We Might Be Masking on Planes for a Good Long While

I was masking on planes for years before the pandemic. So much nicer to not have a cold or flu in addition to jet lag at my destination.

Every media discussion of disease transmission on airplanes (that I've seen, at least) manages to miss or misrepresent the issues as presented in scientific studies.

I posted before a detailed article from Boeing[1] about cabin air quality, but the basic point is that air taken in while cruising undergoes high heat and pressure, so that fresh air input to the cabin is essentially sterile and has <1% relative humidity. The fresh air is mixed 50/50 with cabin air that has been HEPA filtered so that pathogen count is extremely low [2]. Because of this, many have taken the stance that there is no point in wearing a mask on a plane.

However, it is well documented that about 20% of passengers on any flight tend to catch a cold (rhinovirus). With the assumption that air dispersal is not the problem, health advice for many years has focused on surface transmission (fomites) and hand hygene. There is much anecdotal evidence to support this approach. For example, several years ago, a flight attendant told me that they used to catch colds extremely frequently, but since they started wearing gloves to collect trash the problem mostly stopped.

While that approach seems to be effective for rhinoviruses, influenza and coronaviruses are airborne and there have been documented cases of in-flight transmission, though they are relatively rare [3,4,5]. Since cases of transmission are so rare relative to numbers of infected passengers, it is suspected that faulty ventillation has been the cause [6].

However, even when the filtering system is working optimally, there is still a greatly increased risk of transmission due to low air humidity. When relative humidity drops below about 35%, two bad things happen. First, airborne viruses remain viable longer and travel much farther. Second, the mucous membranes in your eyes and nose lose their ability to block pathogens [7,8,9]. During a flight, cabin air RH is generally 5%-20%.

It has been understood at least since the 1970s that low humidity was a significant health risk factor on flights. There was some experimentation in the 1980s with humidifying systems for planes, but they were heavy, costly, hard to maintain, and higher humidity increased corrosion of the aircraft structure [10]. So they decided to just pretend the problem didn't exist.

To defend yourself from infection on a plane, you have to protect both your nose and eyes, both from pathogens on your hands and in the air. In addition to strict hand hygiene, I use eye/nose moisturizers and protect my eyes with Ziena moisture chamber glasses. That routine has kept me from catching colds on long flights for several years now.

[1] https://news.ycombinator.com/item?id=30332290

[2] https://pubmed.ncbi.nlm.nih.gov/8596601/

[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2984860/

[4] https://pubmed.ncbi.nlm.nih.gov/27253070/

[5] https://pubmed.ncbi.nlm.nih.gov/34480171/

[6] https://pubmed.ncbi.nlm.nih.gov/15767002/

[7] https://portal.research.lu.se/en/publications/common-cold-tr...

[8] https://pubmed.ncbi.nlm.nih.gov/17953482/

[9] https://aaqr.org/articles/aaqr-20-06-covid-0302

[10] https://link.springer.com/book/10.1007/b12603