Cabin air can be extremely dry and cause dehydration and a reduced immune system. This results in increased risk of getting infections during and after a flight. Steve Bridgewater looks at air filters, humidifiers and how best to keep passengers fresh and hydrated

[This article first appeared in the November/ December 2021 issue of Airline Cabin Management]

As more attention is placed on health, safety and sanitation, passengers are understandably starting to look more closely at what’s inside their aircraft. Maybe more importantly, they want to know what’s kept out. 

In a recent briefing paper, the International Air Transport Association (IATA) said: “The overall risk of contracting a disease from an ill person on board an aircraft is similar to that in other confined areas with high occupant density, such as a bus, a subway or movie theatre, for a similar time of exposure. That said, the risk on aircraft is probably lower than in many confined spaces because aeroplanes have filtration systems equipped with HEPA filters.”

Humidity

HEPA (High Efficiency Particulate Air) filters have similar performance to those used to keep the air clean in hospital operating rooms and are designed to trap 99% of microscopic airborne particles as small as bacteria and viruses. Once it has been filtered, the air is recirculated and provides higher cabin humidity levels.

Without filtration, flights at higher altitudes – particularly those with low occupancy – can see cabin air humidity drop substantially. This increases the risk of eye, mouth and nose disorders as well as cracked lips and dry hands.

According to Peter Landquist, VP senior advisor sales at CTT, which provides humidifiers and dehumidifiers for retrofit and line-fit on commercial aircraft as well as private jets, dehydration is a degenerative process. Noticeable symptoms include changes to the sense of taste and the viscosity of saliva, but passengers’ mucous membranes will also swell, the volatility of odour molecules decrease and the ability to vaporise in the nose decreases and nasal cavities quickly dry. 

“You often get a cold after a flight,” says Landquist, “but the probability is that you didn’t catch that cold ‘on’ the flight. It is more likely that you caught it after you had disembarked and were mixing with other people before your mucus membranes had dried out.

“These membranes contain silica that is our first line of defence against infection. It catches the germs and virus and transports them down to our stomach where they are destroyed by acid. When it is dry it doesn’t work as effectively, and the germs can reach our receptors and we can catch viruses.”

Humidity is therefore important for comfort but is also vital for our immune system, improves our wellbeing, sense of taste and ability to relax and sleep. Several studies also suggest a relationship between insufficient humidity and the transmission of infections, such as Corona viruses. Dry air creates an environment where droplets remain airborne longer, and therefore can possibly transmit infections to a larger extent.

Our normal living environment has a typical humidity ratio of around 30% and a well-filtered cabin with high passenger density can see levels as high as 20%. However, for less-occupied aircraft flying at high altitudes, in-cabin humidity can drop significantly. “The only contributor to humidity in flight is the passengers and crew,” continues Landquist. 

“The paradox is that the higher the passenger density, the better the humidity – so the problem is particularly acute in first and business class cabins where relative humidity can drop to 5-6% and 6-10% respectively – that’s lower than the desert or the Arctic.”

Relative humidity is at its worst in the cockpit and crew rest areas where a figure of 0% is often recorded, so CTT’s Humidifier Onboard system is provided as standard for crew rest areas on the Boeing 787. While it is optional for flight deck equipment, the take-up rate is in excess of 85%.

The equipment is also offered as an option on the Airbus A350 where it has been selected by Aeroflot and China Southern for use in premium class areas. Other airlines have selected it for flight deck as well as cabin and flight crew rest areas. The Boeing 777-X will also have it as an option for first and business class areas as well as crew areas. The system is also available as a retrofit to existing airframes and can be installed (under an STC) when a cabin upgrade is underway.

“Rain in the Plane”

CTT’s Humidifier Onboard unit increases humidity to around 25% – which is still quite low in comparison to the normal environment. “We could increase it further,” explains Landquist, “but firstly that would involve carrying large amounts of water and the weight penalty associated with it. The unit itself would also need to be heavier and more expensive.

“Critically, it would also mean that airlines would need to employ a drying system to counter the condensation formed within the cabin, something that can create ‘rain the in the plane’!”

