Find out what will and won't help when it comes to protecting yourself and others from airborne transmission of SARS-CoV-2 with portable air cleaners.
This fact sheet explains the basics of air filtration and what to look for (and avoid) when purchasing an air cleaner. We have linked particular terms and ideas to pop-up boxes with definitions or additional information.
What do air cleaners do?
Air cleaners (also called air purifiers or scrubbers) use high efficiency particulate air (HEPA) filters to remove 99.97% (H13 or H14) of aerosolized virus particles in the air. Consisting of a fan and a layered filter, they work very effectively to clean the air of aerosols. They can provide an extra layer of protection in indoor settings where occupancy is high and there’s increased risk of airborne transmission.
How do I know if I need an air cleaner?
A CO2 monitor showing levels greater than 800 ppm indicates that the air indoors is not being refreshed fast enough with outside air?. Improving outdoor air ventilation of our spaces through opening windows/doors, creating cross ventilation, improving HVAC settings and filters needs to performed – but air cleaners are an extra tool in our arsenal to reduce the risk further and to get the effective air changes to where we need them to be.
In a pandemic, we are aiming for 4–6 Air Changes per Hour (ACH),? or much higher if 90%+ clearance is desired.
Unfortunately, this target is hard to achieve with our current buildings and existing ventilation systems, as well as how we currently use our indoor spaces and while maintaining heat and energy efficiencies.
You can calculate your risk of airborne transmission by downloading the Queensland University of Technology's Airborne Infection Risk Calculator.
What to consider when purchasing an air cleaner
Things to look for
If you decide an air cleaner will help reduce risk in your space, these are what you should consider when purchasing one:
- A HEPA (only) air cleaner
- The Clean Air Delivery Rate needs to be sufficient for the room volume
- Maximum tolerable noise – fans are noisy, and it can sometimes make sense to have two quiet (<40dB) portable air cleaners rather than one large cleaner (>50dB)
- Cost
- Australian-made products and expected delivery times.
Things to avoid
- Directional fans without any filtration that blow air from person to person could lead to unintended transmission
- Ionisers, plasma/ozone/photocatalytic oxidation/precipitators and UV purification or disinfecting add-ons.
These are unproven/untested technologies, and in some cases dangerous technologies?, significantly degrading the air quality by producing ions, ozone and oxidation products. Ozone and ions can trigger asthma so these technologies should be avoided. Air circulation without filtration has been shown to lead to super-spreader events.
Australian product comparisons
There are a number of devices available to the Australian consumer market. We have collected here the products that are safe – that is, they only use HEPA filtration and don’t operate as direction fans without any filtration. Many of our favourite brands didn’t pass these criteria, but you will find many did, including the locally-manufactured InovaAir devices.
View a larger version or download a PDF (PDF 88.1 KB)
The values used to produce this plot are from the manufacturers' specifications. We will update these figures and add new products as new information becomes available.
Last updated: Friday 13 August, 2021.
View a larger version or download a PDF (PDF 98.1 KB)
The values used to produce this plot are from the manufacturers' specifications. We will update these figures and add new products as new information becomes available.
Last updated: Friday 13 August, 2021.
View a larger version or download a PDF (PDF 98.8 KB)
The values used to produce this plot are from the manufacturers' specifications. We will update these figures and add new products as new information becomes available.
Last updated: Friday 13 August, 2021.
View a larger version or download a PDF (PDF 97.7 KB)
The values used to produce this plot are from the manufacturers' specifications. We will update these figures and add new products as new information becomes available.
Last updated: Friday 13 August, 2021.
Declaration: Robyn Schofield and the University of Melbourne does not receive any funding from air cleaner manufacturers.
Indoor air does not have regulatory standards in Australia. Currently no standardised and independent testing of air cleaning devices is required or performed to protect consumers. If you found this information useful and would like to support our University of Melbourne ventilation and filtration work, please consider a donation.
CO2 levels are approximately 400ppm (400 parts per million air molecules) in outside air. Humans breathe out 37,000ppm with each breath. For more information, see Exhaled CO2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and Activities by Zhe Peng and Jose L. Jimenez, and Robyn Schofield's interview with ABC News Radio.
The New Zealand Ministry of Education released a great resource for ventilation, CO2 and schools: Designing Quality Learning Spaces: Indoor Air Quality and Thermal Comfort.
ACH = Q/V where Q is the ventilation rate (m3/h) the rate of air supplied at the air inlet and V is the volume of the room.
To achieve 99% clearance, >30 effective ACH is required; in our study at the Royal Melbourne Hospital, this was achieved with two air cleaners taking 5.5 minutes to clear a single patient room.
See Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission by Joseph G. Allen, DSc, MPH; Andrew M. Ibrahim, MD, MSc for more information on how experts have come up with these targets.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has prepared a Position Document on Filtration and Air Cleaning, finding only limited data documenting the effectiveness of gas-phase air cleaning as an alternative to ventilation. Todd Crawford, et al., demonstrated the degradation of indoor air quality caused by corona technology with air cleaners in ASHRAE Journal; New York Vol. 60, Iss 12 (Dec 2018): 64–67.
The California Air Resources Board also released a list of potentially hazardous ozone generators sold as air purifiers, recommending that they not be used, and Marwa Zaatari and Marcel Harmon penned an open letter to school district facility managers and administration leadership to address the use of electronic air cleaning equipment in school facilities.
