Humans normally dwell at the bottom of the atmosphere where sea level air pressure is about 14.7 pounds per square inch or about 30 inches of mercury as measured by a barometer. The air we breathe is roughly 21% oxygen, 78% nitrogen, and 1% other gases. As we breathe, air is brought into the lungs and the oxygen we need is forced through the thin-walled sacs of the alveoli and then passes into the bloodstream. The pressure of the atmosphere is what pushes the air through the walls of the alveoli.

The blood transports the oxygen to the cells where it is burned to fuel our body, and the byproduct, carbon dioxide, is then carried back to the lungs and is exhaled. The build-up of CO2 in the lungs is what triggers the breathing reflex. Nitrogen is also carried throughout the body by the bloodstream, some of it going into suspension in the cells. Nitrogen is an inert gas and under normal pressures has no effect on the body. Hypoxia is a condition in which the body, or part of the body, is deprived of adequate oxygen. Perhaps the most common type of hypoxia experienced by pilots is hypoxic hypoxia, where there is not enough oxygen available to the brain.

Just like a mountain climber, a pilot will encounter reduced air pressure as they climb to higher altitudes. Although the ratio of oxygen is the same, the pressure of the atmosphere is reduced, resulting in less air being pushed into the lungs. Pilots compensate for this in one of two ways: either by pressurizing the air inside the cockpit and cabin of the airplane or by breathing 100% oxygen through a mask. High-performance aircraft such as airliners and corporate jets use their engines to pump compressed air into the cockpit and cabin, keeping the relative pressure high enough to force air into the lungs. Pilots in unpressurized aircraft breathe supplemental oxygen from oxygen tanks. However, breathing 100% oxygen from a mask typically only works up to about 40,000 feet. Most smaller aircraft don't carry oxygen systems or have pressurized cabins, so pilots of these aircraft need to be aware of the reduced oxygen with an increase in altitude.

In fact, the FAA has imposed limits on how high a pilot can go without being on supplemental oxygen. With the decrease in oxygen in your bloodstream, there is a decrease in your vision and mental functioning. That is also why they recommend going on oxygen at 10,000 feet in the daytime but only 5,000 feet at night, as your eyes need extra oxygen to function at night. Stagnant hypoxia occurs when there is a decrease in blood flow to the cells in your body. Simply put, your body cannot circulate your blood. This is first evident in your vision and then your brain function.

Fighter pilots and aerobatic pilots experience stagnant hypoxia when the G-forces created by quick changes of acceleration prevent the blood from flowing normally. This interrupts the supply of oxygen to the eyes and brain, which results in a blackout. Special straining and breathing techniques can be used to reduce the effect of stagnant hypoxia. Hypeemic hypoxia occurs when the blood is unable to accept and transport oxygen. For pilots, this may occur if carbon monoxide is entering the cockpit, perhaps from a leaking cabin heater. Smokers also suffer hypeemic hypoxia whenever they inhale smoke.

The hemoglobin in your blood absorbs carbon monoxide 200 times more rapidly than oxygen. When carbon monoxide binds with your red blood cells, they have no room to carry the oxygen your body needs. Carbon monoxide is a byproduct of all internal combustion engines and combustion heaters and is odorless and colorless. Many aircraft have carbon monoxide detectors to alert flight crews if carbon monoxide enters the cockpit. Histo toxic hypoxia occurs when there is enough oxygen in the blood, but the body cells are unable to make use of it. This form of hypoxia can occur when the body has been poisoned by drugs or alcohol.

It should be noted that drinking just one ounce of alcohol has the same effect as climbing 2,000 feet in terms of oxygen, and the lack of oxygen creates visual and mental impairment. Symptoms are lightheadedness, a feeling of euphoria, dizziness, blue fingernails and lips, also known as cyanosis, tingling in the extremities, and headache. The symptoms experienced differ by person, and each may not be experienced by everyone. Prolonged hypoxia will result in unconsciousness, and at high altitudes, it may result in an untimely death.

If you notice any of these symptoms, it is important to increase your intake of oxygen, check your cabin pressurization or oxygen system, and if it is malfunctioning, you need to descend to a lower altitude quickly. If you suspect carbon monoxide poisoning, turn off any cabin heaters, open all fresh air sources, and land as soon as possible. When under stress or experiencing anxiety, some people will breathe too rapidly. This way of breathing results in hyperventilation, where too much carbon dioxide is being exhaled and creates a lack of CO2 in the blood.

