2-4. Temperature changes in the troposphere can be accurately predicted using a mean-temperature lapse rate of -1.98 degrees Celsius per 1,000 feet. Temperatures continue to decrease until the rising air mass achieves an altitude where temperature is in equilibrium with the surrounding atmosphere. Table 2-1 illustrates the mean lapse rate and the pressure decrease associated with ascending altitude.

2-6. The first subdivision of the stratosphere is termed the isothermal layer. In the isothermal layer, temperature is constant at -55 degrees Celsius (-67 degrees Fahrenheit). Turbulence, traditionally associated with the stratosphere, is attributed to the presence of fast-moving jet streams, both here and in the upper regions of the troposphere.

2-7. The second subdivision of the stratosphere is characterized by rising temperatures. This area is the ozonosphere. The ozonosphere serves as a double-sided barrier that absorbs harmful solar ultraviolet radiation while allowing solar heat to pass through unaffected. In addition, the ozonosphere reflects heat from rising air masses back toward the surface of the Earth, keeping the lower regions of the atmosphere warm, even at night during the absence of significant solar activity.

2-9. Noctilucent clouds are another characteristic of this atmospheric layer. Made of meteor dust/water vapor and shining only at night, these cloud formations are probably due to solar reflection.

2-11. Another characteristic of the thermosphere is the presence of charged ionic particles. These particles are the result of high-speed subatomic particles emanating from the sun. These particles collide with gas atoms in the atmosphere and split them apart, resulting in a large number of charged particles (ions).

Where Pt represents the total pressure of the mixture, PN, PO2, PCO2, … represent the partial pressures of each individual gas, V represents volume, and T represents temperature. To determine the partial pressure of the gases in the atmosphere (or any gaseous mixture whose concentrations are known), the following mathematical formula can be used:

2-20. For the aircrew member, Dalton’s law illustrates that increasing altitude results in a proportional decrease of partial pressures of gases found in the atmosphere. Although the percentage concentration of gases remains stable with increasing altitude, each partial pressure decreases in direct proportion to the total barometric pressure. Table 2-3 shows the relationship between barometric pressure and partial pressure.

2-22. Decreases in the partial pressure of nitrogen, especially at high altitude, can lead to a decrease in the solubility of N₂ in the body. This decrease in N₂ solubility can result in decompression sickness.

2-33. Hemoglobin makes up about one-third of every red blood cell. Composed of several polypeptide chains and iron-containing heme groups, hemoglobin attracts oxygen molecules through an electrochemical magnetic process. Just as opposing poles on a magnet attract, so does the iron content (Fe²⁺) within hemoglobin attract oxygen (O₂^2-).

2-34. When the blood supply is fully saturated with oxygen, as in arterial blood, blood takes on a bright-red color as oxyhemoglobin is formed. As blood passes through the capillaries, it releases oxygen to the surrounding tissues. As a result, deoxyhemoglobin forms and gives venous blood a dark-red color.

2-35. Red blood cells are produced in the red bone marrow. The number of RBCs in circulating blood is relatively stable; however, environmental factors play a large role in determining the actual RBC count. Smoking, an inadequate diet, and the altitude where one lives all contribute to fluctuations in RBC count. In fact, people residing above 10,000 feet may have up to 30 percent more erythrocytes than those living at sea level.

2-48. Under normal conditions, the measure of relative acidity or alkalinity (pH level) within the body is 7.35 to 7.45. During respiration, the partial pressure of carbon dioxide elevates, the acidity level increases, and the pH value lowers to less than 7.3. Conversely, too little CO₂ causes the blood to become more alkaline and the pH value to rise. Figure 2-5 shows how the amount of carbon dioxide in the body affects the pH level of the blood.

2-58. Carbon dioxide and oxygen move in and out of alveoli because of the pressure differentials between their CO₂ and O₂ levels and those in surrounding capillaries. This movement is based on the law of gaseous diffusion: a gas always moves from an area of high pressure to an area of lower pressure. Figure 2-8 illustrates the exchange of CO₂ and O₂ between an alveolus and a capillary.

2-60. Carbon dioxide diffuses from the blood to the alveoli in the same manner. The partial pressure of carbon dioxide (PCO₂) in the venous return blood of the capillaries is about 46 mm/Hg, as compared to a PCO₂ of 40 mm/Hg in the alveoli. As the blood moves through the capillaries, the CO₂ moves from the high PCO₂ in the capillaries to an area of lower PCO₂ in the alveoli. The CO₂ is then exhaled during the next passive phase (exhalation) of respiration.

