Anyone who has flown longer than a few hours knows the feeling: eyes that won't stop feeling gritty, a throat that feels sandpapered, skin that turns tight and dry somewhere over the ocean. The usual explanation is "airplane air is dry," and that's true, but it's also vague. A Swedish research team decided to actually test it, on real intercontinental flights, with the humidifier randomly and secretly switched on or off. The results, published in 2006, are one of the few controlled experiments ever run on this question in a real aircraft cabin rather than a lab.
The setup: 16 flights, one aircraft, a coin flip on humidity
A team from Uppsala University, Dan Norbäck, Torsten Lindgren, and Gunilla Wieslander, flew round-trip between Stockholm and Chicago eight times between December 2001 and October 2002, always on the same Boeing 767-300, always following the same cabin crew there and back after an overnight stay (Norbäck, Lindgren & Wieslander, 2006, Scandinavian Journal of Work, Environment & Health).
The aircraft had a ceramic evaporation humidifier installed near the front of the cabin. On four of the eight round trips, it ran on the way to Chicago and was switched off on the way back; on the other four, the sequence was reversed. Which flights got humidification, and in which direction, was randomized, and neither the crew being examined nor the physicians examining them knew the sequence until after the study, a double-blind design that rules out people simply reporting fewer symptoms because they expected the humidifier to help.
Eighty crew members were invited; 71 (89%) completed medical exams and symptom questionnaires on both the outbound and return flight of their trip, enough for the researchers to run formal statistical tests rather than just describe impressions.
How dry is "dry" at cruising altitude
Before the humidifier did anything, the researchers measured the cabin as-is. Relative humidity at cruise was:
- 12% in the first row of business class (range 10–14%)
- 11% in the last row of economy, row 39 (range 10–12%)
- 10% in the cockpit (range 6–15%)
Turning the humidifier on raised those numbers to an average of 22% near the front of the cabin, 14% in the back of economy, and 13% in the cockpit, increases of roughly 10, 3, and 3 percentage points respectively. Even humidified, cabin air stayed well below the 30–40% relative humidity generally considered comfortable indoors, so this was a study of a modest improvement on an extreme baseline, not a study of turning dry air into humid air.
What changed when the air got (slightly) less dry
The researchers tracked 20 symptoms on a 6-point severity scale, plus objective physical measurements, and compared each crew member's own scores across their humidified and non-humidified legs. Several symptoms improved by a statistically significant margin when the humidifier was running:
- Dry eyes (p = 0.006)
- Skin dryness (p = 0.002)
- Headache (p = 0.01)
- Nasal obstruction (p = 0.02)
- Sneezing (p = 0.03)
- Facial skin irritation, itching, rash (p = 0.03)
- Watery eyes (p = 0.04)
- Eye irritation, itching, burning (p = 0.05)
The two symptoms measured with instruments rather than self-report told the same story: tear-film break-up time, a standard clinical measure of how long someone can comfortably keep their eyes open before needing to blink, improved by 2–3 seconds with humidification (p < 0.001), and the narrowest part of the nasal passage widened by 5–7% (p = 0.02), meaning crew could breathe through their nose more freely.
Two things notably didn't change: peak expiratory flow, a basic lung-function measure, showed no significant difference, and fluid intake was the same regardless of humidification, so the improvement wasn't just people compensating by drinking more water.
The obvious worry: doesn't adding moisture breed germs?
The researchers ran parallel air-quality sampling alongside the symptom tracking, measuring airborne bacteria, mold, particulate matter, and volatile organic compounds during both humidified and control flights.
Concentrations of viable bacteria and mold were low throughout, comparable to previously published aircraft air-quality data, and did not increase with humidification. If anything, the numbers moved the other way: airborne particulate matter (PM10) measured 1 µg/m³ during humidified flights versus 6 µg/m³ during control flights in business class (p < 0.01), and mold, bacteria, and microbial VOCs were numerically 50–80% lower during humidified conditions. The researchers attribute this to the humidifier's design: a ceramic evaporation system that stays completely dry when not in use, unlike spray or reservoir-based humidifiers, which have a known history of harboring microbial growth in other settings.
What this study does and doesn't tell you
This measured cabin crew, not passengers, and it's one aircraft type from 2001–2002. Cabin air systems and airline practices can change over two decades, so this isn't a claim about today's fleet, only about what was measured on this trial. The humidity increase was modest too: even at its best, humidified cabin air here (22%) was still drier than the low end of typically comfortable indoor air (30–40%), so this shows that some improvement helps with some symptoms, not what happens at higher humidity levels the study didn't test.
It's also worth sizing up the study itself: a well-designed, randomized, double-blind trial of 71 people on one aircraft is real, solid evidence, and the researchers describe it as the first intervention study on air humidification in aircraft, meaning there isn't yet a deep stack of replications to compare it against. And this is a study of comfort, not disease: dry eyes, headache, skin irritation, and measurements like tear-film stability and nasal patency, not a test of whether humidification prevents illness. If you fly frequently and have a chronic eye, skin, or respiratory condition, that's worth raising with your doctor directly.
The honest takeaway
In the one controlled, double-blind experiment run on this question, modestly raising humidity on a long-haul flight, from around 10–12% up to 13–22% depending on cabin location, measurably reduced dry eyes, skin dryness, headache, nasal obstruction, and several other symptoms in the cabin crew, without any detected increase in airborne mold or bacteria. It didn't fix everything: lung function and thirst were unchanged, and it's one study on crew rather than passengers. But it's genuine, measured evidence for something a lot of frequent flyers have long suspected from experience.
This content is for general informational purposes only and is not intended as medical advice. Always consult a qualified healthcare provider with questions about a medical condition.
Sources
- Norbäck, D., Lindgren, T., & Wieslander, G. (2006). Changes in ocular and nasal signs and symptoms among air crew in relation to air humidification on intercontinental flights. Scandinavian Journal of Work, Environment & Health, 32(2), 138–144. https://doi.org/10.5271/sjweh.989