I fly often enough that I’ve stopped being surprised by the way I feel stepping off a long-haul flight. Dry eyes. A tight, papery feeling in the nose. Lips that need balm before the seatbelt sign is even off.
For years, I assumed this was simply what travel felt like. It turns out there’s a specific, measurable reason for it—and conditions can be more extreme than most passengers realize.
The Numbers Behind the Feeling
A study published in the Scandinavian Journal of Work, Environment & Health put instruments aboard a commercial aircraft to determine exactly how dry cabin air becomes.
Researchers followed commercial aircrew members on eight round-trip flights between Stockholm and Chicago. They measured cabin conditions directly using instruments placed in business class, economy class and the cockpit.
During normal, unhumidified flight conditions, relative humidity inside the cabin measured between 10 and 12 percent.
For comparison, indoor environments that people generally find comfortable commonly have relative humidity between 30 and 50 percent. A winter home with the heat running—already known to feel dry—may still contain considerably more moisture than the air measured at cruising altitude.
This wasn’t a single anomalous reading. Researchers recorded the conditions across multiple flights, in several parts of the aircraft, during a data-collection period spanning nearly a year.
Dryness Was Measurable in the Body
The research team didn’t stop at measuring the air. Investigators examined 71 crew members using objective clinical measurements and a 23-item questionnaire covering symptoms affecting the eyes, nose, throat and skin, as well as headaches.
Clinical assessments included tear-film stability, which measures how quickly the protective layer of tears over the eye begins to break up, and nasal patency, a measure of how open the nasal passages are.
To determine whether raising cabin humidity changed those outcomes, the researchers installed a ceramic evaporation humidifier aboard the aircraft and conducted a randomized, double-blind trial.
On some flights, the humidifier operated during the journey to Chicago but not on the return trip. On others, that sequence was reversed. Neither the participating crew members nor the physicians examining them knew when the humidifier had been active.
When operating, the system raised relative humidity by approximately 10 percentage points in business class, which was closest to the device. Humidity rose by about three percentage points in the rear economy cabin and cockpit, where the humidifier’s effects were weaker.
Even those modest increases produced statistically significant improvements.
Crew members reported fewer problems with dry or watery eyes, eye irritation, nasal obstruction, skin dryness, facial irritation and headaches. Tear-film stability improved by two to three seconds. The minimum cross-sectional area of the nasal passage—a physical measurement of how open the airway is—increased by approximately 5 to 7 percent.
Each of those changes followed a relatively small increase in humidity sustained for the duration of a single flight.
Why Cabin Air Becomes So Dry
At cruising altitude, the outside air is extremely cold and contains very little moisture. That air is drawn into the aircraft, compressed and used to pressurize and ventilate the cabin.
The result is air that begins with very little water vapor. Adding meaningful humidity back into a pressurized aircraft at altitude is mechanically challenging and is not standard across most commercial cabins.
The study’s authors noted that relative humidity inside commercial aircraft commonly ranges from 5 to 25 percent. The driest conditions tend to occur during long intercontinental flights at cruising altitude—precisely the flights during which passengers and crew spend the greatest number of consecutive hours breathing cabin air.
What This Means for Frequent Flyers
I don’t share this information to make anyone anxious about flying. I share it because there is a physical explanation for something many frequent flyers already recognize: air travel places an unusual and measurable demand on the eyes, nose and skin.
Those tissues are being asked to function for hours in conditions well outside the humidity range most people encounter during daily life.
The researchers’ conclusion was straightforward. Raising cabin humidity, even modestly, appeared to improve comfort and reduce dryness-related symptoms among people breathing that air for extended periods.
For frequent flyers, that isn’t necessarily a minor detail. It can be part of the difference between stepping off a flight feeling depleted and arriving ready for what comes next.
Dr. Jedd Myers, MB BCh, MBA, is a physician, healthcare entrepreneur and co-founder and CEO of the HealthImpact Group. An advocate for the role of health in performance and longevity, he is also an endurance athlete who has completed numerous ultramarathons, cycling events and six Ironman triathlons.
This article is for general informational and educational purposes and does not provide individualized medical advice. Consult a qualified healthcare professional about persistent or concerning symptoms.