Note: The exchange of O₂ and CO₂ between tissue and capillaries occurs in the same manner as it does between the alveoli and capillaries. Figure 2-9 shows the exchange between tissue and a capillary.

2-69. Aviation personnel commonly experience mild hypoxia at altitudes at or above 10,000 feet. Those who fly must be able to recognize the possible signs and symptoms. Being able to recognize these signs and symptoms is particularly important because the onset of hypoxia is subtle and produces a false sense of well-being. Crew members are often engrossed in flight activities and do not readily notice the symptoms of hypoxia. Usually, however, most individuals experience two or three unmistakable symptoms or signs that cannot be overlooked. Figure 2-14 lists the signs and symptoms.

2-82. The expected performance time is from the interruption of the oxygen supply until the crew member loses the ability to take corrective action. Table 2-5 shows that the EPT varies with the altitude at which the individual is flying. An aircrew flying in a pressurized aircraft that loses cabin pressurization, as in rapid decompression, has only one-half of the EPT shown in Table 2-5.

2-94. Hypoxic hypoxia can be prevented by ensuring that sufficient oxygen is available to maintain an alveolar partial pressure of oxygen (PAO2) between 60 and 100 mm/Hg. Preventive measures include—

• Limiting the time at altitude.
• Using supplemental oxygen.
• Pressurizing the cabin.

2-95. During night flights above 4,000 feet, crew members should use supplemental oxygen when available. Supplemental oxygen is necessary because of the mild hypoxia and loss of visual acuity that occur.

2-96. The amount, or percentage, of oxygen required to maintain normal oxygen saturation levels varies with altitude. At sea level, a 21 percent concentration of ambient air oxygen is necessary to maintain the normal blood oxygen saturation of 96 to 98 percent. At 20,000 feet, however, a 49 percent concentration of oxygen is required to maintain the same saturation.

2-97. The upper limit of continuous-flow oxygen is reached at about 34,000 feet. Above 34,000 feet, positive pressure is necessary to maintain an adequate oxygen saturation level. The positive pressure, however, cannot exceed 30 mm/Hg because—

• Normal oxygen masks cannot hold positive pressures of more than 25 mm/Hg without leaking.
• Excess pressure may enter the middle ear through the eustachian tubes and cause the eardrum to bulge outward, which is painful.
• Crew members encounter difficulty in exhalation against the pressure, resulting in hyperventilation.

2-98. Pressurization, as found in the C-12 aircraft, can prevent hypoxia. Supplemental oxygen should be available in the aircraft in case of pressurization loss.

2-99. The prevention of hypoxic hypoxia is essential in the aviation environment. There are, however, other causes of hypoxia. Carbon monoxide uptake (hypemic hypoxia), the effects of alcohol (histotoxic hypoxia), and reduced blood flow (stagnant hypoxia) are also hazardous. Avoiding or minimizing self-imposed stressors helps eliminate hypoxic conditions.

• Dizziness.
• Muscle spasms.
• Unconsciousness.
• Visual impairment.
• Tingling sensations.
• Hot and cold sensations.

The signs and symptoms of hyperventilation and hypoxia are similar, making them difficult to differentiate. The indications given below help to distinguish between the two.

2-112. Although it is difficult, an individual affected by the symptoms of hyperventilation should try to control the respiration rate; the normal rate is 12 to 16 breaths per minute. To treat hyperventilation, the aviator should control breathing and go to 100 percent oxygen. If symptoms continue and conscious control of respiration is not possible, the individual should talk or sing. It is physiologically impossible to talk and hyperventilate at the same time. Talking or singing will elevate the CO₂ level and help regulate breathing.

2-113. When hypoxia and hyperventilation occur concurrently, a decrease in the respiratory rate and the intake of 100 percent O₂ will correct the condition. If hypoxia is severe, the aviator must return to ground level before becoming incapacitated.

2-115. Dysbarism refers to the various manifestations of gas expansion induced by decreased barometric pressure. These manifestations can be just as dangerous, if not more so, than hypoxia or hyperventilation. The direct effects of decreased barometric pressure can be divided into two groups: trapped-gas disorders and evolved-gas disorders.

2-129. Temporal Bone and Jaw Problems. Although upper respiratory infections are the main causes of narrowing of the eustachian tube, there are other causes. Crew members with malposition of the temporomandibular joint (temporal bone and jaw) may have ear pain and difficulty both in ventilating the middle ear and in hearing. In these cases, movement of the jaw (or yawning) relaxes surrounding soft tissues and clears the opening of the eustachian tube.

Note: To avoid overpressurization of the middle ear, crew members should never attempt a Valsalva maneuver during ascent.

