Human factors is a combination of aviation medicine, psychology, engineering and ergonomics. It encompasses all of these factors trying to understand the man/machine interface in the aircraft. It has its roots in aviation accident investigations resolving these where no clear technical cause could be found when aircraft became more and more reliable over the years.
The air in the atmosphere around keeps us alive and the aircraft flying. The higher we go the thinner the air gets and this has an effect on the aircraft but on the pilot too. As with every breath the pilot takes, less and less oxygen is available to support the biological processes in his/her body.
Humans live on the surface of the planet where the atmosphere is the thickest and most of the oxygen is available, ascending means less air thus less performance until you succumb to altitude sickness if nothing is done about this. Carrying extra oxygen is one of the solutions.
The lowest layer of the atmosphere is called the troposphere and contains almost all water vapor and approximately 75% of all molecular mass of the atmosphere, its height varies from 18 km at the equator to 8 km at the poles. It is in this layer where humans live and breathe.
The atmosphere consists of 78 % nitrogen, 21 % oxygen and the rest (1 %) include argon, hydrogen, neon, helium, krypton, xenon, radon, ozone and carbon dioxide. About half of these gases are below 5000 ft and 75 % is below 11000 ft. Standard sea level pressure is 760 mm/Hg, at 18000 ft it is 380 mm/Hg (50 %) and at 34000 ft it is 190 mm/Hg (25 %).
During climb the air pressure decreases rapidly and the temperature reduces with 2°C and 1" per 1000 ft, as a result the air becomes less dense. Any air trapped inside the body has a higher pressure and 'wants' to get out. Air in the stomach and intestines will not have any trouble getting out but any trapped in the middle ear or sinuses could be blocked if the pilot has a cold. The Eustachian tube normally equalizes pressure from the middle ear to the throat.
Humans need oxygen to stay alive. About 21 % of the air is oxygen and this ratio remains constant regardless of altitude. The pressure of oxygen is 21 % of 760 mm/Hg -> 160 mm/Hg, this is the partial oxygen pressure outside the body. Once inside the lungs this reduces to 102 mm/Hg because the lungs contain water vapor and carbon dioxide. This 102 mm/Hg is the minimum oxygen pressure a human needs to function normally.
During respiration oxygen is transferred to the blood through the lungs and moved by a special molecule, hemoglobin. The heart pumps the blood through the body to every cell, but the brain has the highest oxygen requirement. After chemical reaction in the cells carbon dioxide is formed and this gas is also transported by hemoglobin in the blood to the lungs to be breathed out. For normal body operation sufficient oxygen is required and if for any reason the amount of oxygen reduces the human body will start to degrade. This state is called hypoxia.
Up to an altitude of 10000 ft the body can cope quite well and will compensate so that the level of oxygen in the blood remains fairly constant. Hemoglobin is responsible for that. Above 10000 ft during the day and 5000 ft during the night, extra oxygen is required to prevent the effects of hypoxia. During mountain flying the pilot must be aware of this.
If the brain cells do not get enough oxygen certain symptoms will develop in parts of the brain, especially those parts that use the most oxygen to function. Those parts are: vision, personality, memory, thinking, reasoning, judgment, and decision making. Under low oxygen levels (hypoxia) symptoms as: fuzziness, slow thought processes, lack of judgment, failing memory and an unnatural feeling of wellbeing (euphoria) will develop. Visual problems include: loss of sharpness, dimming, tunnel vision and loss of color (grey-out).
Lack of oxygen can be detected as a blueish colorization of finger nails and lips.
If hypoxia is suspected in the pilot or passengers then either get them on oxygen or descent below 10000 ft (if flying in an unpressurized aircraft). Keep in mind that a person suffering from a mild hypoxia is put on oxygen their symptoms will momentarily worsen, do not take them off the oxygen!
There is a certain time period in which to recognize hypoxia and be able to act on it, or control will be lost and the pilot loses consciousness and will die. It is this time which is called time of useful consciousness, where the pilot is able to take corrective action. At about 20000 ft this time is some 30 minutes, at 25000 ft it is 3 to 5 minutes. The pilot may stay longer conscious but the ability to do anything about it has gone. This time period varies per person and the physical effort needed and the state the body is in, fatigued, illness, drugs and more.
This is one of the hypoxia symptoms. It causes too much carbon dioxide (CO2) to be blown out of the lungs, which in turn changes the acidity of the blood and results in other physiological changes in the body. Symptoms are lightheadedness, tingling in hands, feet and around the mouth, leading to further anxiety. Fainting could result.
Treatment of hyperventilation is simple: slow down breathing (but make sure it is not hypoxia, or else you will kill them!), or breathing into a paper bag will help build up the carbon dioxide level in the blood. If in doubt, put them on oxygen just to be safe.
Not needed for respiration but found in the body in a dissolved state. Flight above 8000 ft altitude (or cabin pressure) can result in nitrogen to come out of solution and form gas bubbles. These bubbles will then move around the body and lodge under the skin, joints, the brain, spinal chord causing all kinds of problems. Known as the bends. Scuba divers ascending too quickly will have the same problems.
Symptoms include: joint pains, itchiness, numbness, tingling and paralysis of parts of the body. Chances are that this will result in permanent paralysis or even death.
The normal treatment for the bends is a very urgent compression in a dive chamber followed by a slow acclimatization to normal sea level pressure.