Researchers create air filter that can kill the coronavirus | Hacker News Reader

Researchers create air filter that can kill the coronavirus

uh.edu

161 pointsakeck105 comments

Why not use a HEPA filter? Surely, if it can filter heavy isotopes, coronavirus won’t fit through. Coronavirus doesn’t last more than a few days without a host, so the HEPA filter will eventually “kill” 100% of the virus as well.

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?

> Surely, if it can filter heavy isotopes, coronavirus won’t fit through.

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...

What amount of trapped particles can be released with air current dislodging them as it flows through? If you have a really dirty filter, will the trapped particles dislodge or will it create a sort of barrier and increase filtering efficiency? Thanks for the awesome write up.

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?

I appreciate learning things that update my physical intuition for how things work. Thanks for sharing this.

Awesome write up.

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.

For anyone interested in a deep dive into the weird space of aerosol science this is the Bible for the scientists I work with

https://www.amazon.com/Aerosol-Technology-Properties-Measure...

Very insightful, thanks for sharing. You win HN today

Applause

HEPA is efficient, but expensive to maintain. I stumbled upon this interesting sounding study [1] from 2013, but realized that thee Corona Virus is between 50 - 200 nm and the effective filter range is between 200 - 600 nm.

[1]: https://pubmed.ncbi.nlm.nih.gov/23263945/

Combine HEPA with UV light (at least the type that is the most effective at destroying the coronavirus) and as long as it can hold the virus long enough exposed to that range, it's probably good enough. Question is, would the UV light somewhat damage the HEPA filter over time?

wow, interesting. I'm male but recently discovered I'm allergic to nickel after 30 years of playing guitar (electric guitar strings are usually nickel composite materials) but I didn't realize it was that common. I kinda self diagnosed it but I'm almost happy it's a common thing.

You should look into stainless steel frets and either non-nickel or coated strings.

What are the symptoms of that allergy?

Appears to be the usual[0]: rash, redness, itching, etc.

[0] Not minimizing this, it’s just that our bodies tend to react to allergens in certain ways.

HEPA wouldn’t kill it, just hold it.

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.

Well it just needs to hold it until it's inactive, right? The virus doesn't seem to stay active on surfaces more than about 3 days at room temperature, so it just needs to hold it for that long.

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?

That's already recommended practice.

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).

Oh, good. This is the first time I've seen this. I'm glad this is considered a possible method. Seems like one and done for so many masks is so wasteful if not absolutely necessary.

How long does bacteria last?!

Essentially indefinitely if conditions are right. If it starts making spores, it can stay ready-to-infect for like a few thousand years.

> People kept saying these masks aren't reusable unless they use UV light treatment or something

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.

Active disinfection probably degrades the material.. Wearing them too long probably does too (moisture from breathing). But I agree you can have say 4 or 5 N95s and rotate them.

I interpreted the quotation marks as implying that it's functionally killing the virus unless you somehow blow it off the filter.

UV light in conjunction with a Hepa filter would be very effective.

HEPA filters are made out of fiberglass, which is vulnerable to UV degradation.

You don't have to aim the light at the filter.

Why not use a HEPA filter?

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?

HEPA filters are good enough for the exhaust from BSL4 labs. They work. But they only work on the air that passes through them, as a barrier between separate environments. Just placing one somewhere in the room probably won’t help much.

You'd be surprised how effective "just putting one in the room" is. You can build a fairly effective cleanroom that way, and cleanrooms are a lot "cleaner" than is required for effectively reducing the viral load for things like COVID19

Indeed, and many (most?) domestic air filters include HEPA filters. I have a Phillips one[1] and it does.

[1] https://www.philips.com.au/c-p/AC1215_70/series-1000-air-pur...

Wouldn't that be defeated if you talk with one of the infected? Like you would be in a huge hotspot compared to the room average.

Absolutely, but compare that with not having a filter operating at all. We've seen that one or a few infected individuals can infect hundreds in a closed space, so if we could reduce that by even a small factor, it's definitely significant.

