Reducing the risk of Covid-19 transmission through the use of air purifiers

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Prof Mark Jermy, Dr Julie Bennett, Dr Jason Chen, Phoebe Taptiklis and Dr Caroline Shorter*

Indoor environments increase the risk of transmission for the virus that causes Covid-19 (SARS-CoV-2) by containment and concentration of the airborne virus. However, to reduce such transmission, the particles that carry the virus can be diluted by bringing in as much outdoor air as possible. When good ventilation is not possible, air purifiers can be used as an additional preventative measure to reduce the number of virus-laden particles. Ventilation and purification of indoor air needs to be used alongside public health measures, such as vaccination, staying home when unwell and mask use. This blog outlines some basic principles of using air purifiers to reduce the risk of Covid-19 transmission.

Examples of air purifiers (source Mark Jermy)

Indoor spaces with low levels of ventilation are high-risk settings for Covid-19 transmission, as an infected person can breathe out virus particles which spread through the air.1 The most effective way to remove these particles is by replacing the contaminated indoor air with as much clean outdoor air as possible, as we recently reported. The Centers for Disease Control and Prevention (CDC) recommend that to reduce the risk of Covid-19 transmission, carbon dioxide (CO2) levels, which are used as a proxy measurement for ventilation, should stay below 800 ppm. If it is not possible to achieve these levels that represent adequate ventilation, cleaning the air with the use of an air purifier will give additional protection.2

Portable air purifiers (also called air filters or air cleaners) contain a fan that pushes air through a filter, which traps pollutants that pass through them. If the filter is effective, it will collect most of the virus-bearing particles, leaving the air free of these particles.3 Air purifiers suitable to use in a living room, bedroom, small office, or work breakroom can be bought for around $300-$1000. Schools in Victoria, Australia have recently purchased 51,000 portable air purifiers to be installed into schools to reduce the risk of Covid-19 transmission.

To work effectively, air purifiers need to be placed so that they can process as much of the air in the room as possible.4 If there is more than one person in the room, they should be left on continuously, allowing up to an hour to filter the room, depending on the size of the room. Air purifiers should be used at their highest fan speed, which will mean they make some noise, similar to that of a desk fan.

The best place to position a portable air purifier ideally is:

  • Roughly in the centre of the room
  • At table top height
  • At least a metre from walls, curtains or furniture
  • Away from open windows (so they are not filtering the already relatively clean air from outside, or blowing the cleaned air out of the room)

Users should avoid placing the air purifier in areas where people are likely to walk and take care of electrical cords, which should be taped down to carpets or floors if they cross passage-ways.

When choosing an air purifier, users should consider the filtration performance and the flow. Many of the exhaled particles which can carry SARS-CoV-2 are microscopic. It is best to choose a filter that is High-Efficiency Particulate Arresting (HEPA) to H13 level or better. True HEPA filters should have been tested to reach the official HEPA standard. H13 filters remove 99.97% of particles down to 0.3 microns in size. Some filters are advertised as “HEPA like”, which does not guarantee their ability to filter small particles. As portable air purifiers are unable to clean all of the air in a room at the same time, a lower standard of filter (E10 or MERV 13) is acceptable. Such a filter will remove at least 75% of the virus-bearing particles with each pass of air through the unit.

Air purifier adverts often give a clean air delivery rate (CADR).4 This is the volume of clean air blown out of the device each hour and is usually given in cubic metres per hour, or cubic feet per hour. The CADR number should be two to four times the volume of the relevant room. More than one air purifier in a room can be used to add up to enough CADR.5 If more than one air purifier is used, they should be placed with at least a couple of metres of space between them (otherwise they might largely just be cleaning the same air twice).

Many portable air purifiers have an ioniser as well as the filter. The health effects of ionizers are not yet clear. Given this, if an air purifier with an ionizer has been purchased, we recommend switching the ionizer off. The fan and filter can continue to run normally. We also don’t recommend the use of electrostatic precipitators in rooms with people present.

Cleaning the air in larger buildings

In buildings that have mechanical ventilation, such as heating, ventilation and air conditioning (HVAC) units, additional systems and filters can usually be fitted. Most ventilation ducts have filters, but these might only be good enough to filter large objects, such as leaves. Fitting systems that recirculate air (rather than bring in outdoor air) with additional filters may be worthwhile, provided the system has a powerful enough fan to blow air through the thicker filter. As with the portable air purifiers, any additional filter should be MERV-13 or HEPA H13 rated.

