People who travel on aeroplanes are probably familiar with the term “pressurisation“. It is a process by means of the which compressed air is pumped actively into an aircraft’s cabin, which is hermetically sealed for all intents and purposes, to achieve conditions that make it possible for any passenger or crew member to breathe normally. Although cabin pressurisation has other structural connotations for the aircraft, it is an essential element to enable a living being to travel in an aircraft at a certain altitude.
Commercial aircraft usually fly at an altitude of around 30,000 feet (about 10,000 metres) because engine performance is optimised at that height as a result of lower atmospheric pressure and colder air. At these conditions, combustion is enhanced and fuel savings are evidently achieved. These conditions, however, are hostile for human beings. That is why pressurisation is necessary; in other words an artificial atmosphere is created within the cabin, where there are three key parameters: pressure, temperature and humidity.
There is no doubt that everything connected with the air we breathe in the cabin has been solved more than satisfactorily.
With regards to air pressure, we have to take into account that it is 1,033 kgf/cm2 at sea level, while its average value at an altitude of 7,500 metres is 0,420 kgf/cm2. If we climb up to an altitude of around 15,000 metres, it barely reaches 0,140 kgf/cm2, a value which would not allow oxygen to reach some of the body’s organs, causing hypoxemia (1) (and foreseeably hypoxia) in just a matter of seconds.
Generally speaking, minimum pressure inside the cabin should not be lower than what it would be at an altitude of around 1,800 meters, that is to say around 0,820 kgf/cm2, and it should be maintained at around that level even when the plane is flying at its ceiling.
Constant pressure is maintained by means of controls which measure the pressure differential inside and outside the aeroplane and some valves known as outflow valves. The pressure may also be changed by the crew depending on the needs of each specific moment. One of these is, for instance, adapting to the conditions of the airport where the aeroplane is scheduled to land. Landing at Fiumicino Airport in Rome, which is at sea level, is not the same as doing so at a height of 4,061 metres at El Alto Airport (La Paz, Bolivia) or at Mexico City Airport, which is at 2,238 metres.
As mentioned above, it is also important to adjust other parameters like temperature and humidity to ensure passengers are comfortable.
In order to attain the right atmosphere inside the aircraft, part of the air which enters through the engines is used. This air is subjected to very high temperatures, thereby ensuring any micro-organisms are eliminated while at the same time drying up the air. The air is then cooled and compressed to ensure that the amount of oxygen per unit of volume rises to a level similar to that on the earth’s surface, as we have already mentioned. This process is carried out in the compressor’s stages. The air has therefore not taken part in combustion or been in contact with agents that could be harmful to health.
Before being pumped into the passenger area, the air goes through a series of high-efficiency filters known as HEPA (High Efficiency Particle Arresting) filters. These eliminate up to 99.97% of microscopic particles and eradicate any possible pathogens and allergens, which are always undesirable. It can therefore be said that we can breathe easy even if we hear somebody sneeze or cough in the cabin. There can be no doubt that the air is healthier than the air we normally breath in any city.
As for humidity, its average value inside an aeroplane is around 12%, which is much lower than the value we are normally used to. Cabin humidification is a costly system given that action has to be taken on air which is constantly being renewed in order to raise the above-mentioned level. There are also other reasons for not doing so, such as increasing the aircraft’s weight or the wear and tear it could cause to some aircraft components.
Smells are another aspect that plays a part in the passenger’s respiratory comfort. Since all the air in the passenger cabin is fully recycled every few minutes, it could be said that the smell is necessarily neutral. The standard configuration of air circulation systems contributes to this. In general, the new air tends to enter through ventilation slits on the upper part of the cabin (apart from the customisable air ducts above the passengers’ heads) and exits through openings located on the floor. Among other reasons, this configuration is designed to reduce smells coming from other more “humanised” parts of the cabin.
There are many other aspects that can affect the passenger’s experience during a flight. There is no doubt, however, that everything connected with the air we breathe in the cabin has been solved more than satisfactorily. Have a good flight.
(1) Hypoxemia is an abnormal reduction in the level of oxygen in arterial blood with a pressure below 80 mmHg. Hypoxia, for its part, comes about when there is an insufficient amount of oxygen in the tissues.
