We investigate the misunderstanding about aircraft cabin relative humidity levels and passenger wellbeing.

The introduction of much stronger primary structures made from composite materials on the Airbus A350 and Boeing 787 has allowed the OEMs to reduce the cabin pressure altitude from 8,000ft to 6,000ft. This is being interpreted by some people as producing a much more comfortable cabin because the relative humidity (RH) levels have increased.

Unfortunately, this is not really the case, as the difference between the two altitudes is only 1-2 per cent. It is true that both airframers have made other improvements to the cabin that has a side effect of improving passenger wellbeing, such as air management systems that renew the cabin air much more frequently, temperature control in multiple cabin zones and draught-free ventilation.

However, there is still a long way to go to get to the generally accepted comfort level of around 20 per cent RH. RH decreases with altitude until, says NASA, it is less than 1 per cent at typical aircraft cruising altitudes between 35,000ft and 41,000ft.

That means the air being drawn into the cabin is extremely dry, and passengers and crew are the only sources of moisture. The result is that economy class is typically around 12 per cent, while business and first have to endure 7 per cent and 5 per cent respectively.

Adding 1-2 per cent to those figures leaves an obvious shortfall and it is clear that those who pay most suffer most. Of course, densification is another new trend and those packed more tightly in the back may be able to ‘subsidise’ those at the front a little more.

But why is improving RH such a big deal?

There are two sides to the answer. One is passenger wellbeing, as mentioned already, the other is the effect of low RH on the passenger’s ability to enjoy the services provided by the airline, increasingly a market differentiator.

Surveys consistently show that sleep is the most important factor for premium passengers, who want to arrive feeling fresh and relaxed at their destination, closely followed by privacy. Hence the recent proliferation of suites across many major carriers.

Another differentiator is the quality of catering, and many airlines are collaborating with celebrity chefs (most recently, JAL announced that one-star Michelin chef Ryuji Teshima of Restaurant PAGES in Paris would supervise a menu for First and Business Class passengers) or sommeliers (AEGEAN Airlines is working with the first-ever Greek Master of Wine, Konstantinos Lazarakis).

Now consider the effects of low RH levels on the human body, which are not pleasant. They include dehydration of the eyes, sinuses and mucous membranes; nasal congestion and sinus pressure; altered perception of flavours; and a significant reduction (up to 30%) in smell and taste. In addition, it can increase susceptibility to viruses.

Altogether a mix of symptoms that reduces the benefits being offered by the airlines to their most valued guests. Some passengers are taking matters into their own hands.

The HumidiFlyer filtered mask is designed to trap moisture from exhaled breath, although the visual appearance of the entire New Zealand rugby team in masks must have been somewhat challenging – they have used it for several years when travelling long haul – and there is probably a question about comfort on very long flights.

Clearly, there needs to be a better solution and that is active humidification of the cabin. There is a wealth of experience to prove the benefits, as humidifiers from CTT Systems have been used in flight decks and crew rest areas for a number of years on Airbus A380 (more than 200 crew rest systems delivered) and Boeing 787 (more than 1,500 crew rest systems delivered and fitted to 85 per cent of flight decks).

The system is also used extensively throughout the cabin on large VIP aircraft, many of whose owners would probably fly first class when using airlines. Another hint to airlines that humidification is required.

The OEMs are taking the hint, though, as the CTT Systems equipment is now an option on the A350 and 777X not only for the flight deck and crew rest areas but also for Cabin Zones 1, 2 and 3 and Cabin Zones 1 and 2 respectively.

And one airline has taken the plunge. In April, China Southern Airlines selected the full range for 20 A350-900 aircraft that it has on order for delivery from mid-2019. This perhaps not so surprising as the airline has a flight deck and crew rest humidifiers on five Airbus A380-800s and 10 Boeing 787-8s in service and for 20 Boeing 787-9s for delivery from 2Q18.

Peter Landquist, vice president sales and marketing at CTT Systems, says “The well-being era, debuting with new-generation widebody aircraft, has made great progress in lowering cabin altitude, reducing noise and vibrations, generating draught-free and fresher air, multiple temperature zones and advanced mood lighting to counter jet lag. The last frontier to significantly improve well being is a significant increase in humidity.”

Editor’s Note: The post was originally published in July 2018.