Coronaviruses aren't anything special, HEPA filters that are in part specifically designed to trap virus particles will trap SARS-CoV particles as well, because that's their job.

A bigger issue is probably that HEPA filters are made of the same melt-blown fiber needed for n95 masks.

In the Article they talk about use in air conditioners. Most ACs don't seam to be able to handle the resistance that HEPA filters would add.

This 100%, I froze my HVAC coil and then had to replace it after it leaked freon from an overly restrictive 'allergen + virus' pleated air filter.

Health risks aside, supposedly filter like this would last longer? HEPA filters need frequent replacement.

Why does prolonged contact with nickel trigger this? I thought this was basically the purpose of allergy shots (expose you to the allergen over a period of time)

I worked in a wet lab and we were warned around the dangers of sensitization to bench-top solvents and reagents.

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)

You can become hypersensitive (read: allergic) to a lot of things, including nickel. It won't happen to everyone but its more common in women that you'd think, perhaps related to jewelry.

HEPA + integrated UV light would eradicate the trapped viruses?

The filter heats air to 200 degrees. I assume the air would have to be cooled afterwards.

Is that going to use an insane amount of energy or is there some trick to heat and cool without that much energy (a la air conditioning).

>>> By making the filter electrically heated, rather than heating it from an external source, the researchers said they minimized the amount of heat that escaped from the filter, allowing air conditioning to function with minimal strain.

The minimal mean minimal to other heating alternatives not being insignificant. Air flowing through a hot item will be warmed significantly, this is how electric heaters work!

If it's heating the air stream to kill the virus, I don't understand the difference between "electrically heated" and "heating it from another source".

I interpreted that to mean that the heating elements are within the structure of the filter, rather than essentially putting the whole thing in an oven.

Regardless,how does that prevent the airstream from heating up?

Air molecules are a lot smaller than the particles getting trapped, so presumably a filter with a low enough density can minimize heat transfer except to things getting stuck in it.

No, the filter is heated to 200C, not the air. They don’t say what the temperature delta of the air is after passing through.

Good point, though if air continually passing through, the temperature delta would be significant.

The article says the virus is deactivated at 70C, so the air itself should probably get to 70C plus perhaps a safety margin.

Come to think of it, if you coupled that with a heat exchanger, the filtered air could pre-heat the incoming air and get cooled in the process. This way the filter would only have to transfer enough heat to make up for the inefficiency of the exchanger.

I'm not sure if this is feasible.

It's probably perfectly feasible - it's the same working principle as a dehumidifier, except temporarily heating instead of temporarily cooling. Just reverse the polarity and you're set!

A filter might be easier to install in an existing forced air system. The main problem that needs to be solved is how to eliminate the virus between now and when a vaccine is available. So solutions need to be something that can be manufactured quickly and dropped in as a replacement for a component of an existing system.

Wouldn't the filter simply be placed on the air intake side prior to passing through the HVAC coil and being cooled? Why heat the air after it is passed through the coil and is cooled?

I remember a product called Sterilair, created back when I was a child, that would simply heat air to 270ºC inside a ceramic honeycomb.

https://www.sterilair.com.br/

It even uses the same materials, though in a much easier to build arrangement: a nickel filament through a tube, and gets convection as a bonus.

This is practically an electric heater. It might be a fire hazard and I wonder if fine burned residue particles (chemicals) escaping the 'heater' and circulated with airflow have further health risks.

Air conditioning exhausts a lot of heat. Maybe it can reuse some of that. I might be missing some weird physics scaling though if you're air conditioning 200F temperature air maybe a normal A/C can't handle that.

Yes the warm air passes through the HVAC coil and is cooled?

Is there a fundamental reason why UV doesn't work on air? Wouldn't a UV approach be much more energy efficient and rapidly scalable?

It does work on air. Some commercial HVAC systems integrate UVC lamps. It needs relatively high power or a long straight run where the air flow and light are both pointed in the same direction.

https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradia...

