The Environment of Flight

I am always amazed when I ask a class of experienced jet pilots undergoing recurrent training how many have been through the FAA's physiological training program. Usually less than half will have taken advantage of this valuable opportunity. We spend many hours in our carefully conditioned cockpits, only inches from an environment that is incompatible with life. We sit in shirtsleeves, comfortably oblivious to the frigid and rare atmosphere through which our fragile shell moves. Rarely do we think about the temperature outside or the time available to react should our carefully maintained environment fail.

Jet aircraft are designed to operate efficiently at high altitudes but the human body is not. Humans are land animals, evolved to exist comfortably close to sea level at a maximum speed (and that for only very short sprints) of little more than 15 miles per hour. Anything else is a foreign, and potentially lethal, environment. Any time we operate above the altitude of acclimatization (the altitude where we normally live), risks exist. No matter how you perceive your performance (and despite all the bravado and tough war stories) the body will still respond to the atmosphere in which it is operating and be affected by gas concentrations and ambient pressures.

The atmosphere is an envelope of air that surrounds and rotates with the earth to an altitude of about 25,000 miles. This envelope is constantly changing, as each weather briefing will reaffirm, and temperatures and pressures fluctuate from day to day and from region to region. From a biological point of view, however, the atmosphere can be considered a constant in relation to its effect on the human physiology.

Although it is often said that the air is "thin" at altitude and lacks the oxygen necessary to sustain life, the actual composition of the atmosphere remains constant throughout the altitude range. The percentage of oxygen in the air is constant at 21 percent, but the actual number of oxygen molecules per unit volume of air decreases with pressure and consequently with altitude. There will always be a constant 21% oxygen available, but it is analogous to the difference between 21% of one dollar and 21% of one cent. The percentage is the same but the value is vastly different. The remaining 79 percent of the composition of the atmosphere is principally nitrogen (78%) with carbon dioxide (0.3%), inert gases (1%) and water vapor.

Physical Characteristics of the Atmosphere

Pressure: Atmospheric pressure is really just the weight of all the air molecules in the atmosphere above the point where the measurement is made. Since there are fewer molecules in the column above the measurement point at higher altitudes, you can see that atmospheric pressure decreases with increasing altitude. Since air is compressible, the atmosphere will be denser near the surface (the bottom of the column) and increases of pressure are greater nearer the surface. Conversely, the pressure decreases less rapidly the higher in the atmospheric column one rises. The greatest density change occurs between sea level and 5,000 feet; therefore, the problems associated with pressure and density change must be considered even in pressurized aircraft.

There are a variety of properties of the atmosphere that will change this pressure besides its weight. Seasonal temperature changes, weather systems, latitude and longitude, and time of day all effect atmospheric pressure. The International Standard Atmosphere (ISA) is a baseline from which to work. We all learned this in private pilot ground school. This standard establishes a mean atmospheric pressure of 29.92 inches (760 mm) of mercury at a temperature of 15°C (59°F) in dry air at sea level. This is also expressed as 14.7 pounds per square inch (PSI) or 1013.2 millibars (MB) at the same temperature.

Temperature: The surface of the earth is warmed by solar radiation that is then reflected back into the atmosphere. This direct and reflected solar radiation does little to heat the atmosphere directly. Instead, the air is heated by the warmth of the earth, and consequently the temperature of the atmosphere decreases with increasing altitude ... until the tropopause is reached at about 35,000 feet. After this point the temperature remains relatively constant. The decrease in temperature, or lapse rate, is for dry air and is 3.56°F (1.98°C) per thousand feet of altitude.

The mixture of gases in the atmosphere is subject to several laws of physics governing the behavior of gases. An understanding of these laws will help in the understanding of the effects of altitude and these gases in the body.

Dalton's Law tells us that the total pressure of any mixture of gases (with constant temperature and volume) is the sum of the individual pressures (also called partial pressure) of each gas in the mixture. Also, partial pressure of each gas is proportional to that gas's percentage of the total mixture. Because the percentage of oxygen in the atmosphere remains constant at 21%, Dalton's Law lets us calculate the partial pressure of the oxygen in the atmosphere at any altitude.

As we'll see shortly, the human body is affected by the pressure of the gases in the atmosphere. The partial pressure of oxygen (and to a lesser extent other gases) available in the surrounding air is important in determining the onset and severity of hypoxia.

