Pilot Hypoxia: Aviation Risks at High Altitude

Introduction

The high-altitude environment is hostile to human life and to most other life forms that have not evolved under such conditions. At altitudes above 5,000 feet, atmospheric pressure begins to drop below the levels required for optimal cognitive and physical function. The reduction of function caused by insufficient blood oxygen levels is known as hypoxia, and it exists in several different forms. At low altitudes, the effects on pilots are already demonstrable, although they may only minimally interfere with the ability to fly. As altitude increases, the effects of hypoxia intensify, dramatically reducing a pilot's capacity to perform. Above 10,000 feet, hypoxia becomes a very serious risk to aviators.

In some respects, the mechanism responsible for altitude-induced hypoxia is widely misunderstood, because it is often attributed to the so-called "thinness" of the air at high altitudes. The common assumption is that reduced atmospheric pressure — by virtue of a reduction in the number of air molecules in the column of air above the aircraft — corresponds to reduced oxygen content in the air (Reinhart, 2008). In fact, the proportion of oxygen in air remains constant at approximately 21%, regardless of atmospheric pressure (Jepperson, 2007).

The Physiological Mechanism of Altitude-Induced Hypoxia

Certainly, the number of air molecules does decrease as a function of altitude, corresponding to proportionally fewer molecules of oxygen per unit of air volume. However, the rate at which this reduction in air molecules increases with altitude is only a secondary cause of altitude-induced hypoxia. Rather, the primary cause is the much greater effect that increases in altitude have on the physiological ability of the pilot to absorb oxygen from the air (Jepperson, 2007; Reinhart, 2008).

Within the blood, very little oxygen is actually dissolved directly into blood plasma. It is hemoglobin that absorbs oxygen from the air sacs (alveoli) in the lungs and then transports that oxygen throughout the body's tissues (Reinhart, 2008). In the presence of reduced oxygen content at normal atmospheric pressure, the body can compensate to a certain extent by increasing respiration rates and inhaling more deeply, because the physiological ability of hemoglobin to absorb and transport oxygen remains essentially unaffected (Jepperson, 2007).

However, decreasing atmospheric pressure directly impairs the ability of hemoglobin to absorb oxygen, regardless of how much oxygen is actually available (Jepperson, 2007; Reinhart, 2008). It is precisely this reduced oxygen-absorption capacity of human hemoglobin — and not a decreased proportional molecular composition of air, or even necessarily the absolute amount of oxygen available — that causes hypoxia at altitude.

Hypoxic hypoxia is known as "altitude-induced hypoxia" because it is caused strictly by exposure to high-altitude atmospheric conditions. In principle, hypoxic hypoxia can begin as low as several hundred feet above the ground and is known to affect hikers, mountain climbers, and athletes who play at high-altitude venues such as Mile High Stadium in Colorado (Jepperson, 2007; Reinhart, 2008). In terms of significant effects capable of reducing flight safety, however, altitudes below 5,000 feet are generally considered safe for un-pressurized flight (USDOT, 2003). That said, pilots who routinely operate at approximately 5,000 feet or lower — such as those flying rotary aircraft — are nevertheless susceptible to mild hypoxia, particularly from prolonged atmospheric exposure. Moreover, given the consequences of reduced pilot capacity in the unforgiving environment of flight, there may in many respects be no such thing as "mild" hypoxia.

Types of Hypoxia

Hypemic hypoxia is known as "anemia-induced hypoxia" because it is caused by biological abnormalities in the blood that interfere with the ability of hemoglobin to absorb oxygen, even in the presence of sufficient quantities and at normal atmospheric pressure (Jepperson, 2007; USDOT, 2003). The main factors that cause or contribute to hypemic hypoxia are typically low iron levels (i.e., clinical anemia) and smoking. Cigarette smoke is rich in carbon monoxide, a molecule that bonds to human hemoglobin several hundred times more readily than oxygen does (Jepperson, 2007; USDOT, 2003).

As a result, smokers generally experience hypoxia sooner — in terms of duration of exposure to high-altitude conditions — and at comparatively lower altitudes than non-smokers. The effects of carbon monoxide exposure are so pronounced that even non-smokers who have been in smoke-filled environments will experience reduced resistance to the onset of hypoxia at altitude the following day, due to residual carbon monoxide still bound to their hemoglobin (Jepperson, 2007; USDOT, 2003).

Histotoxic hypoxia refers to hypoxia caused specifically by toxins in the blood that interfere with the ability of hemoglobin to absorb oxygen, even in the presence of sufficient quantities and at normal atmospheric pressure (Jepperson, 2007; USDOT, 2003). Alcohol is the most likely toxin to affect pilots, but other substances such as cyanide, certain narcotics, and various medications — including some available over the counter — can also cause histotoxic hypoxia.

Stagnant hypoxia refers to insufficient oxygen delivery caused by underlying circulatory problems that reduce blood flow, and therefore the efficient transport of oxygen, even when air quality, atmospheric conditions, and hemoglobin oxygen absorption are all normal (Jepperson, 2007; USDOT, 2003).