Why is airplane air so dry?
Outside air at a plane’s altitude is very cold and less dense. Cold air can hold less water compared to warmer air. At a plane’s flight height (around 10 km) the outside air can hold about 10% what it can at ground-level.
Outside air is warmed, filtered, and then cycled through the cabin air about 20 times an hour, depending on the plane model. Air inside the cabin is also filtered and recycled. The mix between outside air and recycled inside air is about 50/50.
Below, the solid black line shows the measured relative humidity inside an airplane’s cabin during a flight. You’ll see that about 15 minutes after take-off the relative humidity drops quickly to its low of about 10% – which is maintained until close to landing.
Why can’t they humidify the cabin air?
That does happen! But let’s go over some of the issues with humidifying a cabin.
As warm air cools, we know that the amount of water it can hold drops as well and that water it can’t hold anymore will condense on surfaces. We’ve all experienced this when we see condensation or “sweat” on a cold glass during a hot, humid day.
The outer layers of a plane are much, much colder than the inside of the plane and that change in temperature can cause condensation. That condensed water can be absorbed into the plane’s insulation which adds weight and more fuel costs, cause corrosion of metal parts, and even affect electrical wiring.
OK so, what about these humidified planes?
Newer plane models, like the Airbus A350 and Boeing 787, can be ordered with on-board humidifiers. They work by passing the cabin air over a water-moistened pad, as the water evaporates – it humidifies the air. That’s not all though, to prevent some of the mentioned problems above, the humidifying system is also paired with a drying system. The drying system warms incoming air and passes it through a silica gel – which absorbs excess water and dries the air. This reduces condensation as cabin air passes through to the outer areas of the plane – where water can cause problems.
These humidified areas aren’t available to everyone though. They’re often standard for areas that the crew members are in, but when it’s available to passengers it often comes with a premium. CTT, one manufacturer of these humidifying/drying systems, says that they’re often only found in business class and other luxury sections.
So what can you do about the dry air in a plane?
While the dry air can feel uncomfortable for our skin, it’s not likely to cause long-term problems.
Trans-epidermal water loss (TEWL) from the skin does seem to increase because of the low humidity, which can lead to dryer skin – especially on longer flights. One experiment found that TEWL increased from 13.0 ± 2.2 g/m²h at 60% humidity to 20.1 ± 4.9 g/m²h during a simulation of a 6-hour flight.
Applying a moisturizer before or during a flight could be of great help. Whether that’s a sheet mask, mist, lotion, balm, or potion. Or if you’re lazy like me and don’t feel like doing anything, your skin will naturally recover once you’re off the plane and back at a normal humidity.
Based on experiments with crew members, we tend to feel the low humidity the most in our eyes (10.8%), mouth and throat (7.0%), and nose (5.0%). Sinus congestion was also a common complaint, occurring in 29.0% of those surveyed.
Moisturizing eye drops (I like the Systane Ultra drops) can help with the eyes. A light-swipe of petrolatum (Vaseline) or another balm inside the nostrils can help with the nose. As for the mouth and throat, keep some water handy! You don’t need to guzzle gallons though, there’s little evidence that low humidity leads to your body becoming dehydrated. One experiment found that for a 6 hour flight, a 60 kg person would need about 450 ml of extra water.
Source: Giaconia, C., Orioli, A., & Di Gangi, A. Air quality and relative humidity in commercial aircrafts: An experimental investigation on short-haul domestic flights. Building and Environment (2013). DOI: 10.1016/j.buildenv.2013.05.006
Source: Lindgren, T., Norbäck, D., & Wieslander, G. Perception of cabin air quality in airline crew related to air humidification, on intercontinental flights. Indoor Air (2007). DOI: 10.1111/j.1600-0668.2006.00467.x
Hashiguchi, N., Takeda, A., Yasuyama, Y., Chishaki, A., & Tochihara, Y. Effects of 6-h exposure to low relative humidity and low air pressure on body fluid loss and blood viscosity. Indoor Air (2013). DOI: 10.1111/ina.12039