Henry's Law states that the amount of gas dissolved in a solution is proportional to the partial pressure of the gas over the solution. A bottle of carbonated liquid demonstrates Henry's Law. When the bottle is uncapped, the carbon dioxide (CO2) in the mixture will slowly diffuse to the atmosphere until the pressure of CO2 in the liquid equals the pressure of CO2 in the surrounding air. The soda will then be "flat." A bottle of soda opened in an unpressurized aircraft at 10,000 feet will foam and overflow. The opposite will happen with soda opened at pressures greater than one atmosphere. A champagne cork won't pop in a diving bathysphere pressurized for deep ocean exploration.

Boyle's Law states that the volume of a gas is inversely proportional to the pressure on the gas as long as the temperature remains constant. A gas will expand when the pressure on it is decreased. This law holds true for all gases, even those trapped in body cavities. A volume of gas at sea level pressure will expand to approximately twice its original volume at 18,000 feet, nearly nine times its original volume at 50,000 feet.

Graham's Law tells us that a gas at higher pressure exerts a force toward a region of lower pressure. There's a permeable or semi-permeable membrane separating the gases, and gas will diffuse across the membrane from the higher pressure to the lower pressure. This will continue until the pressure of the gas is equal, or nearly equal, on both sides of the membrane. Graham's Law is true for all gases and each gas in a mixture behaves independently. It's possible to have two or more gases in a solution diffusing in opposite directions across the same membrane and, in fact, this is what happens to make oxygen transfer possible in the cells and tissues of the human body.

How Does Altitude Affect Your Body?

Okay, so much for the physics lesson. How does this all affect me? When we talk about the effects of altitude upon the human body and altitude sicknesses we tend to think in terms of "high altitude" and classify that as somewhere in the flight levels. Not always so. Your body will be adversely affected by prolonged exposure to any altitude above that at which you've been living. A resident of coastal California will not perform nearly as efficiently at any task in Denver, where the ground is a mile high, as will a native Denverite, but most would not consider being on the ground at Denver as being at "high altitude." Your body and brain, however, will have a different perspective on the matter. The discussion that follows applies to all pilots -- jet jocks, helicopter pilots, recreational pilots and hang-glider aficionados.

How the Human Body Uses Oxygen

Let's start our discussion of the effects of altitude on our bodies by first discussing how the body normally acquires, transports and uses oxygen. The importance of all those gas laws should become clearer. We're all aware that oxygen is necessary to sustain combustion or oxidation. It is necessary in the human body for the same reasons -- to support the oxidation of fuels needed to provide energy for life.

Very little of the oxygen carried by the blood is carried in dissolved form in the plasma. Most of the oxygen -- almost 98% -- is transported by the hemoglobin molecules in the red blood cells. The ability of hemoglobin to combine with and transport oxygen is dependent upon the pressure of oxygen in the surrounding environment. Higher pressures of oxygen enable the hemoglobin to take up larger quantities of oxygen. Lower oxygen pressures will result in an increasing tendency by the hemoglobin to give up oxygen. This variable combining characteristic is what allows the blood to acquire oxygen in the lungs and transport it to the tissues where it is used in metabolism. This characteristic of the hemoglobin also results in what is known as the oxygen dissociation curve (see graph below). While we've seen that oxygen pressure decreases a bit less than linearly with altitude, the ability of the hemoglobin to hold oxygen follows a much different curve. There is a big change for the worse in the hemoglobin's ability to combine with oxygen that occurs in the low twenties.

Air entering the lungs at sea level enters at a pressure of 760 mm Hg. This results in a partial pressure of oxygen in sea level air of about 160 mm Hg. (that's about 21% of 760 mm). The blood flowing through the lungs isn't exposed to atmospheric air though. Blood comes in contact with alveolar air -- the air mixture contained in the tiny air sacks of the lungs -- which is only 14% oxygen. (This is because of the addition of water vapor to the air you breath in plus the carbon dioxide that has diffused from the blood returning from the tissues.) The partial pressure of oxygen in alveolar air is about 14% of 760 mm Hg or 106.4 mm Hg. Carbon dioxide, which is 5.5% of alveolar air (as contrasted to less than 1% in the atmosphere) exerts a pressure of 41.8 mm Hg.

The hemoglobin in the blood returning from the tissues carries oxygen at a pressure of about 40 mm Hg. Graham's Law governs the diffusion of oxygen from the higher pressure of the alveolar air to the blood and the diffusion of carbon dioxide from the blood to the alveolar sacks. The opposite transfer takes place when the oxygen rich blood reaches the tissues which carry oxygen at an average pressure of 20 mm Hg. This lower pressure will allow the hemoglobin to release oxygen which will then diffuse into the tissues. At the same time, carbon dioxide is diffusing from the tissues into the blood. (An average pressure for CO in the tissues is 50 mm Hg.; however, this is dependent upon the activity level of the tissue.) Getting hypoxic yet from all this high altitude discussion??

