During a flight, when the plane ascends into the sky, it transports us into an environment with conditions vastly different from those we are accustomed to on the ground. The primary differences are the lower air pressure, and the significantly reduced oxygen concentration compared to what we're accustomed to at ground level. These factors account for many of the unusual sensations and phenomena we experience during flights.
The Earth's surface is covered by a thick layer of atmosphere—a mixture of gasses that, while light, are not massless. The weight of these gasses exerts pressure on us, creating what we refer to as air pressure. One of the standard units for measuring this pressure is called an “atmosphere”, where one atmosphere equals the average air pressure at sea level. The lower we are, the taller the column of air above us, and therefore, the greater the air pressure on us. In contrast, as we ascend to higher altitudes, the column of air above us shortens, resulting in lower air pressure.
Therefore, at the Dead Sea, which is the lowest point on Earth, the air pressure is about 1.05 atmospheres, while at the summit of Mount Everest (just under nine kilometers above sea level), the air pressure is only about 0.3 atmospheres. We typically don't notice the weight of the atmosphere pressing on our bodies, as we have evolved to live and function under the air pressure at the Earth's surface.
In addition to the lower air pressure, high altitudes also present the problem of a lack of oxygen. Oxygen comprises about 21% of the gas composition in the atmosphere, a proportion that remains constant up to an altitude of around 100 kilometers. However, the partial pressure of each gas, including oxygen, decreases in direct proportion to the overall air pressure, so as air pressure drops, the oxygen pressure also drops. In other words, although the percentage of oxygen in the air remains at 21%, the thinner air at higher altitudes means fewer oxygen molecules (and fewer molecules of other gasses) are present in each unit volume of air. This is why mountain climbers often use oxygen tanks during the final stages of their ascent to high peaks.
A passenger plane typically flies at an altitude of about ten kilometers above sea level, where the air pressure is very low, and accordingly, the partial pressure of oxygen is also low. To ensure passenger comfort, passenger planes are equipped with artificial pressurization systems, but the pressure inside is still lower than at ground level and is similar to the air pressure at an altitude of 2–2.5 kilometers. The pressure cannot be higher, as this would create an excessive differential in air pressure between the interior and the exterior, and the fuselage may be crushed. Consequently, we experience rapid changes in air pressure during takeoff and landing.
Crushed Bottles and Popping Bags
These changes in pressure often lead to surprising outcomes. You may have noticed during your last flight that empty plastic bottles tend to get deformed during landing, while during takeoff, sealed snack bags (such as those containing peanuts, pretzels, etc.) tend to puff up, sometimes even to the point of bursting open.
These phenomena occur due to the pressure differences, in line with the ideal gas law. This law states that, with constant temperature and a fixed amount of gas, the volume of the gas is inversely proportional to its pressure: as pressure decreases, the volume increases, and vice versa.
In a passenger plane, the temperature does not change significantly, thanks to air conditioning. A bag packed on the ground contains a certain amount of air, and since it is sealed, the amount of gas inside remains constant. During take-off, the pressure drops, thus according to the law, the volume increases. The same thing happens to a bottle—if we seal it at high altitude, the amount of air inside remains constant, and during landing, the pressure increases, and the volume decreases, compressing the gas inside, and causing the bottle to deform.
Ear Pressure
The same principle that affects bottles and snack bags also explains the pressure we feel in our ears during flights. The eardrum, a thin membrane, separates the outer ear from the middle ear. Behind the eardrum is a small cavity. While the eardrum blocks fluid movement, it vibrates in response to sound waves from the outside, transmitting these vibrations from the outer ear to the middle ear, and then to the inner ear, allowing us to hear.
For this to occur, the air pressure on both sides of the eardrum must be equal. Typically, this balance is maintained through the only connection between the ear and the external environment: the cavity behind the eardrum is connected to a narrow canal called the Eustachian tube (also known as the pharyngotympanic tube). This canal extends from the ear to the skull, connecting the middle ear to the nasopharynx (the upper throat the mouth, and the back of the nasal cavity). The Eustachian tube regulates the air pressure within the middle ear, equalizing it with the external air pressure,
Pressure balance occurs automatically when we swallow or yawn. You may have noticed “clicks” in your ears when you swallow saliva or eat—these clicks are small air bubbles moving along the Eustachian tube. These bubbles continuously travel to the middle ear, balancing the internal air pressure.
However, during takeoff or landing, the external air pressure changes much more rapidly than the rate at which the Eustachian tube can equalize the internal air pressure. The air trapped in the cavity behind the eardrum behaves like air trapped in a bottle or bag. This pressure differential causes the eardrum to stretch excessively, which can cause discomfort or ear pain. This stretching also impacts sound transmission, creating the familiar sensation of “cotton candy” in your ears.
The Eustachian tube usually opens on its own when we swallow saliva, but if there is any blockage—such as fluid in the ear, a swollen throat, or blocked sinuses, the gas transfer cannot occur and we may experience pain until the pressure returns to normal. Think of the squeezed bottle, and you’ll have an idea of the pressure your ears endure during landing.
Watch Out: Leg Blood Clots
A blood clot is a solid lump of blood components that form following damage to blood vessels and prevents loss of blood through the wound. It consists of red blood cells, platelets, and protein fibers. Occasionally, a clot may develop in the wrong location - on the inner side of a vein, typically in the legs. Such a clot can later detach, and travel through the bloodstream to the lungs, and even to the heart or brain.
Studies have found that passengers on long flights (over four hours) are at a greater risk of developing blood clots. The main cause of this is likely prolonged immobility, as a similar risk is also found in people who work long hours in a seated position
The World Health Organization (WHO) has published a series of studies to determine whether there are specific flight-related factors that increase the risk of blood clot formation, beyond immobility. One study, which examined approximately 2,500 Dutch pilots over several years, found that the incidence of blood clots among them was similar to the incidence in the general Dutch population. Moreover, no correlation was found between the occurrence of blood clots and the number of flight hours. While the results suggest that the risk is not high, a low risk cannot be ruled out, especially considering that pilots are a particularly healthy group of people.
In later experiments performed by the WHO, healthy individuals and others at increased risk of blood clotting spent eight hours in conditions of low air pressure and reduced oxygen levels, similar to those found on standard flights. The researchers discovered that among healthy participants, there was no difference in the likelihood of developing a blood clot. In contrast, individuals at increased risk of clotting, due to factors such as hereditary tendencies or contraceptive use, were affected by the low-pressure and oxygen-deficient conditions in addition to the basic impact of prolonged immobility.
The hypothesis is that the interactions between the risk factors and the conditions experienced during flight lead to increased release of thrombin—a protein that promotes clot formation. The WHO continues to conduct studies to investigate this phenomenon further.
So, how can these issues be prevented? The main recommendation is to keep moving during the flight—for example, get up every so often to stretch your legs and walk to the restroom. It's also advisable to chew and swallow frequently, keep your respiratory pathways open (for example, with a nasal spray), and bring a snack that can offer a practical demonstration of the ideal gas law in action.