Humidity is an important factor for the climate to be perceived as pleasant. The body has adapted to humidity levels common on earth. Climate feels best if the humidity is between 40 – 60 % RH. But our experience range is down to 20 – 25 % RH which is the normal humidity in our homes and workplaces in winter time. At a humidity of 15 – 20 % RH, the discomfort from the dry air increases. This falls-behind of our comfort level simply because it is below levels on earth. We feel different, although it can be difficult to pinpoint exactly what is wrong. It affects well-being, taste, smell and the ability to relax and recover. We face difficulty to sleep and risk of catching a cold increases. At extremely low humidity below 10 % RH, these effects become more apparent and acute; driven by dehydration of our mucous membranes, skin and eyes.
Extremely dry air on board
Air humidity in an aircraft gradually drops in the cabin to reach a level of 5 – 15 % RH after about three hours. After six hours, the dry air has noticeable negative effects that linger and contribute to jet lag. Humidity is the lowest in the cockpit and crew rest compartments, where air humidity drops to unprecedented low level (below 5 % RH). In first class where the number of passengers is low, it is around 5 % RH and in Business Class it is 5 – 10 % RH.
The fresh air lacks moisture
Air on board an aircraft becomes extremely dry due to the fact that fresh air intake at cruising altitude lacks moisture content. The only source adding humidity is people. The air circulates in sections in the cabin, which means that the need for humidification is greatest in sections with fewest passengers. In other words, air is the driest in premium classes where passengers sit more spacious, in flight deck and in crew rest compartments.