Wherever people are, humidity is an important factor for the climate to be perceived as pleasant. The body has adapted to the levels found on earth and it feels best if the humidity is between 20 – 60 percent RH. The normal humidity in our homes and workplaces can in winter time be down to 20 – 25 RH. At a humidity of 15 – 20 percent RH, the discomfort from the dry air increases. These are levels that are below what is common on earth. It affects how we feel, although it can be difficult to pinpoint exactly what is wrong. Well-being, taste, smell and the ability to relax and recover are weakened. Difficulty to sleep occurs and the risk of catching a cold increases. At extremely low humidity below 10 percent RH, these effects become more apparent; driven by dehydration of our mucous membranes, skin and eyes.
Extremely dry air on board
When we sit in an airplane, the humidity gradually drops in the cabin to reach a level of 5 – 15 percent RH after about three hours. After six hours, the dry air has noticeable negative effects that linger and contribute to jet lag. The lowest is the humidity in the cockpit and the staff’s rest areas where it moves down to zero. In first class where the number of passengers is low, it is often around 5 percent RH and in Business Class 5 – 10 percent RH during long-haul flights. Thus, the air in the plane is drier than anywhere on earth.
The fresh air lacks moisture
The fact that the air on board an aircraft becomes extremely dry is due to the fact that the fresh air that is taken in from outside at cruising altitude in principle lacks moisture. The humidity that is supplied naturally comes essentially from the passengers on board. The air circulates in sections in the cabin, which means that the need for humidification is greatest in sections with few passengers. It also means that the air is driest in premium classes where passengers sit sparsely, but where the expectation of comfort is higher and in staff rooms where the dry air is a work environment issue.