[chris (OP)]

I received this email from listener John, who had heard a piece on this week's edition of 5live science and The Naked Scientists concerning the safety of flying and the transmission of respiratory infections aboard aircraft.

We discussed this issue in the light of the recent coronavirus outbreak on the flight from Zante.

John has raised some good points and provoked a thorough answer from me, which I feel may help other readers. Hence I hope John won't mind my reproducing his email here.

John wrote:

I am possibly unusual in that I listen to both the 5 Live Podcast and the Naked Scientists one!

I take issue with your report in today's broadcast on the safety of flying during Covid. Your interviewee failed to consider one vital point which means that the thrust and conclusion are flawed.

By way of background my father died of Covid-19 in late March while in Tenerife. He us still there and I have, at some point, to deal with his Spanish affairs. I have therefore done a lot of investigation into the safety of flying, particularly as I have underlying medical issues.

Your interviewee mentioned the air filters on planes. After asking many times one airline eventually told me "All our aircraft are fitted with super-efficient, state-of-the-art ventilation and filtration systems to control air quality and circulate fresh air around the cabin every few minutes. These systems are similar to those you would find in hospital
settings and meet the highest standards of clinical hygiene."

So I then asked them what size the filters are in microns. Their response was "the Boeing requirement for a HEPA filter is at least 99.97% efficiency on a 0.3 micron DOP (dioctylphthalate, an aerosol used as a standard for testing) particles tested at the rated flow Per MIL-STD-282, Method 102.9.1, DOP-Smoke Penetration and Air Resistance of
Filters. The HEPA filters recommended in the Reference /A/Multi-Operator Message are rated to HEPA Class H13 (or higher)".

So I responded with the observation that as the Covid-19 virus is 0.125 microns or less then their air filters will be ineffective against it.

At this point the airline stopped corresponding with me.

But the point is that the air filters do not filter out Covid-19 but they do circulate the air every few minutes. Therefore if you have say 100 people who do not know each other in an sealed tube (aeroplane) for hours at a time then the risk is very high and not low as your report stated.

This was my reply:

Dear John

Thank you for writing, and for your careful listening. I'm sorry to hear about your father.

You're right concerning the sizes of the infectious particles: flu and coronaviruses are about 100nm (0.1 micron). However, the information that the airline has given you is not helpful because it did not address your question.

The short answer is that I can reassure you that the HEPA filters are actually very good indeed and in particular in the particle size range that includes respiratory viruses (like coronaviruses), and therefore their effectiveness at scrubbing viruses from air is not in question.

The more detailed answer is that there are two lines of evidence that support the effectiveness of the HEPA filters used on flights: 1) test data on particles and filter performance; 2) actual observed infection spread on flights and in other environments where this filtering regime is used. Obviously, 2) is affected by a range of factors other than just filter performance and therefore is a less stringent measure.

In terms of 1) this paper from NASA (2016) is quite technical but the text at the bottom of page 6 referring to the graph on page 7 is the crucial piece of data.

You'll see that the capture efficiency (blue line) for particles of the 0.1micron scale is nearly 100%.

https://ntrs.nasa.gov/citations/20170005166 - Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials

In terms of 2), this is observational data and WHO audits, which suggest that 70% of transmission cases are within 2 rows of an infected individual. Given how "noisy" these data are likely to be (allowing for factors like people wandering around, contact at the airport, in the toilets, at customs etc), this shows that the "on-board" transmissions appear to be well controlled by the ventilation system. Indeed, in situations that have been studied where aircraft have turned off the air systems owing to malfunctions, infection rates were much higher and the spread much wider.

I hope this goes some way to explaining what Julian Tang was aiming to convey.

Sorry, again, to hear about your circumstances and thank you for listening to our programmes.

Chris

[alancalverd]

Problem is that

1. if you are sitting next to an unmasked carrier, you will inhale some of his exhalate long before it reaches the HEPA filter

2. HEPA filters will eventually saturate, so whilst they may remove new bugs, they will infect the air with old ones - the air is 50% recirculated. They are changed according to the aircraft manufacturer's schedule which is based on days and flight hours, not viral load.

[Bored chemist]

Anyone trying to maintain a clean-room environment will tell you that people are filthy.
They shed dust, skin, hair + stuff.
That's what clogs the filters and that's what gives rise to the filter changing regimen.
The additional load from some (on most flights, hypothetical) viruses is not important.

If they were changing the filters at a sensible rate before then that's good enough.

[evan_au]

I imagine that most of the virus breathed out or emitted during speech will be in the form of fine droplets, which will be much larger than the virus particle.

Perhaps the filters are catching droplets?

[alancalverd]

HEPA filters are specified at 99.97% removal of 0.3 micron particles, so they will probably trap some if not most dry COVID viruses (0.15 μm diameter), but as you say, human particle exhalate is mostly larger droplets anyway.

Problem with aircraft filters is that the service changes seem to be a bit random. I can imagine that an old filter containing moist skin cells and bacteria could actually be a breeding ground for an adaptable virus.   

[Bored chemist]

for an adaptable virus.

In general, viruses are very bad at adapting.
People don't get distemper and their dogs don't get colds.

The human cells in a filter are dead or dying. They are certainly not reproducing so there's no way for a virus to take advantage of them.
It's possible that phages are having a party among the bacteria in the filters but they are generally too dry for much bacterial growth. (Air on planes is notoriously dry)

[Petrochemicals]

I should say the rate of air change is the primary factor, planes seem very airy, most likely they are changing the air once a minute if not more, this will mean that viruses do not have much dwell time. The air is also exchanged for outside air pretty regularly (i did not know planes bothered with filters, seems like alot of weight for no reason) which is why you dont suffocate and get fume events in the cabin.

https://en.m.wikipedia.org/wiki/Fume_event

[Bored chemist]

most likely they are changing the air once a minute if not more

No.
Every few minutes if you count "recycled" air, and only half that if you don't.

