Extremely dry air onboard aircraft
During flight humidity gradually decreases in the cabin to 5 – 15% RH. As a reference: Humidity in inhabited places on Earth is in the range of 20 – 100% RH.
- First Class often has around 5% RH and Business Class 5 – 10% RH.
- Cockpit is even worse with almost 0% RH (no moisture content in the air!).
Cabin air in airplanes without continous humidity boost from an active humidification system will always be unhealthfully dry for humans. Air must be fresh with enough oxygen (read: low level of carbon dioxide). Therefore air is re-circulated with approx. 50% and mixed in with fresh air. But air at cruising altitude has no humidity. Consequently, air gradually gets more dry as fresh air intake exchanges humid ground air with super dry air.
The only source adding humidity is people onboard. Most long-haul aircraft offer different classes with different seat-density. The humidity level varies in the aircraft due to the fact that environmental control systems are dividing the aircraft into sections with dedicated supply and exhaust of air. In practise, the air is the driest in sections with lower passenger density (read: passenger paying the most suffer the most from dyr air issues such as impaired taste, sleeping disorders and dry air induced fatigue).