Ultraviolet (UV) sterilizers for air (also called germicidal UV, UV germicidal irradiation, GUV or UVGI) can inactivate the viruses in air. Despite the term “germicidal irradiation”, these systems do not produce the ionizing radiation that is a danger with radioactive materials. The only radiation they produce is UV light. UV systems can be fitted to the ducts of mechanical ventilation systems to sterilise recirculated air. Companies that specialise in heating, ventilating and air conditioning (HVAC) can supply and fit these units. The cost is likely to be $20,000-$200,000 (it varies with complexity of the duct system) for a space where there are 20 people or more.

Alternatively, and sometimes less expensive, UV lamps can be mounted high on walls, shining across the room rather down onto people. This type of system is called an upper room UV and are often used within hospitals. If properly installed, these systems are very safe.6 There are also systems that use UV to sterilise surfaces, such as benchtops or equipment. These are better suited to environments such as laboratories where infectious materials are handled regularly. Extra care is required with these systems so that people are not exposed directly to the UV light. Just as with portable air cleaners, we don’t recommend the use of ionizers or electrostatic precipitators, or any system which directly exposes people to UV light.

Air purification alone is not enough to fully protect people from Covid-19 transmission, mainly because purification units/systems are not able to clean the entire room’s air, at the same time. However, when used alongside other public health measures, including vaccination, staying home when unwell, ventilation, and mask wearing, air purification can be part of a plan to reduce the potential for indoor airborne transmission of Covid-19.

Author details

Prof Mark Jermy and Dr Jason Chen are based the College of Engineering, University of Canterbury. Dr Julie Bennett is based at the Department of Public Health, University of Otago, Wellington. Dr Caroline Shorter is based at the Department of Medicine, University of Otago, Wellington. Phoebe Taptiklis is based at MOTU.


  1. Allen JG, Ibrahim AM. Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission. 2021; 325(20): 2112-3. DOI: 10.1001/jama.2021.5053.
  2. Rodríguez M, Palop ML, Seseña S, Rodríguez A. Are the Portable Air Cleaners (PAC) really effective to terminate airborne SARS-CoV-2? Science of The Total Environment. 2021; 785: 147300.
  3. Conway Morris A, Sharrocks K, Bousfield R, et al. The removal of airborne SARS-CoV-2 and other microbial bioaerosols by air filtration on COVID-19 surge units, medRxiv, 2021.09.16.21263684;doi:
  4. Harriman L., Stephens B., Brennan, T. 2019. “New Guidance for Residential Air Cleaners.” ASHRAE 61 (9): 14–23.


5. For example, if the floor area of a room measures 4 metres x 5 metres, the floor is 4×5=20 square metres. If the ceiling is 3 metres high, the room volume is 20 x 3=60 cubic metres. If an air cleaner has a CADR of 300 cubic metres per hour, it will give 300/60=5 extra air changes per hour. This is in addition to the air changes that the ventilation system makes, which is usually at least 1-2 air changes per hour, so this room now has 6-7 air changes per hour, which is good. A room twice this size could have two air cleaners. If CADR is stated in cubic metres per hour, make sure your room volume is also measured in cubic metres. If CADR is stated in cubic feet per hour, either convert it to cubic metres per hour, or measure the relevant room volume in cubic feet.

6. If properly installed by a good company, these indoor systems have almost no risk of causing skin cancer. This is because the lamps and reflectors are installed so that they only shine through the upper air, and not into the places where people work. Also, the damaging UV in ultraviolet light is UV-B and the steriliser systems use UV-C, which is a different set of wavelengths, and which does not reach through to sensitive parts of the skin.

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5 thoughts on “Reducing the risk of Covid-19 transmission through the use of air purifiers

  1. Kia ora,
    Thank you for the through work detailing all the ways you can monitor the air quality.
    With Early Childhood centres opening soon while covid is in the community I am trying to do some research on the best and most economic ways to ventilate centres as poor ventilation is a cause of the spread of the covid 19 virus. Our children are under 5s and not being vaccinated are in need of all the protection we can have.
    Often our sleep rooms can be without a lot of windows as they can be inside the building or facing noisy streets, they can have 24 or so children packed into them. Often there are rotations of sleepers with babies in the morning and afternoon and 2- and 3-year-olds after lunch.
    Room sizes can be between 12sqM for sleep rooms to 80sqM or more. Play rooms can be used for sleeping as well. With ECE being underfunded we might have to think of cheaper ways to ventilate. I was thinking about how heat pumps might be used for this purpose. For those who have them, it might be an option. What type of ventilation systems or air filters might best work for us at Early Childhood Centres? We can even speculate that some air conditioning could even help spread the virus so safe fresh air systems are needed.
    Looking forward to some quotes of costs to buy and install so I can share details with different centres. Thanks for the air filler ideas.
    Thank you.
    Hugo van Stratum.

