[blagie]

Erm, this isn't quite correct:

1) MERV14 will capture 75%-84% of particles in the 0.3-1μ range.

2) A filter which captures 70% of particles with an air flow of 100cfm will capture the same amount of virus as a filter which captures 100% of particles with an air flow of 70cfm. Both will clean the room just as fast. For air filters, lower filtration + higher airflow is usually a better design option. Going from 70% to 95% to 99.9% means you'll have a more expensive, power-hungry, and more noisy product over one which just has a little bit more air velocity [1].

3) The rating is for 0.3μ since that's the hardest size to capture. A filter will actually capture more particles below 0.3μ.

4) COVID19 virus particles are around 0.1-0.2μ, but that's beside the point. They're travelling on water molecules. Those are much bigger.

5) Even if there were a virus particle somehow floating around, a viral load of one virus is very unlikely to get you sick.

From an engineering standpoint, something around MERV14 is almost certainly the sweet spot for a COVID19 room air filter.

[1] High-filtrations makes sense in places like vacuums, face masks, and other places where the goal is to have clean air coming out. Vacuums shouldn't blow up dust. That's a different engineering design goal than a room air filter. If you'd like to see the impact of loading on a fan, put your hand behind one, and hear how much noise goes up. MERV14 has a much lower load than HEPA.

[aarongray]

1) You're correct.

2) It will not clean the room just as fast, it will take longer to clean the room.

4) COVID-19 particles do travel on water molecules, but they are also airborne. The CDC has admitted this and there is a growing body of research proving this to be true as well.

5) This has not been proven.

[blagie]

You're making statements with no backing, logic, or argument behind them.

2) The percent of material removed by a filter per unit time is the product of (1) filter efficiency with (2) what percentage of a room's air passes per unit time.

4) "Airborne" is generally via microscopic droplets. The CDC's guidance changed from large droplet transmission (which is relatively short-distance and short-time) to airborne. This doesn't mean individual viruses are floating around without any H20.

5) No one credible believes 1 virus particle is likely to infect you, except by very bad luck. Most citations give claims in the 100-1000 particle range. Low initial infectious dose also /appears/ to correspond to less aggressive infections. This has not been rigorously proven (and it's hard to do), but has a strong theoretical basis:

- One virus particle is unlikely to make it past the mucous layer, unless you're super-unlucky.

- If it does, your innate immune system can usually handle minor infections before they escalate.

- If it can't, your adaptive immune system has more time to respond. You're looking at a few days before it kicks in. With a lower initial infectious dose, you'll still have that much less virus when it kicks in.

If you'd like to contradict any of this, please provide citations. I'll read them. I'm glad to be proven wrong. Perhaps I'll learn something.

[jefftk]

> > A filter which captures 70% of particles with an air flow of 100cfm will capture the same amount of virus as a filter which captures 100% of particles with an air flow of 70cfm. Both will clean the room just as fast.

> It will not clean the room just as fast, it will take longer to clean the room.

Here are two different models:

A. Air moves sequentially. First you filter all of the air once, then you filter all of it another time etc. In this model, a filter with 100% efficacy will get everything in a single pass, and the CFM determines how long that pass takes, while a filter with lower efficacy will never get it all, but will get pretty close after a few passes. In this model you want high filtration.

B. Air moves randomly. At each minute, the purifier selects air from the room at random, filters it, and spits it back out. In this model, a filter with 100% efficacy at 70 CFM is exactly equivalent to a filter with 70% efficacy at 100 CFM, and you will often want to trade off efficacy for flow.

I think real rooms are generally much closer to (B) than (A), though of course somewhere in the middle?