Yup. You can even buy them for home AC systems. Both of our AC units include UV lamps, and we bought them years ago. And they include whole-home HEPA filters, too.

And our room IQAir HyperHEPA filter units go down to 0.003 microns, which is an order of magnitude smaller than virus particles.

But that only helps you at home, and it only helps you with those particles it captures before you breathe that air. If you’re between the air filter and the virus particles in question, then you’re more likely to be the filter that catches those particles.

What could go wrong with a 400F heating element on in your air ducts at all times. Wouldn’t it constantly be lighting hair and dust particles on fire???

One of the primary causes of housefires is leaving bathroom fans running.

Really?? I always do this and now I'm worried, got a source? A normal fan or the fans with the heat lamp bulbs integrated?? Cause I can see those starting a fire with the intense heat it generates.

[citation needed]. A quick google search shows some warnings about this happening (due to lint and dirt building up and the motor overheating) but I’d be very surprised if it’s even in the top 10.

It's what the firefighter told my parents when the bathroom ceiling fan burned our house down.

How efficiently are the aerosols and droplets sucked into the AC system? The principal method of spread is by these vehicles, person-to-person, at close range. Imagine a church with most pews filled. If the AC provided absolutely pure air (forget for the moment about how this is created), it would do little to prevent virus spread between unprotected individuals.

You could, in theory, build a system like what they use in datacenters. Drill holes in the floor and ceiling and create enough vertical airflow that any particles you expel will be sucked either up or down before they reach the next person.

That would make a very uncomfortable church to sit in, though.

I'm basically taking this from an earlier hn discussion about the lack of a documented early outbreak in Las Vegas: apparently the casinos have systems powerful enough (and surely involving clever placing of inlets and outlets to enforce a coherent vertical stream) to allow indoor smoking without driving away non-smoking customers. This has since become my mental model of what it would take to make an indoor environment as safe(ish) in the pandemic as the outdoors.

The indoor air quality at casinos with smoking is better than a bar with smoking, and it's not enough to drive me away from walking through the casino to get to the hotel, but your clothes still smell like smoke when you get to your room.

I would put it as tolerable, maybe if I enjoyed gambling or drinking, I'd be ok with being on the casino floor for longer. Better to go to the enlightened casinos with no smoking though.

For my mental model, I usually think of underground mine ventilation. Requirements are typically in the range of several cubic metres per minute per person - enough ventilation to be able to feel a light breeze.

There are plenty of commercial air purifiers that will at least trap SARS-CoV-2 particles, and they can be added to air conditioning systems.

The IQAir HealthPro Plus, which can be bought for $899[1], filters particles as small as 0.003 microns (Coronavirus is approximately 0.1 microns[2]). And their Perfect 16 product is designed for HVAC systems[3].

[1] https://www.sylvane.com/iqair-healthpro-plus-air-purifier.ht... [2] https://abcdust.net/how-large-is-a-corona-virus-virion-compa... [3] https://www.sylvane.com/iqair-perfect-16-air-purifiers.html

This company claims its air filter kills the virus too.

https://cleanairexp.com/

The difference is that the one linked above was tested at the Galveston National Laboratory and works well enough that it also kills 99.9% of anthrax spores.

Is there any evidence that the virus can survive passage through a normal air filter/AC system and infect humans?

Another solution I have thought about is simple increasing the amount of fresh air brought into an indoor space. Laboratories, wood-working shops and other spaces that can produce unhealthy air have extremely high Air Change per Hour (ACH)[1] requirements. It's wasteful[2] since you also have to condition that air, but it seems like a simple solution to ensuring we dilute air swarming with potential virus.

[1] https://www.engineeringtoolbox.com/air-change-rate-room-d_86...

[2] You can limit it a little bit with demand control ventilation, which monitors C02 exhaled by occupants and tunes ventilation accordingly.

> It's wasteful since you also have to condition

Heat exchangers can help, but it's still obviously less efficient.

