Aviation Hygiene

Aviation Hygiene, a branch of professional hygiene, studies the nature, working conditions, and their effects on the health of flight personnel, and is engaged in developing practical measures for organizing flight work on hygienic principles. The beginning of Aviation Hygiene dates back to the first hygienic observations and studies made during balloon ascents and flights (19th century), but it became a specialized branch of hygiene only in the last decade, especially in connection with the use of aviation for military purposes in the World War of 1914-1918. The poor study of hygienic conditions of flight activity and the associated unsatisfactory selection of flight personnel, which were the cause of mass accidents, served as an impetus for the development of Aviation Hygiene in a number of states participating in the world war. In Italy, England, France, and the United States of America, special scientific research institutions were created during this war, which developed significantly in subsequent years and created extensive literature on issues of Aviation Hygiene. The main professional hazards of flight activity, which are the subject of study of aviation hygiene, are related to the need for flight personnel to perform work at high altitudes, under conditions: a) reduced atmospheric pressure, b) reduced partial pressure of oxygen, c) decreased air temperature, d) increased humidity, e) high speed of movement, and f) significant wind. The greatest significance is the reduction in partial pressure of oxygen, which is confirmed both by experiments in low-pressure chambers simulating high altitudes and by the practice of flight activity. As one ascends into the upper layers of air, the percentage of oxygen remains unchanged, but barometric pressure falls, and with it the partial pressure of oxygen also falls. The consequence of this is insufficient oxygenation of the blood, and then insufficient supply of oxygen to the tissues (anoxemia). The gradually increasing lack of oxygen is especially dangerous because it causes severe phenomena with extraordinary suddenness. The decrease in partial pressure of oxygen at altitude affects respiration in that the partial pressure of oxygen in the alveolar air decreases, and consequently, gas exchange also decreases. The natural reaction of the body is deepening, and sometimes acceleration of breathing, as a result of which a larger volume of air enters the alveoli, they expand, and oxygen comes into contact with a larger surface of pulmonary capillaries than under normal conditions. Pilots at an altitude of 4¹/₂ km breathe more deeply, often with their mouths open. The effect of altitude on cardiac activity is manifested by a gradual increase in pulse rate up to an altitude of 4 km, after which a rapid increase occurs (Schneider, Liitz and others). If the subject is given oxygen or returned to normal conditions, the pulse quickly returns to normal. The degree of increase depends on the pulse rate of the subject under normal conditions, on the altitude and duration of flight, and on the general psychophysical state. The view of increased pulse rate as a compensatory phenomenon indicating acceleration of blood circulation is not confirmed by recent research, since blood circulation does not accelerate at altitude (Schneider, Barcroft and others). Consequently, increased pulse rate is a pathological sign. In people who poorly tolerate oxygen deficiency, a sudden sharp weakening of the pulse is accompanied by a state of fainting. Blood pressure does not always show changes. With good compensation, systolic and diastolic pressure at moderate altitudes hardly change. With poor compensation, systolic and diastolic pressure can sharply fall and lead to fainting. An increase in pulse pressure is apparently an unfavorable sign. According to Schneider's experiments, venous pressure decreases at altitude, but he did not note changes in capillary pressure. A number of researchers point to changes in blood composition: an increase in the number of red blood cells and an increase in hemoglobin during flights at high altitudes. It is possible that the increase in the number of red blood cells observed in studies is the result of a change in blood concentration or the appearance of so-called "reserve cells". The use of oxygen prevents and weakens the phenomena of anoxemia. However, there is a limit to which a person can ascend even with oxygen. This level is to some extent individual and is determined by the preservation of performance at a certain altitude. As a general rule, it is considered that ascending above 5¹/₂ km is dangerous, and above 4^{1/₂ km the use of oxygen is recommended, especially during prolonged and repeated flights. The average limit at which a person loses consciousness without the use of oxygen is recognized as an altitude of about 8 km. However, this level depends, in addition to the individuality of the pilot, on the speed of ascent. The absolute limit to which a person can ascend (with oxygen used in the usual way), according to Bauer, lies around 12-14 km, since beyond these limits the partial pressure of oxygen in the alveoli is too low to sustain life. When ascending to great altitudes, phenomena of vascular collapse may be observed: pallor, cold sweat, decrease in muscle and vasomotor tone, muscle twitching, fainting. The effect of altitude on the nervous system is initially expressed by euphoria, which is replaced by a decrease in attention span and loss of ability to control voluntary musculature. At very high altitudes, memory, judgment, sharpness of hearing and vision decrease; drowsiness, faintness set in. The altitude factor also has a certain effect on the organ of vision. Wilmer and Berens point out that at high altitude the field of binocular fixation narrows, the accommodative power often decreases, accommodative fatigue sets in more quickly, and a number of other disorders occur, all of which are more frequent and more sharply expressed in persons with abnormal eyes. The effect of wind arising from the operation of the propeller has a great influence on the pilot, causing changes in breathing, disrupting its rhythm, accelerating it or causing unevenness of individual respiratory acts, enhancing the disturbances that occur due to reduced barometric pressure, accelerating and slowing the pulse with disruption of rhythm and changes in the nature of the pulse wave, and increasing the cardiac impulse (Agazzotti, Galeotti). The movement of air contributes to cooling of the body. The effect of cold on the pilot depends on the season, climate, speed of flight, and altitude. The first signs of cooling (sensation of chill, gooseflesh and pallor) in cases where clothing is not sufficiently warm are intensified: shivering appears, limbs become