Why are Airplanes so Dry? (The Simple Answer)

by Dr. Petra Illig, MD, AME

A flight usually marks the beginning of an adventure. A honeymoon, a hard-earned family vacation, a visit to relatives on the other side of the country. As you ascend through the clouds towards your destination, a world of promise and adventure awaits.

However, halfway to your destination, you notice a subtle yet pervasive discomfort - the arid air inside the airplane cabin. This often-overlooked aspect of air travel affects our comfort and well-being.

For the discerning traveler, it is essential to unlock the mystery of dry airplane air, so you can arrive at your destination feeling rested and relaxed.

What is the humidity of an airplane cabin?

The relative humidity of air in airplanes typically ranges from 5% to 12%! The only source of humidity on most flights is from the air that is exhaled from the passengers and crew. Therefore, the air is driest in the first class and business sections. On a crowded flight in the economy section, the relative humidity may reach up to 20% due to more people exhaling moist air.

Not only is airplane air dryer than the Sahara Desert, but it’s also the driest air most humans will ever breathe!

Relative Humidity

Relative Humidity (RH) is measured on a 0% - 100% scale.

In simpler terms, relative humidity tells us how 'full' the air is with water vapor, relative to the maximum it can hold at its current temperature.

Imagine the air around us is like a sponge. If the sponge is holding 70% of the maximum water it can absorb, that's similar to the air having a RH of 70%.

It's not entirely saturated (which would be 100%), but it's quite moist. An RH of 70% would feel damp or slightly muggy.

Average Relative Humidities:

• Amazon rainforest = 88% RH in the rainy season (77% in the dry season).
• Comfortable home = between 30% and 60% RH. This is the ideal zone for humans.
• Sahara Desert = 25% RH.
• Airplane Cabin = 0% to 12% RH

 

Why is Airplane Air so Dry?

Extreme dry air in airplane cabins is due to:

• Dry bleed air
• High altitude of the airplane
• Lack of humidifiers on the airplane

Dry Bleed Air

To understand why airplane air has a lack of humidity, we must first understand where the air comes from.

Where does airplane cabin air come from?

Many passengers (incorrectly) believe the air in the aircraft cabin is recycled throughout the entire journey of the flight. In reality, the air is only 50% recycled air, with the other 50% being brought on board from outside.

This air is called "bleed air" because it's bled off from the engines.

External air is compressed to a safe breathing density by the jet engines. The air is then mixed with cabin air filtered through a HEPA filter in a mixing manifold, and distributed throughout the cabin via air ducts. Half of the cabin air is continuously expelled overboard, ensuring a constant flow of fresh air inside the cabin.

One of the reasons why airplane air is dry is because bleed air completely lacks moisture; the air outside the aircraft has almost no water vapor in it.

High Cruising Altitude of Airplanes

Most commercial airliners fly at around 35,000 feet or approximately 10,600 meters above sea level.

For reference, Mt. Everest is 29,035 feet tall (8,849 meters).

At this altitude, the air outside of the plane is typically -45 degrees Fahrenheit (-43 degrees Celsius)!

Colder and less dense air cannot hold as much water vapor as warmer, denser air.

Why is air at higher elevations so dry?

Air is dry at high elevations due to a combination of factors: air pressure and temperature.

At higher elevations, there is less air above us exerting pressure due to gravity. If you're a SCUBA diver, you'll know that the deeper you dive, the more pressure on the body.

Air works the same way.

Air in Earth’s atmosphere is densest at sea level, the entire weight of the Earth's atmosphere is above, compressing the air molecules into a denser arrangement.

As you go higher in altitude, there's less air above you, and thus less pressure to compress the air, leading to lower density.

Colder, less dense air cannot hold as much water vapor as warm, dense air.

Lack of humidifiers

At no point is bleed air properly humidified. Humidifying the dry air would require the airline companies to carry enormous amounts of water on the plane.

Water is heavy, and therefore would increase the fuel consumption of the plane... and increase costs.

Some new aircraft, such as the Boeing 787 Dreamliner, promise a more comfortable humidity level for passengers, although it is still below the comfortable range of 30% to 60% relative humidity.

What Are the Effects of Low Humidity in Airplane Cabins?

Extreme dry air in long-haul flights often leads to:

• Dry skin/airplane skin
• Itchy eyes
• Dry mucous membranes
• Loss of sense of smell and taste
• Parched mouth
• Exacerbated effects of jet lag
• Dry Throat
• Other side effects such as motion sickness.

According to the World Health Organization, the drop in humidity levels (RH levels) experienced during a flight manifests in several subtle yet noticeable ways, especially on long-haul flights.

Dry Skin

Our skin, a natural barometer of the environment, loses its moisture, leaving it feeling dry and less supple.

Many passengers report a duller complexion and an itchy, tight sensation of their skin. This is commonly referred to as "airplane skin."

Dry Eyes

Our eyes suffer too; the low humidity speeds up the evaporation of our tear film, leading to discomfort, especially for those indulging in a book or a film during their flight, as they tend to blink less often. Passengers who wear contact lenses also report substantially dryer eyes.

Dry Mucous Membranes

Our mucous membranes in our nasal passages and throat dry out due to the lack of humidity. The Mucociliary Clearance System protects us from harmful germs and bacteria, which means dry air increases our chances of getting sick.

Ever wonder why you get a cold after flying?!

Loss of Sense of Smell and Taste

Dry mucous membranes also affect our sense of smell and taste, in a similar way a common cold might.

Research by the Fraunhofer Institute for Building Physics, undertaken for Lufthansa, provides intriguing insights. It reveals that our taste for salt and sugar diminishes significantly at high altitudes – salt by 20 to 30 percent and sugar by 15 to 20 percent.

A report from AeroMexico offers a more dramatic perspective, suggesting that we lose nearly 70 percent of our taste sensation while flying.

This intriguing aspect of flying highlights the relationship between our environment and our senses.

Jet Lag

Jet lag is commonly associated with crossing multiple time zones, leading to a disruption in our body's internal clock or circadian rhythms. However, travelers can experience symptoms similar to jet lag even without crossing time zones, according to the Mayo Clinic.

This is often due prolonged inactivity, cabin pressure, stress and anxiety, irregular sleep and eating patterns, and dehydration caused by low humidity. 

Dehydration Caused by Dry Air

The lack of moist air in an airplane cabin also leads to dehydration through “insensible water loss” due to respiration.

The dry air breathed in by your lungs essentially acts as a sponge, sucking up the moisture from your body.

On a 10-hour flight, men may lose approximately two liters of water, and women around 1.6 liters. Meaning on a transatlantic flight, a passenger may lose up to 5% of their bodily water.

Not to mention, most passengers tend to not drink enough water while flying and instead drink alcoholic beverages and coffee. Both are diuretics and move fluid out of your body.

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Dr. Petra Illig, MD is a board-certified emergency physician turned senior Aviation Medical Examiner who has served commercial airlines and private pilots alike. She combines her licensed pilot experience with decades of specialized aeromedical practice to address cabin-environment health, flight physiology and certification standards.