Yes, and another big saver would be certain types of heat recovery system[1] that will allow you to recover the heat added to the supply air from the exhaust air without mixing (and thereby polluting) the air streams.

[1] https://en.wikipedia.org/wiki/Heat_recovery_ventilation#:~:t....

https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article

> During January 26–February 10, 2020, an outbreak of 2019 novel coronavirus disease in an air-conditioned restaurant in Guangzhou, China, involved 3 family clusters. The airflow direction was consistent with droplet transmission. To prevent the spread of the virus in restaurants, we recommend increasing the distance between tables and improving ventilation.

The virus didn't come through the ac system in that case. The AC was just blowing air from the infected person to others. This solution would have been useless.

That's a difficult thing to test directly, so I think most people reason it out. In this case, you think about the virus, its interaction with air particles, and the interaction with a typical AC system with and without (unheated) filtration. I think the issue with ordinary filtration is that the virus is very small and can pass through most filters; by heating it the virus only needs to collide with the hot filter once, or (I think) get knocked around by fast air molecules to become unviable (is "killed" the right term for a virus that may not be "alive"?)

Long story short, I think this invention looks great and I for one would use one, especially in cold climates where I already want to heat up the air.

> Long story short, I think this invention looks great and I for one would use one, especially in cold climates where I already want to heat up the air.

If it also works on influenza, it would be a fantastic way to heat buildings during flu season, and the investment could pay for itself even after the current COVID pandemic.

Yes. Why do you think hospitals have HEPA filters in the first place?

FYI: Legionnaires' Disease is spread through HVAC systems that use cooling towers:

https://en.wikipedia.org/wiki/Legionnaires'_disease

That’s because the cooling towers incubate the bacteria that causes Legionnaires.

COVID is caused by a virus that won’t reproduce outside of mammals.

Aren't salt crystals killing it to? Many had home-made masks with salt based on studies.

Can’t every UV-C equipped filter kill the virus? Why would a heat based system be preferred?

Do viruses actually die? I thought they are not living.

They deactivate. But it’s a distinction without value for casual conversation.

If you write an executable to a file, does the program still exist? Or is its existence as a program depend on it being loaded in memory and being executed?

They can deteriorate to the point that they can no longer reproduce, so, close enough?

You are right. 'die' refers to falling apart.

I am not an expert but I believe the reproduction of a virus is more 'chemical' than 'biological'.

It's very much biological. Viruses, like our cells, consist for a large part of protein, which degenerates when above 65 degrees for some time.

they can be lysed and degraded

neither are processes

So, I can just vent through my oven?

This may destroy viruses but it does not kill them as viruses are not alive.

so for this to work, all the air in a room must be heated to 250C for a short time while it passes through the filter.

How much energy does that use?

No, the filter gets heated to 250 °C to destroy trapped viral particles. The air itself won't get heated up much, its contact time with the filter is too short.

You'll have to heat up the air, or at least all particles in it, to at least 65 degrees to deactivate the virus.

The viruses get trapped in the filter and burned up there by the 250 °C, so no need to heat up the air to 65°C.

Is there any good reason to research this at all since the virus is mainly transmitted through surfaces and is not in itself airborne? Sure there are drops that can last long enough in the air to infect people but it’s far from the biggest point of infection.

Pretty sure you've got it backwards there.

Surface transmission is what's supposed to be 'while possible, it's not too likely', and respiratory droplets are supposed to be the main way it spreads (with recently 239 scientists begging WHO to acknowledge it can be spread through the air also, not just the larger droplets, which fall to the ground a lot faster).

Here's what CDC says: "The primary and most important mode of transmission for COVID-19 is through close contact from person-to-person. Based on data from lab studies on COVID-19 and what we know about similar respiratory diseases, it may be possible that a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes, but this isn’t thought to be the main way the virus spreads."

https://time.com/5863220/airborne-coronavirus-transmission/ https://www.cdc.gov/media/releases/2020/s0522-cdc-updates-co...

AFAIK you have it exactly backwards.

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