(i did not know planes bothered with filters, seems like alot of weight for no reason)

You may be shocked to learn this but- among other things-
IT'S TO REDUCE THE SPREAD OF INFECTIOUS DISEASES

Most people reading this thread probably consider that quite a good reason.

which is why you dont suffocate and get fume events in the cabin.

There's a reason why fume events have a name and a wiki page; and it's not because they don't happen.

[evan_au]

The Army Chemical Corps developed a combination mechanical blower and air purifier unit, which incorporated cellulose-asbestos paper

Perhaps the prototype HEPA filters in WW1 protected people from immediate death from poison gas, and subjected them to a long-term risk of asbestosis?

an adaptable virus

I understand that the Novel Coronavirus genome incorporates an "auditing" mechanism which is used during viral replication.
This means that the rate of mutations is much lower than other viruses that don't incorporate such a mechanism.

Humans have multiple mechanisms to audit genes during cell division and DNA repair, otherwise we would all die of cancer before reaching adulthood...

[alancalverd]

People don't get distemper and their dogs don't get colds.

Today's fashionable virus sees to have adapted well to its new host. Or maybe the news hasn't reached you yet!

[Bored chemist]

People don't get distemper and their dogs don't get colds.

Today's fashionable virus sees to have adapted well to its new host. Or maybe the news hasn't reached you yet!

Yes, so it's not going to do well in the bacterial/ fungal/ other microbiological/ cells in an aircraft's hepa filters.
That was exactly the point I was making.
Let's clarify something.
Covid is so specifically adapted that it isn't just a matter of just "only targets people and bats".
It only targets the cells of the respiratory tract of people.
It doesn't usually, for example, target liver cells.

So the idea that it's going to leap into bacteria and back during a flight across the Atlantic is... shall we say "a low priority"

[alancalverd]

Covid is so specifically adapted that it isn't just a matter of just "only targets people and bats".
It only targets the cells of the respiratory tract of people.

and bats and cats and ferrets, so far, so it's moderately adaptable.

So the idea that it's going to leap into bacteria and back during a flight across the Atlantic is... shall we say "a low priority"

One transatlantic trip is a very short time in the life of a HEPA filter. Prior to COVID, most airliners spent an average of 15 hours per day in the sky, with the filters changed every 12 months. That's a lot of generations in a warm, wet environment - Darwin would have been beside himself with joy if HEPA and electron microscopes had been around!

[Bored chemist]

That's a lot of generations in a warm, wet environment

You don't get a wet environment by dragging aircraft cabin air through it.

I suspect the bugs are spores or dead.
Neither is any use to a virus.

[alancalverd]

You don't get a wet environment by dragging aircraft cabin air through it.

Not if the passengers are dead, but most of us exhale and sweat enough H₂O (around 40 ml per hour) to mist up the windows and drip from the ceiling if the aircon is switched off, so something like 120 liters of condensible water pass through the filters every day, over and above the required 50% relative humidity of what is circulated. 

What is found in practice, anyway, is that anyone within 3 seats of a person with a "conventional" respiratory infection, has around 30% probability of developing symptoms after a 5 hour flight, so whatever the theoretical efficiency of the aircon system, there is a good reason why  airlines are now monitoring embarking passengers and at Frankfurt, at least, testing all arriving passengers for COVID and tracking everyone from an infected flight.

[Bored chemist]

"At cruise altitudes, the outside air contains very little moisture, and the main sources of humidity in the cabin air are respiration and evaporation from the skin of occupants. The steady supply of dry outside air is more than sufficient to flush the human-generated moisture from the cabin and maintain a low moisture content in the air, typically 10–20% relative humidity at cruise altitudes. Such values of relative humidity are below comfort guidelines"

from
https://www.ncbi.nlm.nih.gov/books/NBK207472

[alancalverd]

Such values of relative humidity are below comfort guidelines"

Which is another reason why the air is 50% recirculated in a passenger can. The whole system is remarkably complicated. The cold dry ambient intake air is compressed from 40,000 ft to 6-8,000 ft density which heats it, so it's then cooled by exchange with ambient, then mixed with recirc plus a bit of water condensed from the rejected stale air to bring the cabin humidity up to something tolerable. The fact that it is frequently uncomfortable (the BAe 146 was never right, even after they got rid of the fuel fumes) is symptomatic of the complexity of the system. If you didn't recirculate you'd have to carry additional water and the system would be heavier and even more complicated! 

[Bored chemist]

If you didn't recirculate you'd have to carry additional water and the system would be heavier and even more complicated! 

I could live with that.
It's the legionella risk from the humidifiers I'd be worried about.

In practice, when you have finished arguing,  the air on planes is dead dry and so are the contents of the HEPA filters.

[alancalverd]

the air on planes is dead dry

I'll have a word with the aircon engineer at Easyjet - he's either been lying to me or not running as tight a ship as he thinks. Or maybe you've been using the wrong airline. If the air is "dead dry" they can sell more beer, but AFAIK not even Ryanair has stooped that low.

[Bored chemist]

you've been using the wrong airline.

FFS it's not me you are arguing against...
https://www.ncbi.nlm.nih.gov/books/NBK207472



"The World Health Organization (WHO) reports that humidity in airplane cabins is usually less than 20 percent. For comparison, the Environmental Protection Agency (EPA) recommends that houses be kept between 30 and 50 percent humidity. Low airplane humidity is, for now, a fact "
https://www.smartertravel.com/low-airplane-humidity/
(My bolding)
Why don't you cut to the chase + cite evidence that cabin humidity is  commonly greater than 50%?