Students around Australia will soon return to classrooms, and each state has a different stance on whether air purifiers will be used in schools to help reduce the risk of COVID-19 transmission.
With parents anxious about children returning to built-up environments, we asked experts to explain the role of air purifiers in improving ventilation.
Do air purifiers help to reduce the risk of transmitting COVID?
Air quality experts say that purifiers, if powerful enough and equipped with a filter, can effectively filter virus particles out of the air, and want them deployed in classrooms.
The purifiers preferred by academics use mechanical filtration, forcing air through a fine mesh that traps particles.
Other purifiers neutralise particles in the air without filtering them.
But not everyone agrees purifiers are the way to go.
South Australia's Education Department said last month that an independent trial of air purifiers found they "do not reduce the amount of CO2 within education spaces in any meaningful way and provide minimal improvement to the quality of the air".
What filters should the purifiers use?
Researchers say high-efficiency particulate air (HEPA) filters are the most reliable at removing particles of any size.
Research from the University of New South Wales indicates air purifiers with HEPA filters can remove 99.7 per cent of tiny particles
Geoff Hanmer, a professor of architecture at the University of Adelaide and the chair of OzSAGE's ventilation group, said running a HEPA filter with enough output in a room would "actually pull virus particles out of the air and make the air safe for people in the room."
The output bit is important. A filter needs to be designed to work in that classroom environment, factoring in its volume and its existing ventilation, as well as the number of students and teachers in it.
Experts say they ideally need to be able to replace the entire volume of air inside a room, a process called air change, at least six times an hour.
Lidia Morawska, director of the International Laboratory of Air Quality and Health at the Queensland University of Technology, said purifiers with HEPA filters were "useful tools for removing the virus from the air", but not enough to prevent the transmission of the virus on their own.
She said understanding the system involved measuring the air quality in classrooms and taking steps to improve it if it was not high enough.
Those steps include ensuring there was enough natural ventilation — through open windows and doors — and that air conditioners were effectively filtering the air in rooms with no, or not enough, windows.
Donna Green, an Associate Professor at the UNSW's Climate Change Research Centre, agreed HEPA filters should be used as part of a layered risk reduction approach.
"Within a suite of measures to reduce transmission, HEPA is a very valuable tool because it will just lower the amount of particles in the atmosphere that you can transmit so you will be reducing risk, both between students and between students and teachers," she said.
Which classrooms should they go in?
Associate Professor Green argues purifiers should definitely be placed in internal-facing classrooms, but ideally in all indoor school spaces.
Professor Morawska said classrooms with windows did not automatically have good ventilation, and air quality monitoring was needed to work out which ones required purifiers.
How many purifiers does a classroom need?
Clare Walter, a doctoral candidate in air quality and health at the University of Queensland, who also runs a ventilation business, said that entirely depended on the space.
"I've assessed some classrooms where one has been enough, while others, just by the nature of the design of the classroom and the building materials, would have required three large filters," she said.
Associate Professor Green said a large filter — which costs about $600-$800 — would be effective for an average-sized classroom, with two needed for a larger classroom or function space.
Given students need to be able to hear their teacher, two smaller, quieter purifiers might end up being a better option than one big one.
Where should they be placed in a classroom?
It really depends on the air flow of the classroom, where the teachers and students are situated, and where the windows are.
Associate Professor Green said getting a purifier in place, and making sure it was not a tripping hazard, were more important concerns than location.
What is ventilation like in schools?
There are no indoor air quality standards in Australia, so experts say it is hard to know without measuring each individual space.
Professor Hanmer said carbon dioxide monitors should be installed to do just that, given most are accurate enough to measure ventilation levels.
He said that would give parents and students confidence that ventilation was sufficient and allow learning to be moved outside, or for windows to be opened, if carbon dioxide levels were too high.
What are the states doing?
It differs widely around the country.
Victoria is deploying 51,000 purifiers in state schools, while NSW schools will receive 19,000 to be used at the principal's discretion.
Tasmania is supplying 4,500 air purifiers to internal classrooms without adequate ventilation, with Premier Peter Gutwein saying learning in outdoor spaces would be "optimised".
Western Australia this week announced that 12,000 air purifiers would be distributed, while South Australia says they're not necessary and will instead focus on outdoor learning.
Queensland's back to school guidelines also don't include installing air purifiers.
Why isn't there a nationally consistent approach?
National leadership is something Professor Morawska said was sorely needed.
"We're in a situation that every state is doing something different, every state has to rediscover the wheel and it's not helping and it's just confusing," she said.
Associate Professor Green agreed there should be consistency.
Associate Professor Green said it would cost about $200 million to install carbon dioxide monitors and HEPA filters in schools around the country.
"That's the cost of Sydney going into lockdown for one day," she said.
Professor Morawska said acting on ventilation now would have huge benefits in the long run.
"We should really take this opportunity and work towards achieving good ventilation systems in the classroom so it provides good indoor air quality in relation to all the pollutants at all times," she said.
"[Then we won't be] in this situation in the next pandemic, not in such a situation during seasonal pandemics or flus or so on, or when the next season of bushfires comes."
