We’ve all felt the effects of dry conditions on board – but can anything can be done about it? Tom Otley reports

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

The advent of air conditioning means we can live comfortably in places we previously wouldn’t have considered, using a humidifier alongside it if the climate is dry as well as hot. But when we’re travelling, it’s more complicated. 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 that the cabin air is 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 Organisation says: “The humidity in aircraft cabins is low, usually less than 20 per cent (in the home it’s normally over 30 per cent). Low humidity may cause skin dryness and discomfort to the eyes, mouth and nose but presents no risk to health.”

The organisation 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 moisturising 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 defences 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 expenses department.


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 per cent 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 per cent, which is still lower than on the ground, but manageable. In first class, however, it can be as little as 5 per cent, 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 seats are full. As business seats have moved towards being fully flat, cabins have become more lightly populated, making the problem worse. In addition, flight duration has increased with aircraft able to perform longer sectors. So while premium flyers 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.


Do new-generation aircraft solve the problem? It’s true that the likes of Boeing’s B787 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 pressurisation. (See page 26 for more on Boeing’s innovations.)

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, and not just for long-haul aircraft. 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. And as airlines try to fit more passengers in, the potential problem of condensation becomes greater – particularly if operating through the winter where the plane gets cold when doors are open, and then warms up during the flight.

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. Of course, whether it is fitted by an airline in part depends on the price of fuel (since that is a factor in recovering the cost of installing the system, and flying its extra weight around).

For long-haul, the Cair system has been installed in cockpits and crew rest areas, as well as on business and private jets, but for commercial scheduled aircraft only Lufthansa has fitted it, in its A380 first class. In part, this is because it is a feature that is difficult for a passenger to identify 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 travellers.


Ironically, the humidifier system may help travellers to 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 per cent to about 15-25 per cent, 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 recognise that a key differentiator of their products is the quality of the food and wine served, as witnessed by the success of Business Traveller’s Cellars in the Sky wine awards (visit businesstraveller.com/cellars-2015 for the winners), the dryness of the air poses a notable challenge.

In 2011, British Airways created a flavour-boosting umami-rich “height cuisine” menu to try to overcome it. Yet if the cabin had normal humidity, we’d be able to appreciate the dining properly.

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 minimising such symptoms was found to be the air filtration systems, and modern aircraft have very effective ones, much better than in offices, for instance.

In other words, you are more likely to catch a cold from a fellow worker than a neighbouring passenger.


Still, can dry air on board affect our natural defences? The Hygiene Council’s John Oxford points out that in humid countries around the equator the flu virus is always present, yet there’s no evidence that people who live there are better at fighting flu just because of the moisture in the air. Likewise, although it is true that the flu virus moves between the northern and southern hemispheres in winter, we catch it not because of humidity but because we spend more time indoors so are closer to infected people. “You are far more likely to catch something in the taxi to the airport, which has no effective air purification in place,” he says.

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

Still, if sitting in a dry atmosphere is not good for us, what can we do about it? Well, the WHO advice is probably worth looking at – a mixture of moisturiser for our skin, a spray for our noses – although note that some research says regular use of the latter can cause problems, and certainly Oxford sees no reason for nasal sprays and doesn’t use them himself.

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


When Boeing launched the B787, it called it the Dreamliner. While this was partly marketing, it was also a reflection of what it hoped the aircraft would represent for flyers based on the research it had conducted over many years.

As was explained to me at the manufacturer’s factory in Seattle in 2009, Boeing knew that regular flyers preferred some aircraft to others for lots of reasons – namely comfort, on-board technology and perceived safety. So the trick was to find out the sort of things that flyers valued, and then design an aircraft that could supply them.

Kent Craver, regional director of passenger satisfaction and revenue marketing at Boeing at the time, told me: “The irony was that when we held focus groups of passengers, we found out that most people did not look forward to the flying experience. As a company, that was an acute challenge – imagine making something that your customers did not enjoy using.”

From that came a desire to help passengers rediscover their love of flying, so windows were made
30 per cent bigger than those on the B777 – allowing people to see much more, which begins the process of reconnecting them to the physical act of flying, as well as enabling those in the centre of the aircraft to see the horizon.

There was also a reduction in cabin altitude. On all commercial aircraft, a balance is struck between offering a breathable atmosphere (something that most travellers would say is quite important), and not over-pressurising the plane (lowering altitude), which is expensive and stresses the fuselage.

For decades, the average altitude on board has been kept at 8,000 feet. After research at Oklahoma State University, Boeing found that most of the low-level effects of altitude sickness, such as headaches, nausea and fatigue, dissipated below 6,000 feet. Hence, the aircraft is now pressured to 6,000 feet, something that the composite structure allows. When combined with the slightly higher humidity it also allows for, Boeing thinks travellers will see the benefits.

The first customer for the B787 was ANA in 2011. Since then, the aircraft has been commercially flown in two models, the B787-8 and -9. It was joined in 2013 by the A350 XWB, Airbus’ answer to the Dreamliner, which employs a new air-conditioning system that splits the cabin into multiple zones, providing fresher air and better temperature control.