You can breathe at 14,000 feet

Aviation medicine: air quality on board commercial aircraft

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Modern airliners move at altitudes that are absolutely hostile to human life. Extremely low temperatures, reduced air pressure, sometimes high ozone concentrations and low moisture content in the air are the typical factors. The aircraft is therefore dependent on a self-sufficient pressure and air conditioning system. This extremely complex system with different objectives must ultimately be able to provide an air quality in which a large number of passengers can travel comfortably over long distances.
In recent years numerous articles have appeared in the media that have dealt critically with the air quality on board aircraft. Passenger complaints about dizziness, nausea, headaches, irritation of the mucous membranes, breathing problems and even collapse have been reported. Another focus of the discussion was the question of the extent to which infectious diseases such as tuberculosis can be transmitted from passenger to passenger in an aircraft.
The air required for the construction and maintenance of a pressurized cabin and for air conditioning is taken directly from the compressor stages of the engines. This highly compressed air, which is extremely hot at around 250 degrees, is then fed into the so-called "packs" of the aircraft via appropriate pipe systems. Here the air is cooled, reduced in pressure and fed into the cabin via distribution systems. The air, which generally flows in laterally through the ventilation slots over the cabin ceiling, is sucked off again below the window seats, so that a uniform circulation takes place from top to bottom inside the cabin.
At the same time there is also a flow of air in the longitudinal direction of the cabin from the front to the rear. The extracted air is either released to the outside environment via special valves, but some of the air is returned to the central air conditioning systems (packs) through filters and reintegrated into the air cycle (recirculation air).
For technical reasons, the air pressure in the cabin is slightly reduced during the flight with increasing altitude, so passengers also experience a rise in altitude, which is limited to 8,000 feet (about 2,400 meters) at the top. The reduction in the total air pressure requires, in accordance with the gas laws, a reduction in the pressure of the individual components in the air. Inevitably, there is a decrease in the oxygen partial pressure, which causes a reduction in the oxygen saturation of the arterial blood.
As can be seen from the oxygen binding curve, the oxygen saturation of a healthy person under atmospheric pressure conditions at sea level is around 97 percent, while at a cabin pressure altitude of around 2,250 meters, this oxygen saturation drops to around 92 percent. This can be compensated for without difficulty for passengers with average health. However, for air travelers who are already at the limit of the oxygen supply due to a cardiac or pulmonary disease (coronary sclerosis, pronounced pulmonary ventilation disorders), lowering the oxygen partial pressure is sufficient to overwhelm the normal compensation mechanisms, such as increased and accelerated breathing and an increase in cardiac output and induce clinical decompensation symptoms. Careful consideration of the individual health status of the air traveler is therefore absolutely essential in the case of these diseases!
From the ozone layer surrounding the earth, the maximum of which is between 15 and 35 km, in addition to the already increasing ozone concentration with greater flight altitude, glove-shaped protuberances, as it were, also occur in deeper air layers. Especially in spring and autumn, ozone concentrations can occur at lower altitudes that are significantly above the MAK value (maximum workplace concentration) and also above the permitted value for short-term exposure. It is interesting that the initially typical odor of ozone (like burnt cable material) disappears as the concentration increases due to the exhaustion of the olfactory cells, i.e. higher concentrations can no longer be perceived.
However, this then leads to irritation of the upper airways, coughing, shortness of breath, feelings of oppression, eye irritation and so on. Modern commercial aircraft are equipped with ozone catalysts (ozone converters), which are able to break down over 90 percent of the ozone flowing into the aircraft.


Different limit values
The concentration of carbon dioxide (CO2) in an aircraft cabin naturally depends on the number of passengers who exhale CO2 as part of their normal lung metabolism. The greater the amount of fresh air that can be supplied per passenger and time unit, the lower the concentration of this gas that occurs in the cabin. It is interesting how differently the mandatory or recommended upper limit for CO2 has been set. While the American Federal Aviation Administration (FAA) specifies up to 30,000 ppm as the highest permissible value, the American Society for Heating, Cooling and Air Conditioning (ASHRAE) recommends an upper limit of 1,000 ppm. It is certain that health-relevant damage only occurs at CO2 concentrations as set by the FAA as the upper limit.