numb, stupor sets in and there is a tendency to sleep. The effect of motion and speed at the speed of modern airplanes of more than 400 km per hour, the pressure experienced by the pilot during flight, especially during turns, reaches significant proportions. In sharp turns, the aircraft tilts so that the wings and the pilot themselves become at a right angle to their original position. When turning, the increase in centrifugal force acting in the direction of the original line of flight is directed from the pilot's head to his feet. The pilot is secured in the aircraft, the centrifugal force cannot throw him out and acts on his body. As a result, everything that can move in him moves, that is, blood flows away from the head to the legs and internal organs. Anemia of the brain leads to clouding of consciousness, dizziness, and even fainting. Staying in the air is associated with frequent and very diverse changes in position, which places high demands on the organs of equilibrium. The extraordinariness and diversity of nervous experiences and tensions experienced by the pilot during ascent, flight, work in the air (especially combat), landing (especially forced), can be classified among the professional hazards not characteristic of other professions. In order to combat the professional hazards of flight activity, Aviation Hygiene gives first place to the rational staffing of flight personnel. It has been established that for flight activity, young people of good constitution with a perfectly healthy heart, healthy upper respiratory tract, lungs and kidneys, with good musculature, good vasomotor regulation, and perfectly normal joints are necessary (Bauer). The best age for candidates for aviation is from 20 to 28 years. For a pilot, normal vision, both central and peripheral, normal color perception, good depth perception, perfectly normal refraction, absence of noticeable disorders of equilibrium of the eye muscles, good accommodative power, absence of pathological changes and anomalies of the eyeball, normal hearing and smell are necessary; pilots must have perfectly normal organs of equilibrium and a well-developed sense of equilibrium, a stable nervous organization guaranteeing the ability to withstand the most serious strains. From the pilot, a quick and correct reaction is required even in unexpected cases. The pilot must be able to tolerate oxygen deficiency at altitude.}

To determine this ability and establish an individual altitude limit ('ceiling'), so-called 'altitude qualification tests' are used. The most modern methods are the low-pressure chamber, Dreyer's apparatus, and the respiratory apparatuses of Henderson and Piers. All of them are based on reducing the partial pressure of oxygen: in the low-pressure chamber - by reducing barometric pressure; in Dreyer's apparatus at normal pressure, the percentage of oxygen is reduced by diluting it with nitrogen; in respiratory apparatuses - by the subject using the same air with the removal of CO₂, which reduces its partial pressure as oxygen is absorbed. 'Altitude qualification tests' make it possible to classify subjects according to the maximum altitudes they can tolerate. In the U.S. Army, for example, a classification into four classes is adopted: Class A is not limited in altitude; Class B is limited to approximately 4,700m; Class C to about 2,500m; and Class D is rejected as unfit for flying activities. Classification according to classes meets practical requirements: air combat is conducted at high altitudes (fighters), daytime bombing and reconnaissance at medium altitudes (bombers, reconnaissance planes), and nighttime bombing and reconnaissance at low altitudes. Flying activity, which requires great strain on all psycho-physiological forces of the pilot, under improper work regimen and poor organization of labor, easily leads to fatigue in its various forms, up to severe forms of vascular-neurasthenia, and to a decrease in professional qualities and accidents. Aviation hygiene seeks to classify various forms and degrees of fatigue, establish precise objective methods for their recognition, and determine the causes causing fatigue and overfatigue. These causes are very diverse and may lie in the irrational living conditions of the pilot, improper nutrition, neglect of physical training measures, imperfect construction of the apparatus, inadequate equipment of the aerodrome, poor quality of fuel and lubricants, excessive duration or frequency of flights, unfavorable conditions for their execution, unsuitable work clothing, absence or imperfection of all kinds of protective devices, equipment, and accessories (oxygen apparatuses, mufflers, protective visors, glasses, etc.). Depending on flight conditions, aviation hygiene requires periodic rest for flight personnel after certain periods of flying work. In combat front-line conditions during the world war, the British considered it necessary to provide more or less extended rest after 150-300 'flying' hours. American hygienists attach extremely great importance to systematic physical exercises and light sports for preserving the pilot's strength. Of sports games, handball, tennis, horseback riding, swimming, and golf are recommended. A gymnasium and sports ground are now considered an essential part of a well-equipped aerodrome. To prevent the harmful effects of tiring and sometimes dangerous flights in the upper layers of the atmosphere due to lack of partial oxygen pressure, such flights are provided with oxygen supplies. For this purpose, a number of special apparatuses have been proposed that automatically deliver oxygen to the pilot at any altitude in an amount corresponding to the respective altitude. Protective glasses have significant hygienic importance. The main requirements for them are that they do not restrict the field of vision, do not distort it, prevent wind from blowing in, fit comfortably, and cannot cause harm in case of accident. To prevent unpleasant and painful consequences of engine noise, special ear protectors are used. To protect the head from cold and wind, a leather helmet is used. Clothing should be comfortable, not restrict freedom of movement, provide good protection from cold and wind, and be quick and easy to remove; in recent years, special electric heating devices connected to clothing have been used. In view of the expenditure of energy, which reaches significant dimensions under flying conditions, pilots should be provided with easily digestible and fat-rich food. The design features of the airplane should be the subject of careful study not only by engineers but also by hygienists, since the convenience of sitting and controlling, the design of protective visors, exhaust pipes, etc., have a great influence on the pilot's health and the expenditure of energy.