I was unaware that filters have to be designed differently for viruses. Would you be able to point to where I can read about that? You are the second person I have encountered that has said something along the lines of “filters might work differently for viruses”. I have, as you might see from my post, looked quite deeply into filters and they are tested with both liquids and solids of various forms and this heterogeneity in challenge aerosols, from what I have read, hardly seems to affect their efficiency.
I work with bacterial viruses in liquids, and when we want to separate the bacteria from their viruses, we pass the liquid through a 0.22um filter. A quick search shows that the bacteria I work with are usually 0.5um in diameter, whereas the smallest bacteria can be down to 0.13um in diameter; however, the 0.22um filter is fairly standard for laboratory sterilization so I assume smaller bacteria are relatively rare. The 0.22um filter can also be used for gases.
But as with my usage, they block bacteria and not viruses. I’m working with 50nm-diameter viruses, but viruses of bacteria are generally smaller than those of animals; SARS-CoV2 is somewhere from 50-140nm.
If you use a small enough filter it would still filter out the viruses; but you’ll need to get a pore size smaller than what is sufficient for filtering out bacteria. (and smaller pores requires more pressure, more prone to clogging, etc.)
(though for air, it’s quite rare for bare viruses to be floating around; they’re usually in aerosols (bacteria are often also in aerosols, which may be easier to filter out)
Filtering liquids is pretty different from air, because a HEPA filter captures very small particles by diffusion. This means the worst performance is typically at ~0.3um (too small for ideal diffusion capture, too large for ideal interception and impaction) and is better on both bigger and smaller particles. The reported 99.97% efficiency (2.5 logs) is at this 0.3um nadir, though.
This seems largely correct but I must admit I have never seen an experiment that clearly demonstrates that diffusion is the main feature. Perhaps such experiments have been carried out but if so I think one would have to do something extremely challenging like filming the process at extremely high FPS rates with something like a scanning electron microscope. My sense is that the “performance curve” of filters is mostly empirically deduced while we are actually only extrapolating when making statements about what exactly causes these empirical results.
For example, another process I intuitively feel is different between air and water is the density and thus the force of the fluid on contaminants. If you travel in a boat, it is so much harder to stick your hand in the water compared to the air. Similarly, a particle that could potentially attach to a filter fiber in water is unlikely to stay attached as the water would exceed such a high force on it that it detaches. This is why one washes one’s car with a water hose, not an air hose.
I would be interested in any experiment that has looked at the micro scale physics involved in air filtration but my impression after looking at a lot of filter literature is that there are few, if any such studies.
Absolutely, if anything I trust decades of consistent, empirical results way more than something arrived at by armchair mathematics, or even worse, a mixture of intuition and extrapolated theories.
I was unaware that filters have to be designed differently for viruses. Would you be able to point to where I can read about that? You are the second person I have encountered that has said something along the lines of “filters might work differently for viruses”. I have, as you might see from my post, looked quite deeply into filters and they are tested with both liquids and solids of various forms and this heterogeneity in challenge aerosols, from what I have read, hardly seems to affect their efficiency.
I work with bacterial viruses in liquids, and when we want to separate the bacteria from their viruses, we pass the liquid through a 0.22um filter. A quick search shows that the bacteria I work with are usually 0.5um in diameter, whereas the smallest bacteria can be down to 0.13um in diameter; however, the 0.22um filter is fairly standard for laboratory sterilization so I assume smaller bacteria are relatively rare. The 0.22um filter can also be used for gases.
But as with my usage, they block bacteria and not viruses. I’m working with 50nm-diameter viruses, but viruses of bacteria are generally smaller than those of animals; SARS-CoV2 is somewhere from 50-140nm.
If you use a small enough filter it would still filter out the viruses; but you’ll need to get a pore size smaller than what is sufficient for filtering out bacteria. (and smaller pores requires more pressure, more prone to clogging, etc.)
(though for air, it’s quite rare for bare viruses to be floating around; they’re usually in aerosols (bacteria are often also in aerosols, which may be easier to filter out)
Filtering liquids is pretty different from air, because a HEPA filter captures very small particles by diffusion. This means the worst performance is typically at ~0.3um (too small for ideal diffusion capture, too large for ideal interception and impaction) and is better on both bigger and smaller particles. The reported 99.97% efficiency (2.5 logs) is at this 0.3um nadir, though.
This seems largely correct but I must admit I have never seen an experiment that clearly demonstrates that diffusion is the main feature. Perhaps such experiments have been carried out but if so I think one would have to do something extremely challenging like filming the process at extremely high FPS rates with something like a scanning electron microscope. My sense is that the “performance curve” of filters is mostly empirically deduced while we are actually only extrapolating when making statements about what exactly causes these empirical results.
For example, another process I intuitively feel is different between air and water is the density and thus the force of the fluid on contaminants. If you travel in a boat, it is so much harder to stick your hand in the water compared to the air. Similarly, a particle that could potentially attach to a filter fiber in water is unlikely to stay attached as the water would exceed such a high force on it that it detaches. This is why one washes one’s car with a water hose, not an air hose.
I would be interested in any experiment that has looked at the micro scale physics involved in air filtration but my impression after looking at a lot of filter literature is that there are few, if any such studies.
While it’s nice to know the mechanism, I think all we really need in this case is the empirically determined performance curve.
Absolutely, if anything I trust decades of consistent, empirical results way more than something arrived at by armchair mathematics, or even worse, a mixture of intuition and extrapolated theories.