[aarongray]

I love the enthusiasm and DIY ingenuity here. That said, the post is tagged with COVID-19. The COVID virus particles are .1 to .5 microns in size, and these MERV-14 filters, while certainly better than nothing, are not going to capture a significant amount of these virus particles. A better approach is to not filter the particles, but actually rip them apart at a molecular level. This has the added benefit of destroying all sorts of other contaminents that even high grade HEPA filters will miss, such as mold mycotoxins. A system like the Molekule is a good example of this approach.

https://molekule.com/technology

[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?

[gonehome]

Brownian motion affects how small particles move around at small scales [0]. This is why HEPA filters can be effective even against smaller particles (which bounce around and get caught).

Specifically about the Molekule - their claims seemed to be empirically false and the device performed worse than a standard HEPA filter (and they were resistant to allowing independent tests at all).

When the tests came out awful their responses were mostly bullshit. [1]

[0]: https://en.m.wikipedia.org/wiki/Brownian_motion

[1]: https://www.nytimes.com/wirecutter/reviews/best-air-purifier...

[picture]

Is this legit? Their website feels kinda sketchy to me, with the huge and somewhat tacky CG pictures, sweeping claims, apparent SEM photography that just looks too good to be true (could there really be zero residue? and the background never change, presumably over time span of at least minutes!) This really reminds me of vaporware like WaterSeer, or one of the recent "turn kitchen garbage into dry dust" appliances that I've seen ads for

[gonehome]

Your intuition is correct - it’s bullshit.

—

In the summer of 2019, we purchased a Molekule Air (the flagship model) and tested it. We bought an Air Mini that fall and tested it in February 2020. At the time we tested the Molekule Air, the company claimed that its “scientifically-proven nanotechnology outperforms HEPA filters in every category of pollutant.”

Our tests proved otherwise. And by mid-2020, that language had been withdrawn, after many of the company’s claims were ruled against in a case before the National Advertising Division and upheld in a later appeal before the National Advertising Review Board. The Molekule Air turned in the worst performance on particulates of any purifier, of any size, of any price, that we have tested in the eight years that we have been producing this guide. The Air Mini outperformed it, but that’s not saying much: It still produced the second-worst performance we’ve ever seen.

Guide author Tim Heffernan asked Molekule CEO Dilip Goswami why the language was removed. He answered, “The point about ‘in all categories’ is that we see a device that outperforms across all of the categories. Right? So we’re not trying to say that individually, on any particular metric, we would be number one. Right? What we’re saying is, when you look across all the categories, we outperform HEPA. Right? And that’s what we’re attempting to convey with that. And so—it’s fair to say that we needed to re-examine some of the language to make sure that it’s saying what we’re intending to say.”

https://www.nytimes.com/wirecutter/reviews/best-air-purifier...

[aarongray]

Thanks for sharing some background on Molekule, I did not know about this.

[aarongray]

That said though, the Molekule team themselves have said that their device is not optimized for filtering particulate matter, but rather for denaturing mold, viruses, and bacteria. See: https://www.reddit.com/r/molekule/comments/6u7d5y/putting_mo...

This is reflected as well in the testing on Molekule's own website: https://molekule.cdn.prismic.io/molekule/4ec92005-d806-4991-...

And, I will say that they have numerous tests by independent labs. They did do some testing with their own lab and with a lab affiliated with them, but this is far from all the testing they have done. So to say that all their tests were tainted or affiliated with them is simply not true. See: https://molekule.com/papers

[aarongray]

Further reading here: https://molekule.science/air-quality-other-pollutants-beyond...

[jefftk]

MERV-14 filters are rated for 75% efficacy in that range: https://www.nafahq.org/understanding-merv-nafa-users-guide-t...

The respiratory particles that transmit covid are a range of sizes, but likely mostly a bit larger than that: https://www.jefftk.com/p/how-big-are-covid-particles