First off, the air you breathe onboard is often drier than a desert. This may come as a surprise, but the engineering behind an aircraft’s air supply is quite complex. Many people assume the plane simply scoops up air from outside and pumps it into the cabin. However, things aren’t that straightforward. Because the air pressure at cruising altitude is so low, we can’t get enough oxygen from it. We need air at a higher pressure.
Modern jet engines have helped solve this problem. They operate on a principle often summarised as “suck–bang–blow.” During the “suck” phase, air is drawn in through a large fan at the front of the engine. A series of compressor stages then squeeze the air, increasing both its pressure and temperature. By the time it’s ready to leave the compressor, this air is around 200–250°C at 40 psi—more than hot enough to cook a chicken!
This heated, high-pressure air is then diverted away from the engine and routed through Air Cycle Machines (ACMs). These machines cool the air down and remove the last bit of moisture. The cooled air, at about 15–20°C, is then delivered into the cabin.
While this process ensures sufficient pressure and a comfortable temperature, it also results in extremely low humidity. Typical sea-level air has a humidity of around 40–60%. In an aircraft cabin, humidity can drop to as low as 2–3%, which is actually drier than the Sahara Desert, where the average relative humidity is around 25%. No wonder your eyes and skin often feel so dry after a long flight!
This very dry environment not only dehydrates us more quickly but also dries out our sinuses and mucous membranes. These membranes play a crucial role in protecting us against airborne illnesses like the common cold. When they’re dried out, they can’t do their job as effectively, which may leave us more vulnerable to germs.
Understanding how aircraft cabin air works helps explain why we often feel dry and dehydrated after a flight—and why it’s a good idea to stay hydrated when travelling by air.