Filtering and "air cleaning" | COVID-19 Aerosol Transmission FAQs (9/9)

9.1. Are portable air cleaners useful? Which types do you recommend?

Yes, portable air cleaners based on filtration are definitely useful. Our virus exposure risk is a function of aerosol concentration and time. For environments where we can’t reduce time, we can reduce the concentration. Portable air cleaners can effectively reduce indoor concentrations of virus relevant aerosol sizes.

Filtration does not have potential negative effects, and works very well to remove aerosols that may contain the virus. As a side benefit, allergies and pollution are also decreased, with additional health benefits.

Follow manufacturer guidelines for frequency of filter replacement. Unless the filter is being used in a space with high aerosol concentrations, in which case the filter should be replaced more often.

9.2. What are HEPA portable air cleaners?

“HEPA filter” is an acronym for “high efficiency particulate air filter.” They remove more than 99.9% of aerosols in an air stream passing through them. HEPA air cleaners are the best type of air cleaners (left picture below). They are simply a box with a fan that draws air through a high efficiency filter. The filter removes nearly all virus relevant aerosols. We recommend those without additional bells and whistles such as UV, ions etc. Those other features add cost and can add some problems. HEPA air cleaners need to be of the right size of the room, see this spreadsheet for helping to choose the right one. However they are costly, often in the hundreds of dollars or euros for a reasonably sized room.

[Our comment: the major downside to a HEPA filter is that it greatly impedes air flow. It simply isn't possible to push a large amount of air through a dense filter at a high speed without making a huge amount of noise, or having an absolutely massive unit to lower the pressure. It is important to consider benchmarks values of air exchange rates. A HEPA filter can offer a large improvement in small rooms that have particularly poor ventilation, but they are not a scalable solution that will work for businesses. The Reflow Labs Airsafe was designed specifically to address this weakness by enabling far higher air flow rates, focusing exclusively on biological threats and foregoing the filtration of particulate matter.]


Portable HEPA filter unit Credit: Amazon) | Improvised box-fan filtration unit (Credit: Jim Rosenthal)

9.3. But if the virus is 0.1 μm, do HEPA / MERV filters (or masks) remove it from the air?

The virus is 0.1 μm (micrometers), but it is NOT naked in the air. The typical respiratory aerosol that contains viruses and is generated when talking is ~3 μm. See this question above.


And in any case all filters work well at 0.1 μm, because brownian motion helps a lot for filters to capture small aerosol . See the chart below from the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) that shows the efficiency of the filtering mechanisms vs. aerosol size. There is a minimum at 0.3 μm because those aerosols are not well captured by either diffusion (which helps for the smaller ones) or impaction / interception (which helps for the larger ones). And filters and masks are often specified at 0.3 μm because that is the easiest measurement, since that is where most aerosols penetrate the filter. Also 0.3 μm is a typical size for pollution and smoke aerosols, but NOT for virus-containing respiratory aerosols, which are mostly larger. This video discusses the subject very clearly.

9.4. Is there a cheaper alternative to a HEPA air cleaner?

Yes, a makeshift fan-filter system (right picture above) often costs one fifth as much as a portable HEPA unit, and can be very effective at removing aerosols from a room. This article in Wired and this first and second articles from the CEO of a filter company discuss the idea of do-it-yourself systems. These may be noisier as well. Some tips for these:

  • You should close the gaps between the filter and the fan with tape to remove leaks
  • Use a deeper filter, which allows more airflow and a more efficient filtering
  • MERV 13 or similar is recommended. A HEPA filter may cause too high pressure drop, causing the fan to work too hard. The fan then may overheat and/or fail prematurely. Only operate this system when someone is present to monitor the system.
  • From the point of view of filtering, it is better to put the filter in the intake side (where the fan is pulling air from). But then any virus is exposed to the touch. So e.g. in a classroom it may be better to put the filter on the exhaust side, so that any virus is not exposed (but then taping it around the fan is even more important)
  • With most fans the system will work better at a lower setting. The full speed setting may lead to too much force on the filter, and overheating of the fan. A lower setting will also reduce noise. The details will depend on the fan / filter combination. If there is too much noise or overheating of the fan, consider a lower filter grade.
  • Monitor the temperature of the fan, and don’t leave the system operating unattended. There could be some risk of fire if the fan motor overheated for a prolonged period of time.
  • See below about how to change the filter.