The resulting symptoms of dizziness, weakness, fainting, and tingling sensations of the lips, hands, and feet are very similar to the symptoms of hypoxia. What's more, the symptoms can cause anxiety, which in turn stimulates more hyperventilation and exacerbates the symptoms further. It is important to diagnose hyperventilation correctly and not confuse it with hypoxia, and vice-versa. The symptoms of hypoxia can also cause a pilot anxiety, which could trigger hyperventilation, aggravating that situation. To a new pilot, some maneuvers such as stalls may be stressful, just as a real emergency would be stressful to an experienced pilot.

Treatment for hyperventilation is accomplished by deliberately slowing your breathing down to a normal rate. If necessary, you can breathe into a paper bag, which allows you to rebreathe your exhaled CO2, helping your body regain proper CO2 balance more quickly. It is possible to suffer from both hypoxia and hyperventilation at the same time. If you're not sure which one you have, it's always safer to put on an oxygen mask and take deeper, slower, controlled breaths. Remember how 78% of the air around us is made up of nitrogen? When we breathe, nitrogen gas gets absorbed by the body, some of it going into suspension in our cells, some of it being exhaled.

At constant pressure, the nitrogen causes no harm. But if we transition quickly to a lower pressure, not enough nitrogen can leave our body through normal respiration. The excess will then bubble out from the cells and can cause damage around joints and nerves. For general aviation pilots and passengers, the encounter with decompression sickness is often due to scuba diving before a flight. During a dive, a scuba diver is subjected to increased pressures; for each 33 feet of depth, the water pressure increases by one atmosphere, or about 17 psi. What this means is that the deeper a diver goes and the longer he or she stays down, the greater the amount of nitrogen that is absorbed by their body.

Divers can go down and stay down long, but need to make decompression stops at designated depths to outgas the surplus nitrogen. Even after making the decompression stops, a diver could still face decompression sickness if they were to continue ascending to higher altitudes, say, in an airplane. This is why pilots need to know that they should not take anyone flying that has been scuba diving until that person has been on the surface for at least 12 hours, or 24 hours for anyone who has done a dive requiring decompression stops. Symptoms of decompression sickness are joint pain, called "the bends," tingling sensations, seizures, and unconsciousness.

Treatment is accomplished by quickly descending to a lower altitude and administering 100% oxygen, if available. Finally, a trip to a hospital or a diver's recompression chamber would repressurize the individual and remove the gas bubbles from their blood. As we climb and descend, the air pressure around us changes. The internal cavities behind the eardrum and the sinuses also need to equalize this, keeping the internal pressures the same. When pressure changes are small and slow, this presents no problem. However, when the pressure changes rapidly or the ventilation passages are constricted due to a cold or sinus condition, then equalizing the internal pressure with the outside air can be difficult.

Pressure on the outside of your eardrum is balanced by the inner ear air pressure. The inner ear equalizes through eustachian tubes, which travel from your inner ear to the back of your throat. When ascending to a higher altitude, the outside air pressure decreases and the higher internal pressure can easily vent out. However, when descending, the eustachian tubes tend to squeeze closed due to the higher outside pressure and do not allow the ears to easily equalize. Some people have voluntary control of the muscles that flex the eustachian tubes and can equalize the pressure with little difficulty. Others must swallow, yawn, or chew to flex the eustachian tubes.

If these methods fail, you can try something called the Valsalva maneuver: hold your nose closed with your fingers and blow through your nose gently to clear your ears. Do not blow too hard, or you could damage your eardrums. These ear-clearing methods must be used before discomfort is felt. It is extremely difficult to equalize your ears if a blockage has occurred, and the pressure differential has become too great. If you or your passengers experience trouble clearing your ears while descending, stop the descent and climb back up to a higher altitude until the lower pressure relieves the discomfort.

You can then descend again, but this time more slowly, clearing your ears continuously to prevent another blockage. Continuing a descent with an ear blockage can result in a ruptured eardrum or burst capillaries inside the inner ear. There is no way to manually control the venting of the sinus cavities. If you have a bad head cold or sinus condition, do not fly. Flying with blocked sinuses can result in severe pain between the eyes and the forehead and may even result in bleeding from the sinus membranes.

Our bodies consist of up to 60% water, and we require two to four quarts of new water each day to keep our systems running properly. Our comfort zone is 68 to 72 degrees Fahrenheit with 25 to 50 percent relative humidity. Sun-baked ramps and the greenhouse heating of air trapped inside aircraft cockpits can run temperatures up to well over 120 degrees Fahrenheit. Cockpit avionics can push temperatures even higher. High humidity will limit the effectiveness of sweating to help cool our bodies. Studies have shown that as body temperature increases, there is an increase in error rate and motor skills, mental functioning, and short-term memory.

