So, I’m reading my newspaper and I see this headline appear as I turn the page:  Airflow in planes may slow virus’s (sic) spread

My first thoughts are “wow”.  Because I know that a 4.7 hour flight from Tel Aviv to Frankfurt with 102 passengers (7 COVID-19 positive on arrival) involved at least 7  more folks to become infected.  None of them were wearing masks. O,r that woman on the London to Hanoi flight that infected 15 others.  And, SARS (similar to COVID-19) yields a 1 in 3 chance for infection on a 5 hours flight. How could this test provide such different outcomes?

This study was authored by David Silcott , Blake Silcott, Ryan Silcott, Peter Silcott, Braden Silcott,  (all associated with S3i LLC- no real website!), Sean Kinahan, Steven Distelhorst, Danielle Rivera, Kevin Crown, Gabriel Lucero (these 5 from National Strategic Research Institute of U Nebraska), Joshua Santarpia and Vicki Herrera (both from U Nebraska), Wayne Bryden, Mike McLoughlin, Maximilian Cetta (these 3 from Zeteo Tech), and Russell Accardi (L2 Defense).  The research was performed at the behest of US Transcom, the agency responsible for transporting our troops and their families.

The actual document summarizes the 300 test runs effected- using a mannequin that was set up to breathe or cough at various places around 767 and 777 planes. Instead of using coronavirus (which would be dangerous to do), the researchers tested using 1 micron fluorescent pellets or 3 micron pellets that had attached DNA on its surface.  Their test results and analysis then were used to assert that any flights shorter than 54 hours would not cause a patient to develop COVID-19.

Except…

Most aerosols- from breathing, from talking, from sneezing, or from coughing have multiple sized aerosols.  And each aerosol can contain from 2 to 60 viral particles.

As stated above, these tests used two different sized aerosols- separately and not together.  And, of course there was no talking during their experiments (which could lead to viral shedding a decade more intense), nor was there any passenger movement- after all, it was just one mannequin!  And, given that only one mannequin was used- the researchers are assuming that only one traveler would be COVID-19 positive.  Moreover, this study never examined lavatory use or conditions.  Or, the effects of other passengers (infected to not) upon the results.

The researchers had the mannequin emit 4000 particles per hour using a normal ‘breathing’ rate of 7.5 L/min; at a frequency of 15 breaths a minute.  Their monitoring device  only sampled at a rate of 3.5 L/min- so the researchers multiplied the results by 2.14  (3.5*2.14=7.5) to “make up” for it.  The problem with such a low sampling rate is that the air is being pumped and filtered at a faster rate than the sampler operates, which means particles could have been emitted that were handled by the air system without ever passing through the detector.

And, the researchers arbitrarily picked 1000 viral particles as the load that would lead to patient infection.  But, the loading that could cause a subject to become ill with COVID-19 could be as low 50 or 100 (or even lower)- which means the study’s assumptions can miss the fact that many more folks are susceptible to infection.  Just like we found in real life (by the examples mentioned above).

 

(We do not know the minimum infective dose for SARS-CoV-2 .  That will first be determined around January 2021, when British health care workers will deliberately infect volunteers with steadily increasing doses of SARS-CoV-2.  The purpose is, ultimately, to infect folks who have been vaccinated to ascertain the vaccine’s  effectivity.)

(Given the fact that this pandemic is the new normal, there is also serious talk about changing the plane’s ventilation systems, too.  If air were to flow into the cabin from the floor up to the ceiling, the air flow (augmented by its heating from the passengers) would ‘sweep’ the aerosols up and away from the passengers.  The top down flow tends to disperse the aerosol around the plane.  We’ll see if this is just talk or they really do change the ventilation pattern.)

We also have to remember that these tests were effected on 767 and 777 planes.  The ventilation systems on these planes actually pump air to effect 32 to 35 air changes an hour.  And, those air flows are split- 50% of the air flow from the actual airplane is passing through  HEPA filters and is admixed with fresh air for the other half of the flow; that  rate is flowing at about 7 ft³/min (52 gpm).

But, this research used EMPTY planes.  No passenger space was tested using full-sized mannequins- instead,  just ONE mannequin (and that was simply a head shaped puppet) was present on the entire plane.  Which changes the space and momentum for free air motion.  Which also changes the fact that other folks are producing air flows that interfere with the top-down air flow that exists in an empty airplane.

Consider this.   A 767 routinely carries 250 passengers (the range is 200 to 300, but the airlines generally stuff 250 to 300 folks on board) and a 777 caries 301 to 368 passengers.

So, if the plane had 100 passengers (an empty flight for these planes), the total airflow from passenger breathing would sum up to some 750 liters per minute- or 100 gpm.  That’s about twice the air flow that the plane provides.   Which means the top-down motion is going to be greatly disrupted.  Actually, each seat would have a maximum air flow (767 airplane) of about 0.2 gpm flow from ceiling to floor (dividing the total air by the number of vents over the seats- and the ceiling vents), while the passenger [assuming s/he is unmoving and facing front] emits 1.9 gpm perpendicularly.  (The  pressure that the ventilation provides versus exhalation from a person does alter the results, too.)  Even so, there probably is a far greater force propelling the droplets forward (exhalation) than there is drawing them down to the ground (ventilation).

And, consider this.  If the plane were full, there’s be 250 people pushing the air forward- and 8 X the flowrate presumably moving horizontally, compared to the plane ventilation that is drawing them downward.  (Also, in this case, the total top down flow is still about 50 gpm, but the transverse flow would be 495 gpm- albeit at different pressures.)

Yes, this test study is great for a plane carrying a single passenger weighing virtually nothing.  But, absolutely useless for a partially filled or fully-stuffed cabin.

So much for the study’s promise of safe air travel.  I wish it were true.