9.5. How do I select the right HEPA air cleaner? (or fan-filter cleaner)

There are products of various quality in the market. Some work well, others have lower quality filters that may not seal so well etc. We cannot give advice on specific models. We suggest that you select ones:

  • with just the filter (if possible, so no ions, UV etc.)
  • that are recommended by certifying authorities (AHAM or CARB in the US, Eurovent in Spain).
  • that you consult also the Harvard-Univ. of Colorado guide.
  • It is important that the size is large enough for the space. Any amount of filtering will help. To see the effect on the probability of infection for a specific situation, you can use the Aerosol Transmission Estimator (more advanced). The Harvard School of Public Health recommends 5 air changes per hour (ACH) for schools. To calculate this number, you can use the calculator in the Harvard-Univ. of Colorado guide, or simply calculate as in this example:

    • Volume of the room = 5 m x 3 m x 3 m = 45 m3
    • Clean air delivery rate (CADR) of the HEPA unit = 200 m3 / hr
    • ACH = 200/45 = 4.4 h-1 (which is pretty close to 5)
  • If you use more than one unit, then sum the ACH
  • Note that the CADR of the unit can vary with the setting (e.g. low / high speed)
  • For cheap fan-filter combinations, estimating the CADR is a little trickier. The manual of the fan (which can often be found at the manufacturer’s website) should have a specification of flow rate (m3/h or similar units) for each setting. Then the filter will reduce the flow rate. We suggest taking 60% of the nominal flow rate without the filter as the CADR in the calculation above.

9.6. Where should I place a HEPA air cleaner in a room?

In general most commercial portable air cleaners draw air in from the side and exhaust it vertically. For these systems, place the portable air cleaner in the middle of the room, raised off the floor if possible. This placement is to make sure as much of the room air gets to the filter as possible. If the air cleaner is placed near a wall, air on one side of the room will be filtered faster than the other side. Ensure the cord is secure so no tripping hazard exists.

For DIY systems, elevation may be more important due to the typical horizontal exhaust. A strong airflow over the floor may resuspend aerosols that have settled. A vertical exhaust is also desirable, and can be done.

9.7. How and when should filters be replaced?

When filters collect lots of aerosols, the airflow through the filter generally goes down. To maintain the effectiveness of the filter in cleaning room air replace the filters according to manufacturer's guidelines. Typically this is six months to a year for HEPA filters and three to six months for pre-filters. In dirty environments (spaces with high aerosol concentrations such as where there are smokers) replacement may need to occur more frequently.

Wear a mask and gloves when removing filters, and change the filter outdoors if possible. Place the used filters in a sealed plastic bag prior to disposal. Any viruses that have been removed from the air will then be in the filter. It is possible that they die within a few hours, as they do in aerosols, or within a few days, as they do on surfaces (to our knowledge this has not been studied). It is not easy to get any viruses trapped in the filter back into the air, but it is possible in principle. So if the filter has been used within a few days in a location where there could be viruses, then just out of an abundance of caution follow the above precautions. And if possible, change the filter after it has not been used for a few days. E.g. in an office or classroom, you could change it first thing on Monday morning, before classes start. Again, we think this is a (very) small risk, but better to be safe just in case.

