Cabin air and dehydration for travel
Cabin air dries you out faster than most travellers expect.
Commercial aircraft cabins are pressurised to an altitude equivalent of around 6,000 to 8,000 feet. At that altitude, the air holds very little moisture. Cabin humidity typically sits between 10 and 20 percent. For comparison, a comfortable indoor environment runs at 40 to 60 percent. At 10 to 20 percent humidity, your body loses water through your breath and skin continuously, without any sensation of sweating. On a four-hour flight, that loss adds up to somewhere between one and one and a half litres. On a long-haul flight of ten or more hours, the loss can reach two litres or beyond.
What is happening
You do not feel thirsty on planes the way you do after exercise. Thirst is a late signal, and in dry air it fires even later than usual because the drying of your mouth and throat that normally triggers it happens more gradually. By the time you land and feel foggy, stiff and flat, you are already well into a dehydration state that has been building since before takeoff. The headache most people get after a long flight is almost always this. So is the difficulty concentrating, the joint stiffness, and the low mood that can linger for hours after arriving.
KYO exists for exactly this gap. The hotel room is where most people first realise they needed to stay ahead of it, not catch up to it. By the time you are checking in, you are already a litre or more behind.
What to do next
Drink 250 to 300 millilitres of water per hour in the air, not per journey. That is roughly a full cup every hour, which is more than most passengers manage. Adding electrolytes matters here because plain water alone does not stay in your blood as effectively when your mineral levels are already depleted. If you have a long-haul flight, take an electrolyte drink on board and start it before the meal service. Avoid alcohol for at least the first two hours of any flight of four hours or more. The cabin is already doing the dehydrating work. Alcohol adds to it.
Sources: EFSA water guidance, Aerospace Medical Association cabin air guidelines, WHO physical activity guidance.
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