Also, ~8-18% of the population (mostly women) is allergic to nickel due to A combination of genetic predisposition prolonged contact with nickel-coated jewelry. Presumably this filter dumps trace amounts of nickel dust into the air. What could go wrong?
Surprisingly, that reasoning doesn't necessarily actually work.
It turns out that there are actually several different mechanisms by which a filter can stop particles.
Big particles, for example, might not fit between the gaps in the filter--think fish in a net. This is called sieving.
Particles that are too small for sieving but are heavier than the surrounding flow keep moving in a straight line when the flow goes around the filter fibers. They collide with the fibers and get stuck. This is called inertial impaction.
The smallest particles that the filter can handle are not held in place by the fluid they are flowing in and so move around a lot by diffusion. This diffusion can lead them to hitting the fibers and getting stuck.
Particles too big for diffusion but too small for inertial impaction can follow the flow around fibers, but in doing so they can still hit the fiber and get stuck. This is called interception.
There are also electrostatic effects with some filter materials that can ensnare some kinds of particles.
When you put this all together, the result is that filters do not work the way we would intuitively expect, where there is some particular size and everything above that is stopped and everything below that makes it through. That would only be true if sieving was the only mechanism in play.
The curves of efficiency vs. particle size for all of the non-electrostatic mechanisms are S curves. As size goes up, sieving, inertial impact, and interception all go up, but at different rates.
Sieving's curve rising section is almost vertical. Inertial impact's is fairly rapid but nowhere near as rapid as sieving's. Interception's is much more relaxed.
Diffusion is also an S curve, but it goes the other way, being high for small particles and dropping for large particles.
When you add them all up you end up with a curve that is high and flat for small particles, then dips down around some particular size, and then rises back up to high efficiency.
There's some nice illustrations and graphs here [1].
This is why 0.3 microns is used when rating HEPA filters. It's around the size that is hardest for them to handle.
[1] http://donaldsonaerospace-defense.com/library/files/document...
https://www.amazon.com/Aerosol-Technology-Properties-Measure...
Isn't this similar to how N95 masks are rated, as well? That is, the mask will stop 95% of the particles at its worst size (if my understanding is correct). HEPA is a much better filter than N95.
Also, it's my understanding that CV itself can't exist in the atmosphere itself, but must be contained in a droplet. (Again, I'm hoping this is correct). If this is true, then it's not about the size of CV, but the range of sizes of water droplets.
Perhaps also the drying effect on droplets caught in a filter, which cause the CV 'death' is true?
Doesn't this mean that the whole argument now degrades to probabilities of a CV droplet getting through and causing a sickness?
1. How many CV exist in a droplet?
2. How many droplets must one encounter to have a good chance of being exposed?
3. How many CV does it take for a person to get sick? (Presuming the immune system will be a factor in eradicating some particles?)
There are other factors of course, the main one would be is how quickly the air in a room is scrubbed by the filter itself. If the air flow is too low, the filter would be as good as useless, because it's not trapping anything.
Also, I recall reading that hepa filters are the most effective at eliminating airborne particulate the higher the amount of time it has to cycle the same air in a room.
Final thoughts, is using the ozone feature effective in trapping coronavirus and what about an integrated UV light that it directed on the HEPA filter, will this eradicate trapped viruses?
[0] Not minimizing this, it’s just that our bodies tend to react to allergens in certain ways.
On the other hand, a silane quat impregnated HEPA filter would likely kill everything that passed through it, and afaik has no substantial toxicity issues.
That's something I never really got about mask reuse either. People kept saying these masks aren't reusable unless they use UV light treatment or something, and it just seemed like if it's considered safe to touch mail after letting it sit in the garage for a few days after getting it, then shouldn't you just have to let masks sit for a few days for them to be usable again (hell, wait two weeks, even).
I'm sure there's good reasons for it, just haven't seen it. I imagine some masks are considered one time use because they degrade enough after that use that they might not protect as well, but then why are people trying so hard to find methods to make them reusable that seem to mainly just involve disinfecting them?
Because SARS-CoV-2 isn’t the only pathogen out there, and a decent mask is basically a magnet for everything tiny that can harm you.
https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/d...
> One strategy to mitigate the contact transfer of pathogens from the FFR to the wearer during reuse is to issue five respirators to each healthcare worker who may care for patients with suspected or confirmed COVID-19. The healthcare worker will wear one respirator each day and store it in a breathable paper bag at the end of each shift. The order of FFR use should be repeated with a minimum of five days between each FFR use.
Disinfecting is required when supply is so constrained that issuing five respirators to every worker plus replacements isn't possible. You can disinfect a mask in minutes and get it back on the floor. Also, letting a mask sit will get rid of most viruses, including COVID-19, but it won't get rid of other pathogens (e.g. bacteria).
Right now nobody is sure if that will work. Maybe: https://www.nytimes.com/wirecutter/blog/can-hepa-air-purifie...
This is envisioned, at the outset, to be installed in the air handling systems of places like hospitals and airports.
I don't know much about HVAC, so I don't know if this was made because HEPA doesn't scale to hospital/hotel/airport size or there's some other reason.
Presumably this filter dumps trace amounts of nickel dust into the air.
Why is that a logical assumption? Do HEPA filters dump trace amount of fiber dust into the air? Do any other types of filters decompose themselves into the air in noxious quantities?
[1] https://www.philips.com.au/c-p/AC1215_70/series-1000-air-pur...
This source might explain it (didn't really read it much)
https://www.ncbi.nlm.nih.gov/books/NBK234810/
Edit: Allegedly people can get sensitized to pesticides on fruits and then exhibit allergic reactions to "fruit xyz" (without knowing it was the chemicals on/leached into the fruit)