9.8. Is germicidal ultraviolet light (UVC) effective as an air disinfection treatment for SARS-CoV-2?

Germicidal ultraviolet light (gUV, also UVC) is an effective technology to use to supplement ventilation. It is especially useful for increasing effective air change rates in spaces that are heavily occupied, and have the potential for unsuspected infectious persons inside. Data is forthcoming on the inactivation rates specifically for CoV-2 but in the meantime comparing rates for coronaviruses it appears to be as susceptible to gUV as the organism that causes tuberculosis is (Mycobacteria tb.).

One application that has been used since the early part of the 1900s is upper-room gUV. Lamp fixtures are hung higher up the walls to irradiate the upper part of the room only. The lamp fixtures have louvers that direct the UV across the room. Studies in the laboratory have shown that these systems can add as much as 17 air changes per hour. This type of system was used in NY classrooms to combat measles. The CDC has authored a guideline for the use of upper-room gUV in healthcare. Germicidal UV can also be applied in ducts of the recirculating air in an HVAC system. This design must consider the velocity of the air passing through the UV to allow for sufficient contact time.

[Our comment: the Reflow Labs Airsafe builds upon these proven approaches and offer the first drop-in solution that does not require such extensive renovations]

New technology at 222 nm is being developed and commercialized. It has a major advantage vs. the traditional 254 nm technology, in that it can be shone directly on people, because it does not penetrate the skin. It is still expensive and not widely available. [Our comment: see our article on FAR UVC. Far UVC is rather promising, but they require proper filtration to avoid emitting harmful mid-range UVC and the ones currently available do not have these filters. Properly filtered lamps are expected winter 2020, but the costs are prohibitive for most businesses]

Dr. Shelly Miller has been studying gUV for over 20 years and here is a link to slides from a presentation she gave in April 2020. For a summary and additional citations, see section 2.4 of the following paper that was published by some of the authors of this FAQ: How can airborne transmission of COVID-19 indoors be minimised?

9.9. Do you recommend portable air cleaners that are not based on filtration?

We do not recommend other types of air cleaners (ions, plasmas, oxidation etc.) for general purpose use, see this thread for more details.

9.10. Do you recommend spraying disinfectants into indoor air to kill the virus?

Definitely not for cleaning the virus in the air. This technique can be useful for disinfecting surfaces, when nobody will be present in the space for at least three air change rate timescales. If you don’t know the air change timescale for the space assume that you need at least 3 hours. See this Twitter thread and the same info as a Medium post for more details on the issues for some of these systems.

9.11. Should we use humidifiers or dehumidifiers?

It is well-known that dry conditions favor the survival of SARS-CoV-2 and similar viruses. This only matters for shared room (and any potential long range) transmission, if the distance is not kept, transmission in close proximity is not impacted by humidity or temperature. In principle humidifiers can be used to increase humidity and reduce the survival time of the virus. However, ventilation and/or filtering of indoor air is much simpler and more efficient. Ventilation expels the virus-containing aerosols outdoors, while filtering captures them and removes them from the air we breathe. Humidifying alone does not physically remove the virus from the air, but it may make the virus decay faster.

Dry conditions may also make humans more susceptible to infection, which would suggest that humidification to avoid dry conditions can be beneficial.

Given its simplicity and efficacy in removing the virus from the air, we recommend that any investment is focused on ventilation and filtering and germicidal UV, when professionals can do it. We recommend humidification for people who already own humidifiers and want to add an additional layer of protection. REHVA’s guidance agrees with this point. The relative humidity should remain below 65% to avoid promoting mold growth.

For these reasons we do not advise running dehumidifiers, unless in locations where humidity would otherwise be very high and where it could create other problems (e.g. mold).

9.12. Should we keep indoor spaces hotter or colder to reduce transmission?

Viruses survive less well at higher temperatures. However, higher temperatures reduce relative humidity, and low humidity leads to increased survival of the virus. Both effects are relatively small over a few degrees of change under typical comfortable ranges. Therefore at present we do not recommend changing ambient indoor temperatures for this purpose. REHVA’s guidance agrees with this point.

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