2-131. After Flight. If middle-ear and ambient pressures have not equalized after landing and the condition persists, aviation personnel should consult a flight surgeon because barotitis media can occur. This is an acute or chronic traumatic inflammation of the middle ear caused by a difference of pressure on opposite sides of the eardrum. It is characterized by congestion, inflammation, discomfort, and pain in the middle ear and may be followed by temporarily or permanently impaired hearing, usually the former.

2-141. Tissues and fluid of the body contain from 1 to 1.5 liters of dissolved nitrogen, depending on the pressure of nitrogen in the surrounding air. As altitude increases, the partial pressure of atmospheric nitrogen decreases and nitrogen leaves the body to reestablish equilibrium. If the change is rapid, recovery of equilibrium lags, leaving the body supersaturated. The excess nitrogen diffuses into the capillaries in solution and is carried by the venous blood for elimination. With rapid ascent to altitudes of 30,000 feet or more, nitrogen tends to form bubbles in the tissues and in the blood. In addition to nitrogen, the bubbles contain small quantities of carbon dioxide, oxygen, and water vapor. Additionally, fat dissolves five or six times more nitrogen than blood. Thus, tissues having the highest fat content are more likely to form bubbles.

2-147. Bends. At the onset of bends, pain in the joints and related tissues may be mild. The pain, however, can become deep, gnawing, penetrating, and eventually, intolerable. The pain tends to be progressive and becomes worse if ascent is continued. Severe pain can cause loss of muscular power of the extremity involved and, if allowed to continue, may result in bodily collapse. The pain sensation may diffuse from the joint over the entire area of the arm or leg. In some instances, it arises initially in muscle or bone rather than in a joint. The larger joints, such as the knee or shoulder, are most frequently affected. The hands, wrists, and ankles are also commonly involved. In successive exposures, pain tends to recur in the same location. It may also occur in several joints at the same time and worsens with movement and weight bearing. Coarse tremors of the fingers are often noted when the bends occur in joints of the arm.

2-148. Chokes. Symptoms occurring in the thorax are probably caused, in part, by innumerable small bubbles that block the smaller pulmonary vessels. At first, a burning sensation is noted under the sternum. As the condition progresses, the pain becomes stabbing and inhalation is markedly deeper. The sensation in the chest is similar to one that an individual experiences after completing a 100-yard dash. Short breaths are necessary to avoid distress. There is an almost uncontrollable desire to cough, but the cough is ineffective and nonproductive. Finally, there is a sensation of suffocation; breathing becomes more shallow, and the skin turns bluish. When symptoms of chokes occur, immediate descent is imperative. If allowed to progress, the condition leads to collapse and unconsciousness. Fatigue, weakness, and soreness in the chest may persist for several hours after the aircraft lands.

2-149. Paresthesia. Tingling, itching, cold, and warm sensations are believed to be caused by bubbles formed either locally or in the CNS where they involve nerve tracts leading to the affected areas in the skin. Cold and warm sensations of the eyes and eyelids, as well as occasional itching and gritty sensations, are sometimes noted. A mottled red rash may appear on the skin. More rarely, a welt may appear, accompanied by a burning sensation. Bubbles may develop just under the skin, causing localized swelling. Where there is excess fat beneath the skin in the affected region, soreness accompanied by an abnormal accumulation of fluid may be present for one or two days.

2-150. Central Nervous System Disorders. In rare cases when aircrews are exposed to high altitude, symptoms may indicate that the brain or the spinal cord is affected by nitrogen-bubble formation. The most common symptoms are visual disturbances such as the perception of lights as flashing or flickering when they are actually steady. Other symptoms may be a dull-to-severe headache, partial paralysis, the inability to hear or speak, and the loss of orientation. Paresthesia or one-sided numbness and tingling may also occur. Hypoxia and hyperventilation may cause similar numbness and tingling; however, these are bilateral—they occur in both arms, legs, or sides. CNS disorders are considered a medical emergency; if they occur at high altitude, immediate descent and hospitalization are indicated.

• Denitrogenation.
• Cabin pressurization.
• Limitation of time at high altitude.
• Aircrew restrictions.

• Descend to ground level immediately.
• Place the affected individual on 100 percent oxygen to eliminate any additional nitrogen uptake and to remove excessive nitrogen from the system.
• Immobilize the affected area to prevent further movement of nitrogen bubbles in the circulatory system.
• Report to the flight surgeon or to the best medical assistance available.
• Undergo compression therapy in a hyperbaric chamber if symptoms persist and when prescribed by a flight surgeon.