In a normal, healthy individual, sea level pressure is sufficient to cause the blood leaving the lungs to be almost totally (97%) saturated with oxygen. At 10,000 feet the saturation has dropped to almost 90% -- still sufficient for nearly all usual life functions. An oxygen saturation of 93% is considered by medical folks to be the low limit of normal functioning. On top of Pike's Peak (about 14,500 feet and 438 mm Hg atmospheric pressure) the oxygen saturation has dropped to about 80%. Many people, if left in this rarefied air for some period, will develop mountain or altitude sickness: vertigo, nausea, weakness, hyperpnea (increased breathing), incoordination, slowed thinking, dimmed vision and increased heart rate. At 25,000 feet the oxygen saturation is only 55% and consciousness is lost. (Note that the partial pressure of oxygen in alveolar air at 25,000 feet is 14% of 281.8 mm Hg or 39.5 mm Hg -- slightly less than that normally found in venous blood returning from the tissues. Which way do you think the oxygen will diffuse at altitudes above 25,000 feet?)

Nowadays, altitude-savvy pilots are starting to carry a tiny instrument called a pulse oximeter that clips on the finger and, by passing a light beam through the vascular bed of the fingertip, measures the oxygen saturation of the blood and displays it on a digital readout. Think of it as a "hypoxia meter" that allows you to see precisely how hypoxic you are at any given time.

Types of Hypoxia

The effects of hypoxia upon flying skills and the symptoms of its onset are the same no matter what the cause of the hypoxia. It is useful, however, to look at some varying causes of this condition so we can be alert to its poossible onset when of one or more of these factors is present.

Hypoxic hypoxia is also referred to by aviators as "altitude hypoxia." This is the hypoxia that results when there is a lack of available oxygen or partial pressure of oxygen in the breathing air. This is the type hypoxia experienced when flying in an unpressurized cabin or when flying at altitude in a jet with a cabin pressurized to a cabin altitude above 5000 feet. Although strictly speaking, we are somewhat hypoxic when operating even a few hundred feet above the altitude of acclimatization, this becomes most evident when flying unpressurized aircraft. In reality, the symptoms of hypoxic hypoxia do not, in the absence of other contributing factors, become significant until about 5000 feet.

Hypoxic hypoxia occurs because there is a smaller and smaller pressure differential between the pressure of oxygen in the inspired air in the lungs and the pressure of the oxygen in the blood and tissues. Remember that the combining power of hemoglobin and oxygen is influenced by this pressure differential. The greater the differential, the more efficient the hemoglobin becomes. As this pressure differential lessens, it becomes harder and harder for the hemoglobin to pick up and transport the oxygen.

Hypemic hypoxia (also calle anemic hypoxia) occurs whenever the blood's ability to carry oxygen is reduced although there is sufficient oxygen at a sufficient pressure in the inspired air. There are a variety of conditions that can cause this to happen.

Any condition that would cause a reduction in the number of healthy, functioning red blood cells (anemia or reduced production of red blood cells, blood loss, deformed blood cells, disease, etc.) will impair the blood's ability to supply the tissues with oxygen. Remember the old advertisements warning about "iron poor blood?" Iron is the functional part of the hemoglobin molecule and it is the iron which renders the hemoglobin absolutely indispensable for life. In addition to a reduction in the number of red blood cells available, anything that would interfere with the ability of hemoglobin to transport oxygen or anything that would displace the oxygen that is bound to the hemoglobin will affect the oxygen available to the cells.

The most common impairment to oxygen transport by the hemoglobin is carbon monoxide. Carbon monoxide combines with hemoglobin 200-300 times more readily than does oxygen and once bound is extremely hard to eliminate. Smokers will find that the carbon monoxide bound to their hemoglobin will lower their altitude for onset of hypoxic symptoms by 2000-3000 feet. This effect is not limited to smokers, however. Anyone exposed to a smoky atmosphere will suffer somewhat. (Remember this next time you volunteer to go along as a designated driver for a group of drinkers. Just sitting in that smoky bar for several hours is going to affect your performance the next day, even without alcohol and fatigue!) Other chemicals, among them sulfa drugs and nitrites (found in food preservatives) can have an adverse effect on the ability of hemoglobin to combine with and transport oxygen.