Hot conditions also increase your debility, hostility, and frustration, increasing the instances of interpersonal friction, loss of tempers, and miscommunication. Without enough water, our physical and mental abilities decline. The first noticeable effect of dehydration is fatigue, which can be followed by sleepiness, dizziness, headache, cramps, and nausea. Drinking enough fluids is especially important when performing physical exercise and when exposed to hot temperatures. Flying at high altitudes and breathing aviation oxygen are especially dehydrating. The cockpit air at high altitudes ranges from three to five percent humidity, and aviation oxygen contains zero moisture to prevent water from freezing up the system at low temperatures.

This means a pilot is losing moisture from their lungs and skin at a rate of two to four ounces per hour. The smart pilot makes sure he or she drinks plenty of water before flying and carries a water bottle on long flights. You often don't feel thirsty until after you approach a one and a half quart deficit, so drink before you feel thirsty. Also, try to stay away from too much coffee, tea, caffeinated sodas, and alcohol. All of these drinks are diuretics, which cause you to urinate more than usual to flush the Associated impurities from your system, and caffeine and sugar can inhibit the absorption of water.

When exposed to high temperatures for long periods or when performing exercises, it is possible to suffer heat exhaustion or heat stroke. Mild heat stroke causes loss of physical and mental abilities. Severe heat stroke can result in unconsciousness and even death. Heat stroke symptoms consist of feeling hot, often without sweating, dizziness and fainting, muscle weakness and cramps, rapid heartbeat and breathing, nausea and vomiting, confusion, disorientation, seizures, and unconsciousness. When flying during hot weather, be sure to stay properly hydrated and try to find shade and fresh air. This can be difficult in a bubble canopy aircraft, so drink plenty of water, wear loose-fitting, light-colored clothing, and a hat if possible.

Use a canopy shade and make use of air vents and other cockpit ventilation and cooling. Stress and fatigue can seriously reduce a pilot's ability to fly safely. Airline pilots are required to have at least 10 hours of rest, which includes eight hours for sleep, before any flight duty. They are also prohibited from flying more than 30 hours per week or more than eight hours in a day without a rest period. The airlines are also required to conduct fatigue education and awareness training programs for all flight crew and dispatchers. Fatigue can seriously diminish a pilot's ability to perform their tasks, and stress can prevent a pilot from focusing on his or her flying.

Symptoms of fatigue include reduced speed and accuracy of performance, lapses of attention, delayed reactions, impaired reasoning and decision-making or risk evaluation, reduced situational awareness, and low motivation to perform optional activities. It has been said that heavy fatigue is more debilitating to pilot performance than three alcoholic drinks. To reduce fatigue and the symptoms of fatigue, get plenty of sleep before the start of your flight. Before and during your flight, drink plenty of water. During long flights, periodically shift your position in your seat, do isometric exercises, and if the cabin allows, get up and walk around periodically.

Although caffeine in coffee and energy drinks has been proven to provide temporary alertness, be aware of the diuretic effects and possible resulting dehydration, and beware the energy crash following a sugar high. When possible, try and take a nap. Many FBOs have crew rest areas with reclining chairs or bunks just for this purpose. Stress, some stress is normal, such as when coping with a very demanding task, such as flying an instrument approach in bad weather. Under the short-term stress, the body releases adrenaline, and your heart rate and blood pressure increase. All of this helps you to concentrate on the important task, and when the stressful task is over, your system returns to normal.

However, under chronic stress, such as relationship problems, financial difficulties, or school or work problems, the stress does not go away. This long-term type of stress may place a burden on the pilots which they are unable to cope with, that causes their piloting performance to deteriorate below acceptable limits. Pilots under severe chronic stress should ground themselves and seek professional help. Motion sickness occurs when your brain feels that there is a disagreement between what is seen and what your body feels. When flying in turbulence or while performing aerobatic maneuvers, your inner ear is supplying your brain with information about movement, changing orientation, and g-forces, while your eyes may be looking at the stationary references of the surrounding cockpit.

This conflict between what you feel and what you see can lead to symptoms of motion sickness like nausea, sweating, feeling hot and faint, and finally vomiting. An individual has little control over these symptoms. In fact, some of the world's best and most famous pilots suffered motion sickness. The good news is that most pilots build a tolerance to motion sickness with experience. There are also some tricks you can use to prevent or reduce the symptoms. Focusing your gaze on the horizon will minimize the disagreement between your eyes and your inner ears. Keep plenty of fresh air flowing into the cockpit, especially against your face. Consider even opening up the windows.

