Cabin air is dryer than the Sahara Desert. Seriously.
You can expect a typical passenger aircraft to have about 12% humidity. The Sahara Desert sees about 25% humidity.
It has to be this way; to support a more comfortable humidity level, the aircraft would have to carry gallons of heavy, expensive water, and would be at risk for excess condensation which can be damaging and corrosive to the parts.
But knowing this any less harmful to your hydration levels.
When you fly, you likely experience some side effects of dehydration. Dry, cracked skin, chapped lips, peeling cuticles, and even nose bleeds are common after spending even a short period of time in the air.
Dehydration can also cause headaches, digestive issues, and fatigue.
Having dehydrated mucous membranes (especially in the nose) leaves us more vulnerable to airborne viruses as well.