may progress over time require review by the FAA before a  medical certificate may be issued.  The important thing to remember is that with very few  exceptions, all disqualifying medical conditions may  be considered for special issuance. If you can present  satisfactory medical documentation to the FAA that your  condition is stable, the chances are good that you will be  able to qualify for an Authorization.  A number of health factors and physiological effects can be  linked to flying. Some are minor, while others are important  enough to require special attention to ensure safety of flight.  In some cases, physiological factors can lead to inflight  emergencies. Some important medical factors that a pilot  should be aware of include hypoxia, hyperventilation,  middle ear and sinus problems, spatial disorientation, motion  sickness, carbon monoxide (CO) poisoning, stress and  fatigue, dehydration, and heatstroke. Other subjects include  the effects of alcohol and drugs, anxiety, and excess nitrogen  in the blood after scuba diving.  Hypoxia means “reduced oxygen” or “not enough oxygen.”  Although any tissue will die if deprived of oxygen long  enough, the greatest concern regarding hypoxia during  flight is lack of oxygen to the brain, since it is particularly  vulnerable to oxygen deprivation. Any reduction in mental  function while flying can result in life-threatening errors.  Hypoxia can be caused by several factors, including an  insufficient supply of oxygen, inadequate transportation of  oxygen, or the inability of the body tissues to use oxygen.  The forms of hypoxia are based on their causes:  Hypoxic hypoxia is a result of insufficient oxygen available  to the body as a whole. A blocked airway and drowning  are obvious examples of how the lungs can be deprived of  oxygen, but the reduction in partial pressure of oxygen at high  altitude is an appropriate example for pilots. Although the  percentage of oxygen in the atmosphere is constant, its partial  pressure decreases proportionately as atmospheric pressure  decreases. As an aircraft ascends during flight, the percentage  of each gas in the atmosphere remains the same, but there are  fewer molecules available at the pressure required for them  to pass between the membranes in the respiratory system.  This decrease in number of oxygen molecules at sufficient  pressure can lead to hypoxic hypoxia.  Sublimation is a process in which a substance transitions  from a solid to a gaseous state without passing through  an intermediate liquid state. Dry ice sublimates into large  quantities of CO$_{2}$ gas, which can rapidly displace oxygen- containing air and potentially cause hypoxia via carbon  dioxide intoxication. Case studies have shown that both illness  and death can be caused by occupational and/or unintentional  exposure when transporting dry ice in small, confined  spaces such as a flightdeck or airplane. Exposure to high  concentration of CO$_{2}$ gas may lead to increased respiration,  tachycardia, cardiac arrhythmia, and unconsciousness.  Exposure to concentration of CO$_{2}$ gas in excess of 10 percent  may cause convulsions, coma, and/or death.  The tendency of dry ice to rapidly sublimate also means that  without proper ventilation, it can rapidly pressurize. For  this reason, dry ice should never be placed inside a sealed  transport container (i.e., leak-proof secondary container)  and must be placed within an outer shipping container or  storage container that allows adequate ventilation to release  the CO$_{2}$ gas and avoid pressurization. Sealing dry ice within a  leak-proof container may result in explosion of the container  potentially leading to serious physical injury or death.  Hypemic hypoxia occurs when the blood is not able to take  up and transport a sufficient amount of oxygen to the cells  in the body. Hypemic means “not enough blood.” This type  of hypoxia is a result of oxygen deficiency in the blood,  rather than a lack of inhaled oxygen, and can be caused by  a variety of factors. It may be due to reduced blood volume  (from severe bleeding), or it may result from certain blood  diseases, such as anemia. More often, hypemic hypoxia  occurs because hemoglobin, the actual blood molecule that  transports oxygen, is chemically unable to bind oxygen  molecules. The most common form of hypemic hypoxia is  CO poisoning. This is explained in greater detail later in this  chapter. Hypemic hypoxia can also be caused by the loss  of blood due to blood donation. Blood volume can require  several weeks to return to normal following a donation.  Although the effects of the blood loss are slight at ground  level, there are risks when flying during this time.  Stagnant means “not flowing,” and stagnant hypoxia or  ischemia results when the oxygen-rich blood in the lungs  is not moving, for one reason or another, to the tissues that  Health and Physiological Factors Affecting Pilot Performance  Hypoxia  Dangers of Transporting Dry Ice  Hypemic Hypoxia  Hypoxic Hypoxia  Stagnant Hypoxia  •  Hypoxic hypoxia  •  Hypemic hypoxia  •  Stagnant hypoxia  •  Histotoxic hypoxia  17-3 