Histotoxic hypoxia is a disruption of cellular respiration. There may well be sufficient oxygen of sufficient pressure in the inspired air to fully saturate the blood and hemoglobin, but the cells expecting and needing the oxygen are unable to use it due to the presence or absorption of cell toxins. The most common toxin found at the cellular level that can cause this effect is alcohol. Although other toxins, notably cyanide and some narcotics, also can cause this disruption of cellular respiration, alcohol is by far the most common culprit.

Now, we are all aware of the hazards associated with alcohol and flying and I'm not suggesting that any true professional would knowingly violate these rules and guidelines. Many pilots, however, may be impaired by alcohol at the cellular level and not be aware of the problem -- or its cause. Remember the iron poor blood mentioned earlier? Be cautious of the "tonics" or "elixirs" offered as remedies. Carefully read the labels on any over-the-counter medications or nutritional supplements you propose to ingest. Although many more manufacturers are eliminating or reducing the alcohol content of the liquid medications, you may be surprised at the percentage of alcohol some still contain. One popular vitamin supplement for "iron poor blood" contains 12% alcohol!

The Effects of Hypoxia at Various Altitudes

Hypoxia is an insidious and progressive condition and is almost undetectable by the pilot. You should always be aware that without supplemental oxygen at sufficient pressure you will gradually and progressively lapse into incompetence while maintaining an absolutely euphoric faith in your own ability.

As blood saturation of oxygen drops there is a steady disruption of life functions. From a blood saturation of 93%, considered the low limit of normal functioning, where visual problems begin to occur, there is a rapid deterioration into unconsciousness with decreasing saturation.

Keep in mind, as mentioned earlier, that although the partial pressure of oxygen in the atmosphere decreases somewhat less than linearly with increasing altitude, the hemoglobin's ability to combine with this oxygen follows a much different and more deadly curve. This property of hemoglobin bears heavily on oxygen requirements at altitude. Let's look at some common hypoxic effects an average, healthy individual can expect with increasing altitudes.

5000 feet. This is considered by most to be a "low" altitude. The retina of the eye is more demanding of oxygen than any other organ of the body -- even the brain itself which demands 30% of the supply. At this "low" altitude this extension of the brain will begin to suffer degradations in function which will be most noticeable at this point in night vision.

Instruments and maps are more easily misread during night flight at this altitude and ground features and lights are more easily misinterpreted. It is always an eye-opening shock to my students when I bring them in over the Mojave Desert after a long flight at a cabin altitude of 8000 feet. After having them note the discernible features on the dark desert floor, I have them breath 100% oxygen for a few minutes. Without exception, all are shocked and amazed to note the features that "jump" out of the blackness after a little O2. Most have heard of this little demonstration before it actually takes place, but none are totally prepared for its dramatic effect. This level of hypoxia is extremely insidious because most pilots feel they are functioning at peak efficiency at this point. Extra vigilance is necessary to prevent missing critical fixes on charts or misreading instruments.

10,000 feet. Night vision is now degraded by 15-25 percent. The blood saturation has dropped to 90 percent and your brain is receiving the absolute minimum supply of oxygen. This is the absolute highest altitude at which you should have any trust at all in your own performance although your judgment is already severely compromised. Euphoria will prevent true self-assessment of your abilities. Physical hypoxic symptoms such as tingling and headache may not become apparent for four hours or more at this altitude, although judgment has long gone by the wayside. Above 10,000 feet blood oxygen saturation and performance degrade steeply.

14,000 feet. Blood oxygen saturation is down to a dangerous 85%. You will be increasingly disabled at this altitude. Vision will dim. You will experience serious degradation of judgment, memory and thought. The impairment of judgment will leave you feeling just fine and confident in your performance, however. If hypoxia is not recognized and corrected at this stage of impairment, it is unlikely that it will be recognized. You are in serious danger.

16,000 feet. Only 2,000 feet higher than the last assessment, but you will behave as though you had ingested a full load of gin and tonics. Your blood oxygen saturation will have dropped to 79 percent and you will be seriously disabled. You will be euphoric, belligerent, disoriented or perhaps all three. You will be irrational, unreliable and dangerous. If you are alone, your chances of survival are decreasing rapidly.

18,000 feet. At this altitude you are incapable of any useful function although you may still feel great! Blood saturation has fallen to 71 percent and your brain is suffering. You will pass out in about 30 minutes.

