Coronavirus Brings Cabin Humidity Levels into Question

 - July 1, 2020, 9:00 AM

This story is part of AIN's continuing coverage of the impact of the coronavirus on aviation.

Jeff Gusky, an emergency room doctor, believes the aviation industry is ignoring vital information that could help prevent infection by coronaviruses like Covid-19. The key is high humidity, he explained, because low humidity levels are a critical factor in the spread of Covid-19. 

“Dry indoor air is like an accelerant that turns small viral outbreaks into massive viral bombs,” he explained. “Dangerously dry indoor air was like an invisible blasting cap that made Wuhan, New York City, and other hot spots explode.”

Low relative humidity in aircraft cabins—as low as 5 percent at altitude—is not healthy for humans. It creates a host of problems, including drying out sinuses and diminishing the ability to smell and taste. But those inconveniences pale in comparison to the vulnerability to disease. It is especially problematic on long intercontinental flights with fewer people on board because much of the humidity that does exist in aircraft cabins is supplied by the breath of passengers. A person exhales about 3.5 ounces of water per hour.

Even before the coronavirus pandemic, medical experts and scientists knew that dehydrated (and thus shrunken) mucus membranes are much less adept at protecting humans from incoming bacteria and viruses of any kind, let alone the super-contagious Covid-19. So keeping cabin air well-hydrated is now more important than ever.

According to a report titled “Seasonality of Viral Infections” by Miryu Morijami, Walter Hugentobler, and Akiko Iwasaki published by the Annual Review of Virology, “An incoming virus first must find epithelial cells to invade the host. Mucus layers can effectively trap the virus before it can enter the host cells. Mucus secreted from the submucosal glands serves as a mechanical barrier and as a chemical barrier by its antimicrobial properties.”

The report added: “Multi-tiered host airway defense systems prevent infection by incoming respiratory viruses. [But] fluctuations of temperature and humidity of the inhaled air have been shown to directly affect the airway mucosal surface defense at multiple levels...The intervention studies in school and nursery children, office workers, and army recruits have shown that increasing humidity from low to median range reduced respiratory infection rates and absenteeism.”

Gusky advises passengers and flight crew to drink a lot of water to hydrate their mucus membranes to avoid viral infections, and he is also alerting airlines, aircraft manufacturers, and health authorities about the benefits of humidity. In Cincinnati, Ohio, a remarkably low rate of infections among the city’s police department is due to members working more outdoors, according to Gusky. “They have the lowest Covid infection rate of any large city police department in Ohio,” he told AIN. “The Cincinnati jail has had zero Covid cases amongst inmates. It turns out that the jail is the safest place to be in Cincinnati re: Covid. The jail’s humidity is a constant 52 percent.” Optimal indoor humidity levels are about 60 percent.

Effect of dry air on mucociliary clearance.
Effect of dry air on mucociliary clearance. (a) Proper mucus hydration is required for the efficient mucous transport. (b) Dehydration caused by dry breathing air leads to increased viscoelasticity of the mucous layer and immobilizes cilia, which are pressed down by the reduced height of the dehydrated periciliary layer. (Source: Annual Review of Virology)

For indoor areas, which typically are heated and cooled without regard for the dangers due to lack of humidity, Gusky proposes a “Viral Safety Index” that would inform people of the risks they face. This index could be disseminated as part of weather reports or by a building’s operator.

For example, he explained, “‘Green’ indoor air is like social distancing on steroids. Green indoor air is in the range of 50 to 60 percent humidity measured on an inexpensive digital hygrometer costing about $10. Covid does not do well in green air. When it’s ‘red’ we humidify and hydrate. When it’s green we can all breathe easier. The Viral Safety Index gives advance warning of dangerously dry indoor air in time to make red air green.

“We spend 90 percent of our time indoors,” he added. “And it’s critically important to keep indoor air green around-the-clock until the crisis has passed. This is especially important for the elderly, for people living in densely populated inner cities, and for anyone with a pre-existing medical condition.”

When it comes to flying, there have been some advances in improving cabin humidity, but extremely dry air remains the case in most pressurized aircraft. At the same time, aircraft cabins don’t like moisture. And when warm moist air inside encounters fuselage skin that can be cooled to 50 degrees below zero, it condenses and can saturate insulation blankets, compromise wiring, and lead to corrosion. This moisture also adds weight, which translates into reduced payload and lower range.

Sweden-based CTT Systems has a solution to the problem with its Cair technology. The system includes not just a well-filtered humidifier for each temperature zone in the cabin, but also its Zonal Drying dehumidifying device that targets the space between the interior cabin shell and the external skin of the fuselage. Combining both systems in one installation humidifies the cabin but keeps the moisture-vulnerable parts of the aircraft dry.

CTT’s Cair system also addresses the problem of corrosion, another reason why aircraft manufacturers have been loath to turn moisture-generating humidifiers loose inside their airframes. It’s one reason why the mostly composite Boeing 787 is able to offer cabin humidity levels as much as three times higher than conventional airliners (up to 15 percent for the Dreamliner, which is still low compared with the 50 to 60 percent recommended for indoor levels).

Unfortunately, CTT’s Cair system is not yet available for most business jets, although it is available for large VIP jets. Acropolis Aviation’s Airbus ACJ320neo is equipped with a Cair system. 

“We have focused, so far, on large VIP aircraft, the ACJ320 up to the [Boeing] 747-8—and commercial airlines—due to the fact that our product requires some space,” said CTT v-p of sales and marketing Peter Landquist. “Large-cabin business aircraft such as the [Bombardier] Global 7500 and [Gulfstream] G700 are very interesting for us. It’s still tight for space, but we are confident that our humidifier system can be fitted in these type of aircraft.”

The Cair humidifiers use a glass-fiber pad with moistened air channels. As water, which could come from the aircraft’s regular potable supply, dampens the pad, dry air passes through, evaporating the water and humidifying the air. The pad also traps contaminants, reducing the danger of spreading bacteria.

The Zonal Drying component—a fan, heater, and a rotor—draws air from the cabin and dries it as it is blown into the gap between the cabin shell and the exterior skin. That keeps the relative humidity low in the localized air surrounding wiring and sensitive aluminum structure. 

“The Zonal Drying system can be installed as a standalone in an aircraft where high cabin humidity is expected, typically low-cost/charter airlines with high passenger load,” Landquist explained. “[Cair] cabin humidifiers, however, are always installed together with a Zonal Drying system. Installation time is 150 to 300 man-hours, depending on how many humidifiers and Zonal Drying systems are needed. The system’s weight is between 35 and 150 kilograms [77 to 330 pounds], depending on system size. Water consumption is from five to 25 liters per hour.”

French supplier Liebherr Aerospace also manufactures aircraft humidification systems, which boost humidification levels to 20 to 25 percent, according to the company. The Liebherr device produces steam using hot bleed air from the aircraft’s engines.