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I understand that half the air in an airliner is re-circulated (per How much air is recirculated (vs. bleed air injected) in modern airliners cabin?). Of this 50% re-circulated air, what it's the chance that viruses pass through its filters?

Disregarding possible recent developments due to the COVID-19 outbreak, were filters normally designed to remove viruses or sterilize the air somehow?

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Modern airliners use HEPA filters to filter the air before re-circulation. This will remove almost all airborne pathogens like viruses:

The overall risk of contracting a disease from an ill person onboard an airplane 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. anywhere where a person is in close contact with others.

That said, the risk on airplanes is probably lower than in many confined spaces because modern airplanes have cabin air filtration systems equipped with HEPA filters.

HEPA or high efficiency particulate air filters have similar performance to those used to keep the air clean in hospital operating rooms and industrial clean rooms. These filters are very effective at trapping microscopic particles as small as bacteria and viruses.

[...]

Air filters can remove very small particles such as bacteria and viruses. Virtually all viruses and bacteria are removed; even the most difficult particles in the range of 0.1 to 0.3 micron are filtered out with an efficiency level of of 99.995%. Contrary to popular belief, very small particles below 0.1 micron are easily filtered out by the mechanism of diffusional interception.

(IATA Briefing Paper - Cabin air quality - Risk of communicable diseases transmission, emphasis mine)

The following image shows the location of this filter in front of the re-circulation fan for a Boeing 737:

Boeing 737 Air Circulation System
(Boeing 737 NG FCOMv2 2.30.5 Air Systems - Air Conditioning System Description)

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IF the airliner has HEPA filtration yes, so pathogens floating in the air that happen to get drawn into the recric system would be trapped and not recirculated, so some of the total pathogen load in the cabin, but not all, would be removed. In theory, if you just added a fixed amount of pathogens at the start and just let it run, the concentration would slowly decrease. Problem is, if there are sick pax on board they are continuously issuing them, so the filtration is only a mitigation.

If an airplane doesn't have HEPA filtration, effectively, forget it. There are HEPA elements made for older airliners with existing recirc filters under PMA that are a direct replacement, but you won't know which airplane you're on has them. Maybe you can ask when a book a ticket.

In any case, I said "only a mitigation" because the pathogens you are going to be exposed to on the flight are all around you in the cabin and what may be removed by the recirc system really isn't going to help you all that much. If you catch something it's usually because you touched a contaminated surface and then touched your face at an entry point, like the eye, after.

This is why wearing face masks can help in preventing breathing in a pathogen directly, but you usually don't acquire them that way anyway. When you catch a cold from someone in the office, it's rarely because they coughed in your face; it's because you touched something they touched, or coughed on, earlier and then rubbed your eyes or scratched your nose.

People who are very disciplined about never touching anywhere near their faces with their hands probably have the best protection from catching these kinds of things.

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    $\begingroup$ Well done for staying with the real question. $\endgroup$ – Harper - Reinstate Monica Mar 7 '20 at 23:54
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    $\begingroup$ Most face masks people are wearing do not protect from anything breathed in. They do help a lot in protecting others from what you breathe/cough/sneeze out. $\endgroup$ – WGroleau Mar 8 '20 at 1:31
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    $\begingroup$ @Harper-ReinstateMonica the "real" question was about airliner filter systems. This looks like general health advice and (albeit undoubtably correct health advice) doesn't answer the question at all. $\endgroup$ – Jamiec Mar 8 '20 at 10:36
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    $\begingroup$ I would reword that to avoid starting the second paragraph with "This is why wearing face masks can help", as a lot of people will just stop reading there. $\endgroup$ – njzk2 Mar 8 '20 at 12:26
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    $\begingroup$ Now that information has been added directly answering the question, the last two paragraph seem pretty tangential and distracting. Even though they made up most of the content of the original answer, I think it would improve the answer's quality to remove them. $\endgroup$ – Will Mar 9 '20 at 9:45

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