20,000 feet. If you have not already collapsed, it will not be long now. Five to 15 minutes is about the time of useful consciousness at this altitude and prolonged exposure can result in death. Blood saturation has dropped to 71%.

25,000 feet. Forget it! Blood saturation has now dropped to lethal levels. Time of useful consciousness is three to six minutes with death following not long after that. Above this altitude, suffering a rapid decompression may also result in a condition divers know as the bends and various other pressure related maladies. Remember, this is only HALF as high as some modern civilian aircraft are certified to fly!

What Determines Your Response to Hypoxia?

It is impossible to tell exactly when hypoxic reactions will begin to affect you. Individual reactions to hypoxia vary greatly not only among people, but in the same person on a day to day basis due to differences in body chemistry, general health and diet. Some of the determining factors are somewhat under the pilot's control and some are dictated by the flying environment itself.

Absolute Altitude. This is the easy one. The severity of hypoxia will depend directly upon the absolute altitude of the environment you're in. This may be the aircraft altitude in an unpressurized cabin or the cabin altitude in pressurized craft. As the altitude of the environment climbs and the partial pressure of the oxygen in that atmosphere drops, the risk of hypoxia rises. Acclimatization (to normal living altitudes) can help only to a limited extent. The Denver folks may have an advantage up to about 15,000 feet, but after that everyone is more or less equal -- equally impaired. This factor is usually somewhat controlled by the pilot; however, mountains or weather can cause a climb to a previously unplanned altitude. This is of little consequence in a pressurized aircraft.

Acclimatization. One question I am often asked concerns the permanent residents in the Andes above 17,000 feet. How do these people remain conscious, much less do work? The answer is acclimatization. The Peruvians who live in these villages have extremely high red corpuscle counts and so have much more hemoglobin with which to transport oxygen. These residents also have much larger pulmonary ventilation volumes and increased cardiac output. Some acclimatization shown by this population is reversible -- they will be lost with acclimatization to a lower altitude and consequently a higher atmospheric pressure. Other of their adaptations, however, seem to be permanent evolutionary responses to life at lowered atmospheric pressures, are not lost with residence at lower altitudes and are passed on genetically to subsequent generations.

The native Denverites we spoke of earlier will have more tolerance to altitude and become hypoxic later and at a higher altitude than coastal pilots because they are already partially acclimated to altitude by virtue of their residence in the Mile High City. They are only 5000 feet above their physiologically adapted altitude at 10,000 feet. The LA pilots are 10,000 above theirs. (Then again, there are some who claim that those of us in LA are permanently in the ozone, but that is probably the topic of a different discussion.)

Rate of Ascent. The quicker you (or your environment) climb, the more rapid the onset of hypoxic symptoms. The climbers who ascend Mt. Everest are well aware of this phenomenon. They spend several weeks in their climb, stopping at several intermediate altitudes to acclimate. An explosive decompression in an aircraft with the resultant rapid climb of the cabin altitude can reduce the time of useful consciousness to one-third to one-half of that normally expected. A rapid ascent can cause the symptoms of hypoxia to quickly accumulate and incapacitate a pilot before awareness of the encroaching disability dawns on the dimming consciousness.

Duration of Exposure. Staying at 8,000 feet for several hours (not uncommon in a jet at flight level cruising altitudes) can cause the same symptoms and incapacitation as staying at a higher altitude for a shorter duration. The symptoms of hypoxia are cumulative and time related, but there is no reliable means to predict the exact relationship or effect. The only certainty is that the higher the altitude the shorter the time of exposure before symptoms begin to occur. This, too, will vary on an individual basis.

Physical Activity. Any physical activity will obviously cause the body to demand more oxygen for normal functioning. The muscles will rob the brain of the marginal amounts of oxygen available in the blood and the time of onset of hypoxic symptoms will be shortened. Although not much physical activity is expended by pilots, the extra amount required to fly in turbulence or with a failed autopilot can dramatically reduce the already minimal oxygen supply to the brain and the retinas of the eyes. This factor is usually not under the pilot's control.

Temperature. The temperature in the cabin has a great affect on an individual's tolerance for and response to hypoxia. Either extreme -- the cold of a cockpit at altitude at night with a failed heating system or the greenhouse environment of a poorly air conditioned pressurized aircraft at high noon -- will cause the body to expend energy in an attempt to maintain its core temperature within acceptable limits. This expended energy is just another form of increased physical activity and will decrease a pilot's tolerance to hypoxic conditions.

