We would buy a mobile air purifier in our home.

Against the smoke of the city, etc.

It would have a HEPA + activated carbon filter. They are marketing it as:

"filtration process captures 99.97% of airborne particles down to 0.1 micron in size."

and having also:

"encapsulated ion particle charging chamber"

But the question:

Would the mentioned filter+ion charger also help cleaning the air from bacteria and viruses?

Ex.: "Coronavirus/2019-nCoV" is about "Coronavirus virions are spherical with diameters of approximately 125 nm" - but I just Googled it, not from an official source. And 0.1 micron is 100 nm.

UPDATE: would it be more effective, if it would have UV-C light in the air purifier?


The aforementioned filter will filter microbes

Yes, it is routine practice to use microfiltration in labs to sterilize solutions from microbes. 0.2 micron filters will remove all typically-found bacteria most of the time, except for the odd bacteria here and there, though this is uncommon. 0.1 micron filters certainly remove all stray bacteria that may make it through upon a single 0.2 micrometer filtration. Here is a nice read on the appropriateness of 0.2 and 0.1 microfilters, which simply states that 0.2 micro filters are usually more appropriate since they additionally guarantee a better flow rate.

The aforementioned filter will NOT ensure filtration of viruses

These filters are not used, suitable nor effective for viruses, however. You ask specifically about the coronavirus. From a 2015 study:

Coronaviruses are members of the Coronaviridae group and contain a single-stranded, positive-sense RNA genome surrounded by a corona-like helical envelope (Ryan 1994). The SARS virus genome consists of 29,751 base pairs. Approximately 41% of the genome is GC base pairs while 59% is TA base pairs. Coronaviruses have a size range of 0.08-0.15 microns; with a mean size of 0.11 microns (see Figure 1).

This means 0.1 micron filters will not protect you from these viruses. They may help you reduce viral count in the air, which may of course be sufficient, but this would not be an adequate measure in a biological safety facility.

The cost-effectiveness of commercial air-purifiers for biosafety vs. pollution

On a advice-related note: I do not think air-purifiers are going to help you in a cost-effective manner to avoid viruses (e.g. Rhinoviruses that cause the common cold, for instance), lest you stay at home all the time. You expose yourself outdoors, and carry plenty of viruses indoors on your body and in your respiratory tract; these will bypass your filtration system on the daily. Being healthy and lucky enough to avoid exposure to viruses outside the house is still the best bet. I think an air purifier has a greater potential to help with smog and urban pollution, those particles are typically larger than the diameter of most microbes.

  • $\begingroup$ Wow, thx. I updated the question, UV-C popped in my mind :) $\endgroup$ Jan 24 '20 at 12:29
  • $\begingroup$ It would depend on the UV-C lamp and how it sits in the purifier. A quick google search copy pasta for you: "the average bacterium will be killed in ten seconds at a distance of six inches from a typical UV-C lamp". And from Wikipedia on microbe inactivation: "dosages for a 90% inactivation of most bacteria and viruses range from 2,000 to 8,000 μW·s/cm2". Air filtration units would have a flow-through, so the lamp(s) would have to illuminate a very thin cross-section of the piping; again, its efficacy would depend on the flow-rate, distance from the lamp, and illuminated area. $\endgroup$
    – S Pr
    Jan 24 '20 at 12:54

When thinking about using a HEPA filter for dealing with air-borne virus and bacteria, there are two important things one need to know:

1- While a sieve traps objects larger than a certain diameter and let the smaller ones pass through, this is not what happens in a HEPA air filter. In this case, due to the principles of fluid mechanics, and also because of brownian movement, there is a critical size (around 0.3 Micron, 300nm) for which, the filtration efficiency is minimum. This means that for both particles larger and smaller than this critical size, a HEPA shows very good filtration efficiencies. As you can see in the graph below, for a True HEPA, even for this "most penetrating" size, the efficiency could be still very impressive (higher than 99.7%).enter image description here

This means that a good quality microfiberglass HEPA filter can be very good in eliminating nano particles the size of a virus.

2- The second point is that very small airborne microorganisms, are in many instances attached to bigger particulate substances in the air, whether liquid micro droplets or solid particles. I do not know so much about the nature of the bonds and whether this happens for the coronavirus or not, but this may be reason for higher risk of respiratory infectious diseases when PM2.5 air pollution gets high.

For these two reasons, I believe that HEPA air filtration could help a lot in indoor spaces, reducing the risk of infection, by reducing the number of particles of any size from the air.

In contrast, when comparing to the application of UV-lamps, there are to negative points:

1- UV lamps have lifetime and the radiation wavelength is so much affected by working hours of the lamp.

2- If air speed and exposure to radiation is not enough, complete degeneration may not happen and we can not be sure if we are not producing a new genetically improved virus!

All Together, I think we should generally be more careful with the active control methods. With passive control methods such as HEPA filtration, we are sure we are not adding anything new to the air!!!

As a reliable reference on the efficacy of HEPA filters on bacteria and nanoparticles you can see the Technical Memorandum of NASA Marshal space flight Center of May 2016:

Perry, J. L., Agui, J. H., & Vijayakimar, R. (2016). Submicron and Nanoparticulate Matter Removal by HEPA-Rated Media Filters and Packed Beds of Granular Materials.

  • $\begingroup$ Many thanks for the infos! :O $\endgroup$ Feb 14 '20 at 10:56

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