Except that field data shows they don’t because they are not used precisely as required - which means the filter doesn’t work effectively if at all and then it becomes infectious waste that isn’t treated and disposed of correctly.
Human system effects dominate - as the field data shows. It’s like HCQ - works in a lab, not in the real world.
Might be useful in tightly controlled medical settings with adequate filtered ventilation. But there’s no hard evidence beyond that at this stage.
The problem with filters is they have a lower limit. PM2.5 cuts out much of the smell, but none of the carcinogenicity (PM0.1 or so). The only filter that really works is a sealed cap. Burning biomatter is just not worth the health impact or the cost and effort to mitigate. It's like capturing CO2: a stupid idea.
Oxygen is smaller and easier to pass the filters than COVID-19 particles. It's possible not to have reduced oxygen flow but protect against the virus, but physically impossible to do the reverse as you suggest.
> > 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?
> Hospitals have turned to portable air filters as an attractive solution when their isolation facilities are full.
Woah! I knew that isolation rooms where full-on filtered and with positive pressure. But I genuinely believed that all hospital HVAC units and major circulation ducts had HEPA filter stages.
Is this true? Is it not mandatory for hospitals to add these filtering stages on all HVAC units or circulation ducts?
Do they really just keep rotating the same contaminated air throughout the whole hospital?
For viruses sure, but other things like air filters make sense. Nothing good comes from breathing combustion engine exhaust or gas stove combustion byproducts.
If you live anywhere near a coal mine or power plant you are also getting a good dose of radioactive dust although I’m not sure a hepa filter is good enough there.
Rosenthal's tweeting/re-tweeting on the matter, & praising the real-world testing done here,
> Great point. There is too much reliance on tests in chambers and test ducts. Then we are assuming these tests apply to real world environments. They often do not. #corsirosenthalboxes are a great way to test filters in occupied spaces. No assumptions - just good data.https://twitter.com/JimRosenthal4/status/1677339876931383296
This is like theoretically true but mostly useless.
There is filtration beyond even HEPA.
For example, MERV-20 is 99.9999% efficient at 0.1->0.2um particle sizes.
It is 99.99% efficient at particle sizes like 0.01um.
The bigger issue is not whether you can get filters to do it, it's whether your system was built for the pressure drop you'd need to do it.
IE UV's advantage is that it doesn't really drop pressure (but does require some dwell time), not that you can't filter air well enough.
You won't often catch covid from stray particles but from the person breathing next to you, which these filters can't help. This study was in a hospital where everyone is wearing serious PPE, so its a super edge case to get rid of covid in the air because the main use case of catching covid has been handle by plain old PPE.
This doesn't have sweeping applicability. It won't stop covid among the general population. Its for maybe hospitals that are on super covid lockdown that want a little more protection for their most vulnerable patients.
We get rid of covid through mass vaccination like any other virus. There's no shortcuts for those who refuse masks and vaccines.
>> In contrast, a plain old filter is likely to be extremely effective. A HEPA filter will catch almost all particles big enough to contain a virus, and a MERV 13+ or better filter is quite good. All you need is a filter with enough surface area that a good filter has low pressure drip.
The problem with installing a MERV 13 filter in a normal HVAC system is that the pressure (usually) isn't rated for it and can cause serious damage to the motor(s) as you noted, which is more common than people think (definitely the case in my home system).
UV-C installation is fairly simple; though I agree they are power hungry and you need to keep them on most of the time with maximum one power cycle per 24 hour period. They're cheap upfront and do work on normal coronaviruses, though.
Still the real problem is very little state-funded research on the topic, or development of some mitigation techniques for businesses that must run indoors. It's once again fairly useless bureaucracy.
Impulsing off topic:
It looks like they do work, just nowhere near the efficiency of a HEPA filter. Ionic filters certainly work in principle, but they really can't compete with devices with higher surface area (any fibrous air filter) or higher power (a CRT screen). Or the humongous surface area of the furniture of the room.
Funnily, in the little clean room lab I worked in at college, we would occasionally run the vacuum cleaners for a few days because they had great HEPA filters, spot improving sensitive zones. Kinda noisy for home use, though.
> Even thought the pores are larger than a virus the masks are statically charged causes smaller things to get trapped.
Even filters that aren't statically charged stop particles that are smaller than the gabs between the fibers. There are actually several mechanisms by which a filter can stop a particle.
1. Big particles don't fit between the fibers of the filter. Think fish in a fish net. This is called sieving.
2. Particles too small for sieving but heavier than the surrounding flow don't make the turns as well as the surrounding flow when the flow goes around the fibers. The particles can get embedded in the fibers. This mechanism is called inertial impaction.
3. The smallest might be too small to actually be affected much by the flow of the surrounding fluid through the filter. The move by diffusion, and many will randomly hit the fibers and get stuck.
4. Particles too big for diffusion but too light for inertial impaction still can run into fibers and get stuck. This is called interception.
5. As you mentioned, some filters have an electrostatic charge which can help trap particles.
The effectiveness of sieving, inertial impaction, and interception all follow S shaped curves that start out low for small particles, then at some point start rising, and then level out. The sieving curve's rise is almost vertical. The rise for inertial impaction is steep but not nearly as steep as it is for sieving. The curve for interception's rise is much more relaxed.
The effectiveness for diffusion goes the other way. Much more effective for very small particles, then above some size drops down and is low from then one.
When you put all these together, you get a curve that is effective at the small end, and at some point as size goes up effectiveness drops, reaching a minimum, and then rises again to reach high effectiveness for particles above some certain size.
The reason 0.3 microns is used for many HEPA filter ratings is that is in the middle of the low part of that U shaped curve, so when you get a filter that removes, say, 99.97% of 3 micro particles, it should actually do better for both larger and smaller particles.
That's not the point though - that part is technically true (EPA filters are 'Efficient Particulate Air' filters and HEPA are 'High Efficiency Particulate Air' filters, and the E and H correspond to those respectively).
The point of the article is that the Wirecutter authors don't understand the physics of air filters and gives the difference more emphasis than what actually matters - it doesn't actually make a massive difference in this particular application. For a purifier that intakes and exhausts in the same space, getting more airflow through the filter per hour can mean over time it's basically the same effectiveness, and using a slightly lower spec filter can be a good design trade-off because it doesn't require as much pressure so it can use less power per unit volume of air filtered.
Of course, in other applications, like bringing air into a cleanroom, it makes a massive difference, but that's not what we're talking about.
> Almost all of these review sites, not understanding the physics involved, believe a HEPA filter sieves particles down to a size of 0.3 microns, which implies that anything smaller passes on through.
To be fair, it took a pandemic for me to go to the literature of mask effectiveness and finally found the "on the filtration efficiency of fiberous filters" paper that showed the u shaped curve. it's not something that they scream from the hills about in their product brochures. That said it should be screamed from the hills.
Human system effects dominate - as the field data shows. It’s like HCQ - works in a lab, not in the real world.
Might be useful in tightly controlled medical settings with adequate filtered ventilation. But there’s no hard evidence beyond that at this stage.
Feynman’s rule still applies.
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