Karolinska Institute in Sweden did a study during the pandemic that had findings of virus particles in every location in the hospital ventilation system, past all filters and air purifying. It was a early finding that indicated the role of recycled air in spreading the virus in places like airports, planes, trains and large buildings like shopping malls. The covid virus is so small and so effective as a airborne virus that even hospital filters had a limited effect.

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vintermann a day ago | parent | next [-]

Smaller doesn't mean harder to filter, beyond a certain size. HN favorite (and pre-covid particulate filtrering proponent) dynomight explained this well in an article on a cheap air purifier:

https://dynomight.net/ikea-purifier/

Also, I'm pretty sure respiratory viruses need aerosol droplets to get anywhere they can harm you, and those droplets are much bigger than viruses anyway.

	▲	belorn a day ago \| parent [-]
		Some respiratory viruses need large aerosol droplets, but covid virus particles remaining viable with a half-life of up to about an hour outside large droplets. The initial theory was that environments like extreme hot and humid climates made it impossible for aerosol droplets to harm people, while dry air increased the risk. This recommendation was change later into the pandemic as new evidence was gathered. There are similar studies on influenza. There doesn't seem a strong consensus on what environments are safe, or how long such viruses can last outside of large droplets. There is a lot of factors in play, including the virus itself.

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chiefalchemist a day ago | parent | prev [-]

So this is why we were told to wear masks that were ineffective at blocking viruses? In short, we effectively went mask-less for the duration and spreading was relatively minimal; certainly not mitigated by disposable masks.

This was also interesting:

Back to School in a Rapidly Changing World

https://play.prx.org/listen?ge=prx_12369_085cdf46-2a33-4f24-...

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JumpCrisscross a day ago | parent [-]

> why we were told to wear masks that were ineffective at blocking viruses?

Masks and HEPA filters can block droplets that contain viruses. Neither filters out actual virus particles.

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vintermann a day ago | parent [-]

Respiratory virus particles outside an aerosol droplet don't last long, but yes, they should be filtered too (not "blocked", it will always be probabilistic)

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JumpCrisscross a day ago | parent [-]

> they should be filtered too (not "blocked", it will always be probabilistic)

The article’s point is we haven’t precisely studied what those probabilities are.

	▲	vintermann a day ago \| parent [-]
		No it isn't actually, those kind of low level technical questions have been somewhat studied. It's the end to end interventions which have been poorly studied.