Self-Imposed Factors. Pilots vary widely in their susceptibility to oxygen deficiency and the same person will show variances from day to day. This is primarily due to many factors influencing susceptibility to hypoxia which are under direct control of the pilot. It is the pilot's responsibility to avoid these factors as much as possible. Your tolerance of oxygen deficiency will be reduced by any one or more of the following factors. The affect of combining these factors cannot be accurately assessed.

• Fatigue. Fatigue is both a exacerbating factor of and a symptom of hypoxia. A mentally or physically fatigued pilot will have less tolerance for hypoxia and its associated decrements in performance and perception because the fatigue will already have degraded performance, perhaps to unacceptable levels. Hypoxia will deepen the fatigue and the cycle continues on a downward spiral of increasing fatigue and degradation of performance. Also, there is a chance that the pilot will not attribute increasing fatigue to the effects of hypoxia and will not take prompt corrective actions.
   
• Alcohol. This cellular toxin is a risk factor even after the blood alcohol level has returned to zero. Of course, as we noted above, any alcohol in the blood or cells will hamper their ability to take up and utilize oxygen. One ounce of alcohol in the blood will raise the body's perceived altitude by 2,000 feet. However, the aftereffects of the alcohol can be just as debilitating. The fatigue caused by the disturbance of normal sleep cycles by alcohol will diminish tolerance to hypoxia as discussed above. Additionally, the depressant effect of this drug will remain after the toxin is cleared from the blood. This will cloud the pilot's judgment and delay the recognition of the problem.
   
• Carbon Monoxide. Again, it is important to remember that carbon monoxide (CO) will combine with hemoglobin in the red blood cells 200-300 times more readily than will oxygen. Once bound it is almost impossible to rid the red blood cells of their cargo of this toxin -- in fact, most carbon monoxide remains bound to the red cells until the cells die and are scavenged by the liver. Of course, the major source of carbon monoxide in the blood stream is cigarette smoke -- either your own or second hand smoke from other smokers in the vicinity.

Probably no other self-imposed risk factor is as deadly -- or as controllable -- as is the CO level in the blood. Smoking a pack of cigarettes in the 24 hours preceding a flight can saturate as much as 8-10 percent of the available hemoglobin. This will raise the body's perceived altitude by as much as 5,000 feet! You can be effectively in Denver on the ground at LAX. You will suffer the effects of hypoxia at sea level. An 8,000 foot cabin of a jet cruising in the flight levels is going to be just the same as cruising unpressurized at 13,000 feet. Is it any wonder that most accidents happen in the landing phase? One wonders how much hypoxia contributes to the accident rate yet is rarely listed as a cause -- especially in modern jet transports.

Cabin Decompression -- Explosive and Otherwise

Our fragile cocoon -- those sheets of aluminum and plexiglass that give us the false sense of security as we cruise in shirtsleeves at FL 430 -- can fail us. It is not a thought that most pilots choose to dwell on, but the possibility is certainly there. Although structural failure like the one experienced by the Aloha Airlines 737 would certainly cause a sudden and explosive decompression, a more likely cause would be the failure of a door seal or a cracked cabin window or pressure bulkhead. Explosive decompression and emergency descent drills are practiced at least once a year by all jet pilots during recurrent training and I am often amazed at the casual attitude shown towards these emergency situations. The actuality will not bear any resemblance to the neat and orderly drill accomplished in training.

An actual decompression will first of all be accompanied by a good deal of noise as the higher pressure air in the cabin rushes out until the cabin pressure, and consequently the cabin altitude, equals the ambient pressure outside the aircraft. This may be preceded by a loud popping sound -- the sound of a champagne cork magnified 100 times. Dust and debris will be picked up and rush toward the opening where the pressurized air is rushing out of the cabin. Smaller items may be sucked outside the aircraft. A fog will form in the cabin since the warmer air in the cabin is capable of carrying more moisture than the cold air outside the aircraft. As the cabin temperature and pressure drop the moisture will condense forming a wet, cold fog. A significant temperature change will occur -- the ambient temperature at FL 430 is -67° F. Confusion will reign. And these are only the effects on the atmosphere inside the craft -- what of the effects on the humans?

First of all, there is a distinction between rapid and explosive decompressions. Any decompression that takes place in less than one-half second is considered to be an explosive decompression. This type of decompression occurs primarily in smaller-bodied aircraft and, thankfully, is not common since it can be rapidly fatal. Human lungs usually require about 0.2 seconds to release their air. Any decompression happening in less than this short amount of time can result in rapid lung decompression and rupture or severe damage. The only emergency procedure available for dealing with this type decompression is to immediately get on oxygen and get the aircraft to a lower altitude as quickly as possible.

