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I was researching how to apply what we know about air purifiers (https://www.microcovid.org/blog/hepafilters) to better air filtration in central HVAC, and I wanted to ask about your research and share mine.
I read the microcovid blog/whitepaper as basically saying central HVAC and MERV filters essentially don't work because they don't remove enough particles. I came to a very different conclusion: let's take MERV-13 filters as an example, since they're the most standard recommendation for HVAC systems and covid.
Let's as an example compare a filter removing 50% of 0.3-1.0 micron particles (MERV-13) with airflow rate = 10 * room volume / hour, vs 99.97% (HEPA) with airflow rate = 5 * room volume / hour. The rate of removing particles from the air is the same in these two cases (this is approximately what CADR measures). Looking at this as a decay rate for particles in the air, we can calculate the half-life of particles in the air as: ln(2) / (50% * 10 / hour) = 8.3 minutes, vs ln(2) / (99.97% * 5 / hour) = same.
So I reason that if the system delivers the same value of (airflow rate) * (filtration efficiency) - this is roughly what products label CADR for smoke - the half-life of particles in the air is the same, and therefore the protection from Covid is roughly the same.
Note - this is based on conceptual reasoning and not data (although presumably the standard MERV-13 recommendation has some data to back it up)
Would love to hear your thoughts on this, and if there's anything I got wrong or missed.
Closing note - thanks a ton for all of your research and work, these sorts of estimates are exactly the sort of thing I like to do myself, and I was actually thinking of throwing together a spreadsheet before I saw your work - which I was very happy to see because there's no way I would have been able to do such high-quality research myself!
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I was researching how to apply what we know about air purifiers (https://www.microcovid.org/blog/hepafilters) to better air filtration in central HVAC, and I wanted to ask about your research and share mine.
I read the microcovid blog/whitepaper as basically saying central HVAC and MERV filters essentially don't work because they don't remove enough particles. I came to a very different conclusion: let's take MERV-13 filters as an example, since they're the most standard recommendation for HVAC systems and covid.
Let's as an example compare a filter removing 50% of 0.3-1.0 micron particles (MERV-13) with airflow rate = 10 * room volume / hour, vs 99.97% (HEPA) with airflow rate = 5 * room volume / hour. The rate of removing particles from the air is the same in these two cases (this is approximately what CADR measures). Looking at this as a decay rate for particles in the air, we can calculate the half-life of particles in the air as: ln(2) / (50% * 10 / hour) = 8.3 minutes, vs ln(2) / (99.97% * 5 / hour) = same.
So I reason that if the system delivers the same value of (airflow rate) * (filtration efficiency) - this is roughly what products label CADR for smoke - the half-life of particles in the air is the same, and therefore the protection from Covid is roughly the same.
Note - this is based on conceptual reasoning and not data (although presumably the standard MERV-13 recommendation has some data to back it up)
You can find my write-up of recommendations based on this reasoning at https://jackc271.substack.com/p/air-filtration.
Would love to hear your thoughts on this, and if there's anything I got wrong or missed.
Closing note - thanks a ton for all of your research and work, these sorts of estimates are exactly the sort of thing I like to do myself, and I was actually thinking of throwing together a spreadsheet before I saw your work - which I was very happy to see because there's no way I would have been able to do such high-quality research myself!
Thanks,
Jack
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