from sea level
increasing altitude (further details to

be found in Section II, 1.2 below). Most individuals in reasonably good
health can well tolerate such a hypobaric environment to altitudes of
8-10,000 ft. because of the body's physiological compensatory
mechanisms: increased heart rate, respiratory rate and tidal volume.
(Exceptions might be those with significant cardiac or pulmonary
disease.) However, as higher altitudes are reached, compensatory
mechanisms will become less effective and eventually ineffective in
overcoming the effects of an increasingly oxygen deficient environment,
resulting in signs and symptoms of hypoxia.

As a countermeasure to hypoxia, aviators must breathe supplemental O~2~
and understand the

concept of equivalent altitudes (Table II). As one ascends from sea
level, it would be necessary

to deliver increasing percentages of O~2~ (by well-fitted aviator's
mask) if a sea level PAO~2~

(alveolar oxygen pressure) of 102 mm Hg is to be maintained.

At 34,000 ft., it would require 100% O~2~ to maintain a PAO~2~ equal to
that of breathing air at sea

level. However, beyond 34,000 ft., even with 100% O~2~, a sea level
PAO~2~ cannot be maintained.

With altitude it will continue to decrease so that at 39,000 ft., it
will be equal to that of breathing

air at 10,000 ft. even though one is breathing 100% O~2~ pressure
breathing is required to maintain a

sea level PAO~2~ beyond 34,000 ft. while breathing 100% O~2.~

TABLE II

EQUIVALENT ALTITUDES

Breathing 100% O~2~ at is equivalent to breathing air at

34,000 ft. sea level

39,000 ft. 10,000 ft.

Armstrong's line at 63,000 ft. is a key altitude landmark to be
remembered even though it is well beyond the range of commercial (the
Concorde excepted) and general aviation operations

(Table I). At this altitude the barometric pressure is only 47 mm Hg
which is also the water vapor pressure in the body fluids. Hence, as
63,000 ft. is exceeded, the body's water vapor pressure will exceed the
barometric pressure, resulting in off gassing or bubbling (boiling) of
body fluids. This phenomenon is called ebullism.

Where does space begin? This is a question that may be more
philosophical than scientific. With increasing altitude, there are
differences in atmospheric characteristics and in the physiological
stresses imposed upon crew and passengers, any one of which could be
posited as the threshold of space. There is simply no clear demarcation
line. From a physiological perspective, there are key landmarks that
argue for the definition of space-like conditions (Table III).

TABLE III

PHYSIOLOGICAL LANDMARKS

[Altitude (ft.)]{.underline} [Effect]{.underline}

10,000 Physiologic Compromise

34,000 Pressure Breathing Required

50,000 Pressure Suit Required

63,000 Ebullism

It might be argued that space begins at about 10,000 ft. where
physiological compensatory mechanisms, such as increased heart rate,
respiratory rate, and tidal volume are active and wherein increasing
percent of oxygen becomes necessary to avoid significant effects of
hypoxia.

Or space might begin at 34,000 ft. where 100% oxygen must be delivered
under increasing pressures in order to maintain a sea level PAO~2~. At
50,000 ft., a pressure suit becomes essential and at 63,000 ft. the
blood begins to vaporize. Consequently, an argument could be made that
on a physiological basis, space begins at any of the above altitudes.
Although there is not unanimity on the question of where space begins,
the most commonly accepted definition is 100 Km (60 miles); the altitude
at which there is insufficient aerodynamic forces to support aerial
flight.

There are three gas laws germane to aerospace medicine: Boyle's Law,
Dalton's Law, and Henry's Law**.**

Boyle's Law states that when the temperature remains constant, the
volume of a given mass of gas varies inversely as its pressure. This is
expressed as V~1~/V~2~=P~2~/P~1~ whereV~1~ is initial volume, V~2~ final
volume, P~1~ initial pressure, and P~2~ final pressure. For example, as
the pressure of a gas decreases, its volume will proportionally
increase. To illustrate this in aviation, as an aircraft ascends, the
barometric pressure will decrease; consequently, the volume of gases in
the body cavities (middle ear, sinuses, lungs, GI tract) will increase.
As the aircraft ascends to a cabin altitude of 8,000 ft., for example,
the barometric pressure decreases from 760 mm Hg to 522 mm Hg or by
about 1/4^th^. This will result in a 25 % increase in gas volume in the
body cavities, which could have clinical implications.

Dalton's Law states that the total pressure of a gas mixture is the sum
of the individual pressures of all the gases in the mixture. This law
can be expressed as P=P~1~+P~2~+P~3~+... where P=the total pressure of
the gas mixture, and P~1~, P~2~, P~3~...are the individual or partial
pressures of each gas. As an example, at sea level where the barometric
pressure is 760 mm Hg, the gases in the lungs (H~2~O vapor, O~2~, N~2~,
CO~2~) each impose different pressures, but the total pressure is 760 mm
Hg. Likewise, at ½ atm. (18,000 ft.), the sum of the pressures of the
mixed gases in the lungs will be 360 mm Hg (See Table IV).

Henry's Law states that the quantity of gas dissolved in a liquid is
proportional to its partial pressure in contact with that liquid. As an
example, with altitude, the ambient PN~2~ decreases below that which is
in the blood and tissues. As a result, nitrogen leaves the tissues for
the blood. If the pressure differential of nitrogen between the
atmosphere and the blood and tissues is too great, the capacity of the
blood to dissolve the N~2~ will be exceeded. As a result, nitrogen
bubbles will form in the blood stream, possibly causing decompression
illness.