Are HEPA filters likely to pull COVID-19 out of the air?

post by John_Maxwell (John_Maxwell_IV) · 2020-03-25T01:07:18.833Z · LW · GW · 3 comments

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Could hospitals install HEPA filters in common spaces and run them 24/7 to decrease COVID-19 spread?

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answer by John_Maxwell · 2020-03-25T01:35:55.709Z · LW(p) · GW(p)

Some quick Googling and skimming of articles:

A doctor quote in this Buzzfeed article points out that HEPA filters are rated to remove particles of .3 micrometers, and a coronavirus is about .1 micrometer. He says this means HEPA filters won't do the job. EDIT: However, Wikipedia states:

The common assumption that a HEPA filter acts like a sieve where particles smaller than the largest opening can pass through is incorrect and impractical. Unlike membrane filters at this pore size, where particles as wide as the largest opening or distance between fibers can not pass in between them at all, HEPA filters are designed to target much smaller pollutants and particles.

The abstract of this preprint states: "Aerosolized viruses are commonly thought to exist as agglomerates, which would increase the particle size and consequently increase their capture efficiency. However, many of the threat agent viruses can be highly agglomerated and still exist as [submicrometer] particles."

I wonder whether the relevant size is not the size of the virus but rather the size of a cough droplet. This paper seems to have found that droplets are greater than .5 micrometers in size. This article distinguishes between "droplets" (>5 micrometers) and "droplet nuclei" (≤5 micrometers) and says that "droplet nuclei" are the only kind that get transmitted over more than 1 meter. It also cites a couple papers which also say that most droplets are larger than .5 micrometers.

So ability to filter droplets is looking pretty good, assuming the filter doesn't break them up.

This online air filter store cites some research indicating that although HEPA filters are only rated for particles that are .3 micrometers, they end up doing a great job of filtering smaller particles also.

answer by [deleted] · 2020-03-25T01:23:16.361Z · LW(p) · GW(p)

Seems highly unlikely to me, for at least two reasons:

  • The SARS-CoV-2 viruses are roughly 50-200 nanometers in size, right around the size where HEPA filters are least effective (both facts from Wikipedia).
  • Given the 2-meter-distance advice, I'd assume most airborn contagion comes from fairly direct breathing/coughing/aerosolization. It's possible that a strong ventilation system could help here, but it would have to circulate a lot of air very quickly.

Edit to add: HEPA filters are already widely used in hospital ventilation systems, so I imagine any low-hanging fruit here has already been plucked.

↑ comment by John_Maxwell (John_Maxwell_IV) · 2020-03-25T01:46:34.943Z · LW(p) · GW(p)

Diffusion predominates below the 0.1 μm diameter particle size, whilst impaction and interception predominate above 0.4 μm.[11] In between, near the most penetrating particle size (MPPS) 0.21 μm, both diffusion and interception are comparatively inefficient.[12] Because this is the weakest point in the filter's performance, the HEPA specifications use the retention of particles near this size (0.3 μm) to classify the filter.[11]

From Wikipedia. This Wikipedia article actually seems pretty encouraging--despite 0.3 micrometers being near the filter's weakest point, it still filters out 99.95% of those particles?

If most transmission takes place over ~2 meters, it could be very helpful to install HEPA filters near e.g. hospital desks/beds/doorways. I don't remember observing ventilation ducts near desks/beds/doorways in the hospitals I've visited.

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comment by Steven Byrnes (steve2152) · 2020-03-25T14:59:44.598Z · LW(p) · GW(p)

Related: This NYTimes article discusses the interaction of HVAC and virus spread.

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↑ comment by John_Maxwell (John_Maxwell_IV) · 2020-03-25T23:17:18.247Z · LW(p) · GW(p)

Thanks!

...Most hospitals, though, use a filter with what’s known as a MERV rating of 13 or higher. And for good reason — they can capture more than 80 percent of airborne viral particles.

For buildings without mechanical ventilation systems, or if you want to supplement your building’s system in high-risk areas, portable air purifiers can also be effective at controlling airborne particle concentrations. Most quality portable air purifiers use HEPA filters, which capture 99.97 percent of particles.

That suggests there could be gains from installing HEPA filters.

comment by Chris_Leong · 2020-03-25T13:21:40.976Z · LW(p) · GW(p)

Is HEPA the highest standard or are there higher quality filters that could remove viruses?