A rapid decompression is one that occurs in more than one-half second, but less than about ten seconds. This is the type decompression experienced in larger-bodied aircraft and is the more common of the two. There is not the high potential for lung damage in this type decompression, however, the noise, confusion, debris and fogging will all be present in varying degrees of severity. Rapid donning of oxygen and descent is still necessary, but other emergency measures may also be available to deal with the situation and lessen the impact of the decompression.

Subtle decompression is also a danger in pressurized aircraft. A gradual loss of cabin pressure (or, more commonly, improper setting of the cabin altitude controls) and an slowly rising cabin altitude may not be recognized by the crew in time to deal with the emergency effectively. In all jet aircraft certified to transport category standards, FAR 25.841 requires "Warning indication at the pilot or flight engineer station to indicate when the safe or preset pressure differential and cabin pressure altitude limits are exceeded. ... and an aural or visual signal (in addition to cabin altitude indicating means) meets the warning requirement for cabin pressure altitude limits if it warns the flight crew when the cabin pressure altitude exceeds 10,000 feet." In addition, these aircraft are required to have means to limit the cabin altitude to no more than 15,000 feet in the case of a failure of components in the pressurization system. This cabin altitude limitation, however, does not refer to structural failures, but only to the components of the pressurization system itself.

Time of Useful Consciosness. The time of useful consciousness is the time your brain is awake enough to be useful and make decisions. This varies from almost indefinite at 10,000 to 9 to 12 seconds above 40,000 feet. An explosive or rapid decompression will cut this time in half due to the startle factor and the accelerated rate at which an adrenaline soaked body burns oxygen.

How Does Decompression Affect the Body?

Of course, the most serious effect of decompression is the resultant hypoxia and more or less rapid loss of effective consciousness. The most noticeable immediate effect of rapid cabin decompression will be the sudden rush of air from the lungs. As noted above, this can be a near fatal experience in explosive decompressions in small aircraft. It will, however, happen. I've had pilots tell me they would be able to hold their breath and prevent this effect. That won't happen. First of all, the surprise of the event will override any defensive measure you may have thought you could put into place. Secondly, the rapid change in pressure differential will make it impossible to hold your breath. (Remember, we can be talking about a pressure differential of 8.8 PSI. Let's put these pressure differentials into perspective. A differential of 8.8 PSI is 1267 pounds per square foot. The pressure differential between the bottom and the top of the wing of a popular corporate jet is less than 1/3 PSI -- and that supports the weight of the aircraft!)

Trapped Gases. Boyle's Law, as you will remember, states that the volume of any gas is directly proportional to the pressure exerted on that gas. In other words, as the pressure drops, the gas expands. Any gas trapped in the human body will expand with a drop in the pressure surrounding the body. This can occur in various places in the body causing varying degrees of discomfort or pain.

Ear Blocks. The trapped gas disorder almost everyone who has ever flown is familiar with is one affecting the ears -- actually the middle ear. Usually, trapped gas in this area is a problem on descents, but discomfort and pain can be present during rapid decompressions. Normally the eustachian tube connecting the middle ear to the nasal passages acts to equalize the pressure between the outer ear and the middle ear. (These two areas are separated by the eardrum) Any pressure differential between these two areas will cause the ear drum to bulge and the compromised flexibility of the ear drum will affect hearing.

Swelling of the eustachian tube, common with "head colds," will prevent normal equalization of pressure between the outer and middle ear. Since the nasal end of the eustachian tube acts a somewhat of a one-way valve allowing air to pass out of the middle ear, this problem is more common on descent than on ascent or rapid decompressions, but can happen, especially in the presence of severe swelling of the mucus membranes lining these passages. Since the greatest change in air density occurs between sea level and 5,000 feet, these problems will be more severe with rapid pressure changes at these altitudes than in the rarer air of the flight levels.

Sinus Block. Sinus block can be more severe than blockage of the eustachian tube leading to the middle ear because the passages between the sinuses and the nasal cavity are much smaller than the eustachian tube. This problem can be equally severe during ascent descent. With a rapid decompression and any inflammation of the sinus passages at all, severe, almost incapacitating pain may be felt. Some have described this pain as feeling as though a nail was being driven into the cheekbone. The only relief available is descent to a higher ambient air pressure to relieve the pressure differential between the sinus cavity and the ambient air.