    • Kia ora Hugo,

      The sleep rooms you describe sound very difficult to ventilate, given the lack of windows or inability to open windows (noise, safety). Unfortunately heat pumps do not ventilate rooms as they do not bring outdoor air inside. They are designed for heating and cooling of rooms. Any air purification system that you consider should meet the performance described in the article and be placed appropriately. Ideal placement of air purifiers may be difficult around small children (safety hazards) and you should also consider the noise any units make. Would it be possible to move the children’s sleep areas to covered outdoor spaces?

    • Hugo, I wish it was someone in the aged care home sector showing as much concern as you are! I seem to have missed any reportage of child care being locations of deadly outbreaks of Covid-19, in fact I thought the aged-care death rate from Covid was a factor of hundreds higher than that in childcare.

      Your concerns are excellent in a broader sense that we have been going the wrong way as a society for the last couple of decades, obsessed with energy efficiency in buildings, and “fresh air exchange” or even air treatment as a substitute, is knocked down the list of priorities. It is quite possible that aerosol buildup indoors plays a role in RSV and other diseases that DO kill infants; and might play a role in a future pandemic that does kill infants and children and so on. This point has been well made by others writing about aerosol de-risking.

      But ventilation and air treatment in aged care homes is a “stable door through which the horse has already bolted” in most of the world – but in NZ we COULD yet put the right emphasis, urgently, on public health advice to “protect the vulnerable” – which is a point that has been spoken about in inverse proportion to the success any public health officials have had in actually succeeding in doing so. Our strategies such as hard lockdowns, are like carpet-bombing Iraq to get Saddam, and still not actually getting Saddam – and carpet-bombing again and again and again in callous disregard for the collateral damage. We have still ended up with a death toll of vulnerable people, actually people living in high risk environments – because the virus ends up getting into those environments anyway one day. The right “environmental” advice would have been like getting Saddam with a precision guided munition.

      By the way, there has been a battle going on behind the scenes since at least April 2020 with experts trying to make the WHO and the global networks of officials focus on aerosols and ventilation and air treatment. I say a lot more in a comment on the next thread on this blog.

  2. There should have been an Apollo Project conceived by the WHO to these ends at least by mid 2020. The case for aerosols being the main means of transmission was obvious to dozens of experts who presented to the WHO on 2 April 2020; and it was also obvious to non-experts like the Dutch Mathematician Maurice De Hond who started posting on it in March 2020. Merely as a curious person with time on my hands during lockdowns, I had recognized this by May 2020 and have been in an increasing state of disbelief as so much time has passed and the issue is still only slowly seeping through the less important experts who seem to be ignored just as much as everyone before them who “got it”, has been ignored already. In some cases, for 16 months.

    Maurice De Hond’s website has been the most up to date source on everything Covid, along with hypotheses from Maurice and others who are obviously way ahead of the official experts. For example, the numerous environmental factors that affect the likelihoood of indoors aerosol buildup, are the major explanatory mechanism for wide variation in Covid outcomes between regions. But everybody wants to make it political, as statistician William Briggs said halfway through last year, everything is down to our political enemies doing the wrong thing, or our political favorites doing the right thing, it is never about “nature”! No matter how outrageously cherry-picked the evidence for the argument has to be!

    You are quite right to point out that ventilation is impractical in some parts of the world some or all of the time – for example, the air may always be too polluted; outdoors may be too noisy; or the air may be too cold, too hot or too wet. But this is most emphatically not the case in New Zealand – we are said to have “dodged a bullet” again and again but what should be getting discussed is NZ’s obvious starring role as the ideal aerosol-mitigating natural and built environment. UV, humidity, temperature range, wind, predominant housing type, ozone content at ground level, air purity due to low population and low industrialization, etc

    I would be hard to imagine greater institutionalized incompetence, than that of “epidemiology” that applies the impact of Covid in Bergamo and NYC in winter, to computer modelling of potential Covid impact in New Zealand. What on earth do “epidemiologists” spend years studying to gain their qualifications, if it does not include environmental filters or multipliers of disease impacts???

  3. We updated the paragraph on filter specifications today, what I’d originally written wasn’t very clear about why filters of the HEPA (H13) standard are best, but filters of the lower E10 or MERV 13 standard are sufficient.

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