For comfort, human beings requires a relative humidity of 40% to 60%, which corresponds to the normal humidity of the atmosphere. The humidity of the air in pressurized aircraft is typically only 5 to 15 %. Such difference results from the air conditioning system of modern aircraft drawing air from outside the aircraft - At airline operating altitudes, the air is very cold and can only hold very little amount of water vapour. Water vapour is added to the cabin environment during respiration, but as the air is continually replaced this will have a limited effect in increasing the cabin humidity.

It is paramount that a person compensates for this very low humidity by drinking sufficient fluids in flight to avoid dehydration - thrist is a late symptom of dehydration. Symptoms of dehydration may include headache, feeling dizzy or lightheaded and muscle cramps. Remember that even slight dehydration can cause a reduction in performance. It is also important to recall that caffeine and alcoholic beverages actually contribute to dehydration. Dry air may cause irritation of the eyes especially if contact lenses are worn and these may have to be removed.