
Researchers create air filter that can kill the coronavirus - akeck
https://uh.edu/news-events/stories/july-2020/07072020ren-coronavirus-filter.php
======
hedora
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?

~~~
tzs
> 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...](http://donaldsonaerospace-
defense.com/library/files/documents/pdfs/042665.pdf)

~~~
sizzle
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?

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joe_the_user
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).

~~~
dbcurtis
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.

~~~
praptak
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.

~~~
dTal
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!

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elif
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?

~~~
catherd
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...](https://en.wikipedia.org/wiki/Ultraviolet_germicidal_irradiation#Air_disinfection)

~~~
bradknowles
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.

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johnnymonster
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???

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

~~~
sgerenser
[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.

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

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aj7
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.

~~~
kijin
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.

~~~
usrusr
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.

~~~
toast0
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.

------
donogh
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...](https://www.sylvane.com/iqair-healthpro-plus-air-
purifier.html) [2] [https://abcdust.net/how-large-is-a-corona-virus-virion-
compa...](https://abcdust.net/how-large-is-a-corona-virus-virion-compared-to-
the-mp10-2-5/) [3] [https://www.sylvane.com/iqair-perfect-16-air-
purifiers.html](https://www.sylvane.com/iqair-perfect-16-air-purifiers.html)

------
guidedlight
This company claims its air filter kills the virus too.

[https://cleanairexp.com/](https://cleanairexp.com/)

~~~
reaperducer
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.

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Causality1
Is there any evidence that the virus can survive passage through a normal air
filter/AC system and infect humans?

~~~
saeranv
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...](https://www.engineeringtoolbox.com/air-change-rate-room-d_867.html)

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

~~~
dehrmann
> It's wasteful since you also have to condition

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

~~~
saeranv
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://en.wikipedia.org/wiki/Heat_recovery_ventilation#:~:text=Heat%20recovery%20is%20a%20method,thus%20energy%20costs%20\)%20of%20buildings.&text=Heat%20recovery%20systems%20typically%20recover,the%20energy%20efficiency%20of%20buildings%20).

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

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

------
glaive123
Do viruses actually die? I thought they are not living.

~~~
thdrdt
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'.

~~~
gpvos
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.

------
mootzville
So, I can just vent through my oven?

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

------
codecamper
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?

~~~
mschuster91
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.

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

~~~
mschuster91
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.

------
Shalle135
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.

~~~
cableshaft
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://time.com/5863220/airborne-coronavirus-transmission/)
[https://www.cdc.gov/media/releases/2020/s0522-cdc-updates-
co...](https://www.cdc.gov/media/releases/2020/s0522-cdc-updates-covid-
transmission.html)

