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The air we breathe

As a frequent flier, you know that the air on an aircraft is dryer than on the ground. Depending on where in the world you live, the humidity at street level is likely to be somewhere between 40 and 50 percent – on an aircraft, it might be as low as 5 or 10 percent.

Most modern aircraft take in air from outside via the engines and, although this is filtered, warmed and purified before being piped in, it is still extremely dry, since air at high altitude has a low moisture content. The result is cabin air greatly lacking in H2O.

Contrary to popular belief, this does not mean that you become internally dehydrated in the same way as you would if exercising in a hot climate, through perspiration and expiration – although you may feel that you are, meaning you will have a dry mouth and dry skin. The World Health Organization says: “Low humidity may cause skin dryness and discomfort to the eyes, mouth and nose but presents no risk to health.”

The organization adds: “The available evidence has not shown low humidity to cause internal dehydration and there is no need to drink more than usual.” It advises that we should use skin moisturizing lotion, a saline nasal spray and spectacles rather than contact lenses to reduce discomfort.

It is this sense of dryness in our nose and throat – or, to be more accurate, the mucous membranes of the nose and respiratory tract – that makes us think that we are dehydrated even if we are not. Since those membranes are one of the body’s first defenses against airborne bacteria, many of us believe we are more likely to become ill after long-haul flights, particularly if the passenger next to us is coughing and sneezing.

Professor John Oxford is a leading virologist and chairman of the Hygiene Council. While he does not believe that low humidity makes it more likely for us to catch a virus, he does point out that one of the principal ways of avoiding what the person next to you has is “social distancing.” In other words, give yourself some room. Good luck with arguing that as a reason for upgrading your travel policy with the expense department.

First Is Worst

Although dry air might not cause harm, it does contribute to discomfort. As you’d expect, the humidity levels on an aircraft differ from cabin to cabin and depend on the duration of the flight. On long-haul flights, I have measured humidity levels and found them to be 10 percent on average. Variations occur because of the number of people exhaling, so the more people there are, the more moisture there is in the air.

It follows that where passengers are most densely packed – otherwise known as economy – levels might be as high as 20 percent, which is still lower than on the ground, but manageable. In first class, however, it can be as little as 5 percent, even if the cabin is full. The same applies to business class, although it depends on the cabin’s size and configuration and how many fellow travelers are seated in the cabin with you.  

In addition, flight duration has increased with aircraft able to perform longer segments. So while premium fliers may be reducing their chances of getting deep-vein thrombosis because they have more room to move around, they may be suffering from the effects of drier air.

Generation Gap

Do new-generation aircraft solve the problem? It’s true that the likes of Boeing’s 787 and Airbus’s A350 offer slightly higher levels of air moisture. Having flown long-haul on both, including a Dreamliner delivery flight from Seattle to Doha with only 50 people on board, I’d say that they do leave you feeling less exhausted, although this may be as much to do with mood lighting, reduced noise levels and increased pressurization.

It remains the case that most aircraft are not next-generation, and while the new planes have improved the situation, they have not solved the problem. Still, one company says it has a product that can: Swedish environmental equipment supplier CTT Systems and its Cair humidification system.

There’s a complication, however. Before you can humidify the air, you also have to be able to control the humidity and condensation in the cabin, and for that you need “zonal-drying” equipment. On the face of it, this may seem strange – to increase the moisture levels, you first have to dry the atmosphere out – but aircraft are extremely controlled environments, and every action causes a reaction.

Luckily, eliminating unwanted condensation has other benefits. Any system that acts to reduce the condensation forming above cabin ceilings and in walls – water that is then absorbed by the noise- and thermal-insulation bags in the fuselage – will also reduce the aircraft’s weight.

Easyjet has run a long-term trial with CTT’s zonal drying equipment to measure how much weight is saved using the system, and how much fuel can be saved as a consequence. It has found that as well as reducing weight and fuel costs, it also cuts maintenance spend owing to fewer moisture-related faults and corrosion, and less frequent replacement of insulation blankets.

The Cair system has been installed aboard some long-haul aircraft in cockpits and crew rest areas, as well as on business and private jets. However, for commercial scheduled aircraft only Lufthansa has opted to put it in its A380 first class. In part, this is because it’s difficult for passengers to identify higher humidity as a noticeable benefit. Put a vintage champagne on the menu or offer a chauffeur-driven car to the aircraft, and it’s clear that money has been spent. But a system that humidifies the air

is unlikely to register with many, but the most seasoned, travelers.

Good Taste

Ironically, it’s the addition of a humidifier system which may help travelers appreciate that glass of vintage champagne in a way they couldn’t previously hope to. Lufthansa says that the system raises the humidity in first class from 5 percent to about 15-25 percent, and that “food tastes better, passengers sleep better and there is a greater sense of well-being.”

The fact that the taste of food is enhanced may well be something we notice, although it could be argued that in the rest of the aircraft, the less responsive you are to the taste of airline food, the more edible it is likely to be. Still, since airlines recognize that a key differentiator of their products is the quality of the food and wine served, the dryness of the air poses a notable challenge.

It should also be pointed out that research in this area is still being conducted. Boeing says that a study it carried out with the Technical University of Denmark in 2005 showed that humidity was not the only factor driving the symptoms associated with dryness, including throat and eye irritation, headaches and occasional dizziness. In fact, the most effective technology in minimizing such symptoms was found to be the air filtration systems, and modern aircraft have very effective ones, much better than in offices, for instance.

Cabin Fever

Still, can dry air on board affect our natural defenses? It’s doubtful, says the Hygiene Council’s John Oxford, “You are far more likely to catch something in the taxi to the airport, which has no effective air purification in place.”

According to Oxford, a prime reason for people to fall ill after a flight may not be anything in the air, but the “fecal-oral route.” This is people who don’t wash their hands after leaving the toilet, then leave traces of their feces on the door handle and everywhere else they touch. Then, if you touch the handle on your way out (having washed your hands) and eat a bread roll upon returning to your seat, you are more likely to fall ill. The quality of air will be irrelevant.

This observation is probably worth bearing in mind, coming from Oxford – someone who, as well as being so senior in his field, hasn’t had flu for 30 years. His advice: Wash your hands, take hand sanitizer with you when you travel and if you see someone coughing and sneezing, try to keep your distance as much as possible.

By Tom Otley