When detecting the onset of air sickness symptoms, stop doing what made you sick. For example, if you're doing stalls or spins or aerobatics, stop the maneuvers and fly straight and level. Try to fly when the air is smooth and cool, such as early in the morning or at altitudes above the bases of the clouds. Concentrate on the task at hand; the pilot flying is usually not the first one to get sick. His mind is busy and doesn't have the time to be confused with secondary information. Remember when you were a kid on the playground, and you spun on the merry-go-round, then jumped to the ground and couldn't walk straight? The world was spinning; you were dizzy and fell over. Well, you were experiencing vertigo.

What happened was the fluid in your inner ear got to spinning along with the merry-go-round, then stop your movement only to have the fluid in your balance sensors keep moving, giving you erroneous information. You can do this to yourself in an airplane too, especially when doing spins or by changing the orientation of your head while turning. When experiencing vertigo, you should look to the natural horizon for correct information. When that is not available, such as on a dark night or while flying in the clouds, you should look to your attitude flight instruments. After the confusing motion stops, normal sensory information will return.

Illusions occur when what you think you see is different from what you really see. For instance, you could confuse the lights of small boats on the water with the stars or mistake a slanting cloud deck with the horizon. The solution for most illusions is to constant cross-check between visual and instrument references. Some other common illusions that pilots encounter are mistaking an approach to a skinny runway as being too high, or having runways that have uphill to downhill slopes, generating erroneous glide path impressions.

Vision, a pilot's most important sense is vision. Understanding how your eyes work will allow you to compensate for their limitations. Light entering your eye is focused by the lens and then falls upon the retina. The retina is made up of light-sensing cells called rods and cones. Most of the cones are concentrated around the center of your vision and detect color and are responsible for higher resolution and detailed vision. The rods, on the other hand, are located outside of the center of vision and are better at detecting movement and are very sensitive at low light levels.

Each eye has a small blind spot where the optic nerve is attached to the retina; here, there are no rods or cones. Since the nerve attaches in a slightly different place in the field of vision for each eye, the brain blends the two images together, and blind spots are normally not noticeable. The blind spot can become apparent, however, when what you are looking at cannot be seen by both eyes. Empty field myopia: when looking into a hazy background or sky without prominent features, a pilot's eyes tend to relax and focus at a distance of only 10 to 30 feet away.

When the eyes are focused so close, it is difficult to detect objects in the distance, such as other airplanes. To prevent this phenomenon, pilots should actively scan by shifting their gaze to different sectors and concentrating on spotting potential distant objects. By actively seeking airplanes in the distance, the eyes will maintain focused at the correct focal length. Since the eye detects motion with its peripheral vision, the search technique that works best is to concentrate momentarily on a sector of about 15 degrees in width, stare without moving your eyes, then shift to the next sector and hold your focus there momentarily before shifting again. At each sector, look on the horizon, then above, then below, before moving to the next sector.

When scanning this way, any moving target will be more obvious to the eye. Having said this, however, it is the target that stays stationary in your field of vision, the one that is not moving in relation to your flight path, that has the greatest collision potential. Night vision: when you turn out the lights at night, you can't see anything at first, but after a few minutes, you can see fairly well in the dark. Why is that? The rods of your eyes are ten thousand times more sensitive to light than your cones and are responsible for your night vision. However, they can be overwhelmed by bright lights and take up to 30 minutes to adapt to low-light environments.

They also do not detect color, which is why colors are hard to see in the dark. Remember how the center of your field of vision is made up almost exclusively of cones? That means at night you have a blind spot in the middle of your view. If you are trying to pick out a small object at night, such as an airport light beacon or the strobes of a distant aircraft, you will have more success if you look slightly off-center so that the rods can pick up the target. Alcohol and drugs: flying at times can be a very demanding activity. It calls for good hand-eye coordination and, even more importantly, it requires accurate perceptions and good decision-making.

Substances that can dull or distort your mental and physical abilities have no place in aviation. Medical studies show that even one ounce of alcohol has a measurable negative short-term effect on your ability to reason and to perform complex tasks. Federal Aviation Regulations prohibit flying within eight hours of consuming any alcohol. The time before flying should be extended longer if you are still under the influence after eight hours. Regulations also prohibit flying if you are under the influence of any drugs, legal or illegal. Some over-the-counter drugs, especially those for colds and allergies, can impair your physical and mental performance. Pilots need to read the packaging carefully and consult with their aviation medical examiner before using any medication and flying.

It is also important for pilots to know that it is not just the flight crew that must not fly under the influence; it is also illegal for a pilot to take a passenger flying that is drunk or suffering under the influence of drugs. A disruptive passenger can seriously compromise the safety of a flight.