Even without humidifiers, an aircraft is prone to condensation as ice formed on the inside of the airframe at altitude begins to melt when the aircraft descends. Most of this will run down into the belly where it can easily be emptied through drain valves, but some accumulates in the cabin insulation blankets. 

According to Landquist: “This is a particular problem for carriers with high density cabins – such as low-cost, short haul airlines – especially those operating in regions where there are long, cold, damp winters. This can have an adverse effect on an aircraft’s wiring and electrical components and, on average, these aircraft can carry up to 300kg of water locked within their insulation.

“At CTT, we have calculated that removing 200kg of water results in a weight-saving that equates to a reduction in CO² emissions by the equivalent of 66 tonnes per year per aircraft within a fleet.”

CTT provides its Anti-Condensation System product as a standalone item or in combination with the Humidifier Onboard unit.

How Clean is the Air?

It’s safe to say that the air within a cabin is much cleaner than it is in most of the world’s cities, and regulatory authorities such as the FAA and EASA require a minimum rate of 0.55 pounds per minute outside fresh air flow per occupant. 

David Conrad, vice president for business development at PTI Technologies Inc, which makes all types of filters for the aviation industry, says: “An aircraft exchanges its entire volume of cabin air 20 to 30 times more frequently than the air in an office, and five to six times more than hospitals. 

“The air circulates through strategically placed vents and upper and lower air inlets around the overhead bins. The air flows downward at 1 m/sec (3.3 ft/sec) from top vents and flows out at floor level. Every row has its own vents. That the air flows vertically, not horizontally, limits the probability of virus dissemination.”

Cabin air systems are usually designed to operate most efficiently by delivering approximately 50% outside air and 50% filtered, recirculated air. This normally provides between 15 to 20 cubic feet of total air supply per minute per person in economy class as well as a sterile and particle-free air supply. As the circulation is continuous, fresh air is always flowing into and out of the cabin. 

The 50/50 mix of outside and filtered air is a deliberate choice as the air at cruising altitude contains almost zero moisture. Using 100% outside dry air in the cabin would be very uncomfortable and retaining filtered air from within the aircraft helps maintain the humidity balance. Furthermore, air that is taken from the engine to pressurise the cabin and provide clean air results in increased fuel consumption.

HEPA Filters

According to the ISO/European air filter efficiency classification, a HEPA filter can be “any filter element rated between 85% and 99.995% removal efficiency.” 

To meet HEPA standard in the USA, filters must remove from the air that passes through it at least 99.97% of particles whose diameter is equal to 0.3 microns. “Scientists have found that particles of 0.3 micron size get through air filters more than larger or smaller particles,” says Conrad. 

“To better understand a size of 0.3 microns it is useful to first have some comparisons. The average diameter of a human hair is about 70 to 75 microns and the human eye can see unaided about 40 microns. The majority of the bacteria species are typically between 0.2 and 10 microns in size. While viruses are even smaller in size, filters tend to trap these particles quite well.”

The carbon elements within HEPA filters are traditionally changed as part of an aircraft’s regular maintenance schedule. If they aren’t, IATA points out that “airflow through the filter might be reduced, but the efficiency of the filter at capturing viruses is not affected”. 

Developments

PTI’s Cabin Air filters are designed to remove both smaller particles such as viruses and larger particles such as lint and pollen, and its HEPA media has a demonstrated efficiency of greater than 99.999%. Today, the units are fitted to Airbus, Boeing and Bombardier airliners and are designed and tested to a standard of 7,500 hours between changes. This is roughly a change interval of C Check to C Check or every 18 months. 

David Conrad sees the business segment for cabin air filtration and cabin air quality changing over the coming years. “While Covid has been a catalyst, it is not the only factor driving change,” he says. “We are seeing airline flight crews reporting adverse health effects from fumes in the cabin, and lawsuits filed over cabin air quality.

“In addition, we are seeing studies and engagements by regulatory agencies such as the FAA and EASA studying cabin air quality and health effects. We believe that we have developed the technology for the filtration technology gap and we are testing it now. The next step will be to integrate it into an aircraft and fly it.”