Drop in humidity
However, it corresponds to the experience of the critical aviation physicians that even if 1 500 ppm are exceeded subjectively perceived as uncomfortable disturbances of well-being can occur. Human vapors and smells play an intensifying role. The breathing depth and breathing frequency, which are stimulated even with slightly increased carbon dioxide concentrations, can lead to a hyperventilation syndrome that develops slowly and unnoticed in sensitive people, which can lead to a short-term circulatory collapse. In summary, it seems advisable to keep the concentration of carbon dioxide as low as possible, but never above 1,500 ppm.
The lower the temperature, the less the air is able to absorb moisture. The standard temperature at an altitude of ten kilometers is minus 52 degrees, and the relative moisture content is only a few percent. Since this outside air is supplied to the cabin via the air conditioning systems, there is a rapid drop in humidity in the cabin.
The passenger, who inhales this extremely dry air and exhales again saturated with steam, makes a decisive contribution to increasing the humidity in the cabin. Naturally, the more people travel per room unit, the higher the humidity to be measured in this area.
The longitudinal movement of the cabin air from front to back also leads to a slight increase in air humidity from front to back. In the first class with large seat spacing, the humidity drops to 4.3 percent, in the economy class, which is much more densely seated, it is up to 14.6 percent. During long-haul flights, this extremely dry air can lead to unpleasant dehydration of the mucous membranes of the upper respiratory tract and irritation of the conjunctiva, especially in contact lens wearers.
A humidification of the air in the context of the air conditioning system is technically not feasible for a number of reasons. Here, not only the weight of the water to be carried play a major role, but also the expected higher corrosion of the airframe and, last but not least, the penetration of defrosted condensation water into sensitive electronic systems after landing. For the passenger, the reduction in the relative humidity means an increased need for liquid, which is about twice the normal drinking amount.
A survey of passengers has shown that 82 percent of all passengers and even 92 percent of non-smokers feel negatively influenced by tobacco smoke. This fact and the meanwhile clear evidence of the carcinogenic effect also with passive smoking have led to the fact that smoking has been considerably restricted or completely banned by corresponding legislations of the governments.


Increase in nicotine concentration
The non-smoking passenger involuntarily inhales tobacco smoke not only when he is sitting in the smoking area, but also when he is three rows in front of or three rows behind a smoking area. Due to the modern air conditioning systems in the aircraft with a high proportion of recirculation, there is a gradual increase in the nicotine concentration in the blood throughout the cabin, even for passengers in the non-smoking section. From a medical point of view, there is an inevitable requirement to generally ban smoking while flying.
There are different ideas about the necessary fresh air supply per passenger and time unit. The German DIN regulations vary in their recommendations between twelve and 23 cubic feet per minute. In fact, with high-density seating in a Boeing 747 (Jumbo), as used, for example, in domestic service with up to 520 passengers, only a size of around 6.5 cubic feet is achieved. Of particular importance is the question of how much of the air that flows through the aircraft cabin is returned to the air circuit via filters ("recirculation air").
While the older generation of aircraft (Boeing 707, Boeing 727) did not have such an air return system, i.e. guaranteed 100 percent fresh air supply from the outside into the cabin, modern aircraft cabins have a recirculation air proportion that can be up to 50 percent. The undoubted advantage of this recirculation air is that it allows the overall humidity to be increased in the interests of better passenger comfort, and the ozone concentrations are also reduced.
On the other hand, an increased carbon dioxide level has to be accepted, and the cigarette smoke is distributed to all sections of a cabin within a very short time. Undoubtedly, economic considerations also play a role. Since the performance of an engine is reduced the more the bleed air is extracted from the engine's compressor stages, the higher the fuel consumption. Due to the factors mentioned, an optimal ratio between fresh and recirculation air is still under discussion. The idea of ​​the aircraft manufacturers to completely dispense with fresh air supplied from the outside by improving the efficiency of the filters within the recirculation air can only be viewed with great skepticism from an aero-medical perspective.
The flow rate of the air inside the cabin is not only a function of the air exchange rate per unit of time, but also depends crucially on the size of the outlet openings or the suction device for used air.


Air flow
The individual adjustment of additional fresh air nozzles above the seats, which was customary in earlier aircraft generations, is increasingly no longer possible in modern aircraft, and an individually controllable air supply, which could be seen as a clear advantage of these systems, is no longer possible. The air flow speeds of the no longer controllable air conditioning systems are sometimes viewed as too high, while others are clearly too low. In an effort to keep the cabin noise level as low as possible, large-area air supply systems have been implemented which have a very low flow velocity. In individual cases, the low air flow speed is evidently insufficient to completely replace the stale air with fresh air ("hanging smoke").
In these aircraft, there may well be subjective disorders because a constant flow of air is necessary to ensure the body's thermoregulation. The removal of heat via the moist skin, preferably the face, is particularly important.
Overall, it can be stated that the flow speed of the fresh air in an aircraft must be set higher than is desirable in normal office workplaces. A large number of passengers in a confined space, relatively high cabin temperatures and, last but not least, an increased metabolic rate with corresponding heat production due to the special psychological effect of flying are just a few of the factors that require an upward deviation from the corresponding norms.
Of particular importance is the extent to which there is a risk of passengers being infected by bacteria, viruses or fungal spores in the cabin air. Particularly with the high recirculation rates, the fear is repeatedly expressed that an infected passenger may pose a risk of infection for all other passengers traveling with them. With knowledge of the technical equipment of the aircraft, however, this risk must be largely denied.
The recirculation air in particular is constantly passed through highly effective filters, the pore size of which is smaller than a micrometer. Due to the adhesion of even smaller particles and the Braun molecular movement of the smallest particle sizes, such as viruses, these HEPA filters (high efficiency particular air filters) have the ability to filter out between 91 to 99.9 percent of all microbial components in the cabin air. These filters are changed at regular intervals.
It should also be taken into account that the extremely low humidity on board commercial aircraft significantly reduces the survivability of infectious agents. In summary, the risk of infection during a flight is to be assessed as significantly lower than, for example, in public transport such as buses or trains.


Author's address
Dr. med. Lutz Bergau
Senior physician at Deutsche Lufthansa AG
60546 Frankfurt

Aviation medicine: air quality on board commercial aircraft

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