Dental Problems. Although dental problems associated with rapid decompression are not as common as ear and sinus blocks, they can occur. Any abscess or infection in the gums or around the roots of teeth will cause pain upon ascent and a rapid decompression can cause disabling pain. Only return to a higher ambient pressure will bring relief and dental care should be sought as soon as possible.

Teeth that have been improperly filled cause problems with altitude or rapid decompressions. The higher pressure under the filling will cause excruciating pain and in rare instances can cause the tooth to explode. An exploding tooth would be distracting, not to mention the pain associated with the failure.

Intestinal Problems. Intestinal problems associated with rapid decompression range from merely embarrassing to totally incapacitating. There is normally about one quart of free air in the intestinal tract. This is air that is swallowed and gases that are produced by digestive processes and fermentation. Diet variations can increase or decrease this average volume. This air, too, will obey Boyle's Law and increase in volume with the decreasing ambient air pressure. That quart of air, at sea level, will expand to more than nine quarts at 43,000 feet. That greatly expanded volume of air can cause severe intestinal cramping and pain or it can be simply a discomfort. Much depends on the fatigue level, apprehension and general physical condition. The expanding gases will attempt to escape through both ends of the gastrointestinal tract.

Evolved Gas Disorders. The bends is a common term usually associated with scuba and other kinds of divers. It is, however only one of several kinds of evolved gas disorders. These are medical conditions, also called decompression sickness or DCS, associated with the release of dissolved gases into the body. If you will recall Henry's Law you will remember that the amount of gas dissolved in a solution is directly proportional to the pressure of the gas over the solution. This is the effect observed when the top is removed from a carbonated beverage. The release of pressure over the solution allows the carbon dioxide to bubble out of solution.

The human body contains a great deal of nitrogen (an inert gas comprising 78% of the atmosphere) dissolved in the blood and other body tissues. This nitrogen has been introduced into solution at sea level, or near to sea level, atmospheric pressure. Any pressure above this will allow the nitrogen to come out of solution. Flight in an unpressurized aircraft or a pressurized aircraft with a cabin altitude above sea level will allow the nitrogen and other inert gases to come out of solution in small bubbles.

Evolved gas affects the human body in a variety of ways, none of them good:

• Circulatory System. The formation of gas bubbles in the circulatory system is perhaps the potentially most serious of the evolved gas disorders. The gas bubbles, properly called aeroembolisms, first block the smallest capillaries, but as the bubbles become larger, larger and larger vessels become involved. Blockage of blood vessels in the heart, lungs, or brain can be rapidly disabling, even fatal. Damage to body organs will very with extent and duration of blockage.
   
• Chest and Lungs. Gas bubbles in the chest (thorax) area and vessels of the lungs cause a condition and symptoms known as the chokes. These symptoms begin with a burning pain in the central area of the chest which progresses to a stabbing pain and is worsened by deep breathing. An almost uncontrollable cough will be unproductive and continued time at altitude will bring on feelings of suffocation and typical signs of cyanosis. The lips, ear lobes and fingernails turn blue and tingle. At this point, if an immediate descent to a higher ambient pressure is not made, collapse and death are possible.
   
• Muscles and Bones. Nitrogen bubbles forming in the fluid filling the joint spaces, especially the larger joints of the shoulders, elbows and knees cause varying amounts of pain ranging from an annoying ache to severe and incapacitating pain. This is the same affliction suffered by divers ascending too rapidly from deep dives. Popularly called the bends, this condition will become steadily worse until equilibrium is reached between the pressure of gases dissolved in the blood and the ambient pressure. Again, the only cure for this affliction is descent into an area of higher ambient pressure. Subsequent exposures to lower ambient pressure than the body is acclimated for will tend to cause recurring pain in the same joint spaces first affected.
   
• Nervous System. Evolved gas disorders affecting the nervous system range from the mildly annoying tingling, itching or cold and warm feelings caused by bubbles of nitrogen formed around the nerve tracts in the skin to the life-threatening air embolism in the brain. Peripheral nerve involvement rarely causes permanent damage; however, the symptoms are almost identical to those presented by hypoxia and hyperventilation. Central nervous system (brain and spinal cord) involvement are much more serious. Early symptoms of nitrogen bubble formation in these areas are usually visual disturbances such as the appearance of flashing or flickering lights, headaches and confusion. More severe and potentially life threatening symptoms can include partial or total body paralysis, loss of hearing or speech, and unconsciousness. The appearance of any of these symptoms heralds a medical emergency and descent to a higher ambient pressure and immediate medical intervention are necessary to preclude permanent disability or death.