Who would have thought a light breeze could be so complicated? Yet talk about the topic of ventilation in schools as a means to reduce Covid-19 transmission and you’ll soon find there’s a lot more to it than opening a window - and even that is not straightforward.

The issue of ventilation has hit the headlines over the past two weeks, starting with the news that a trial of ventilation technologies would take place in 30 primary classrooms in Bradford to see how they can help reduce the build-up of Covid-19 particles in the air.

Project lead Professor Mark Mon-Williams, from the University of Leeds, told Tes that the trials are critical to help our understanding of how devices such as air purifiers can help reduce the build-up of Covid-19 in classrooms and stop it spreading.

“We will be able to look across the data and see what is happening by really drilling down to see whether these technologies are keeping children safe,” he said.

The Department of Health and Social Care, which is funding the trial, added that it would play a key role in help ensure schools are Covid-safe. “We want to ensure schools are both safe and comfortable for students and staff - and have been clear that good ventilation is crucial,” a spokesperson said.

However, a few days later the teaching unions have voiced their frustration in an open letter to the government to that these trials are starting far too late - and are unnecessary.

“This project should have been undertaken more than a year ago and is just delaying protections which could be put in place now,” they say. “The benefits of ventilation in the control of airborne diseases are already well understood and accepted, and we already know that air purifiers work.”

What we already know 

This second point certainly has some merit.

For example, long before researchers in the UK began their work, academics in Germany had conducted and published findings from their own experiments into how efficient air purifiers in classrooms are at reducing particle transmission.

The study, led by Dr Joachim Curtius, from the Institute for Atmospheric and Environmental Sciences at Goethe University, involved placing air purifiers around a classroom and then monitoring air quality during eight single lessons and two double lessons in a class of 26 students - and comparing this to a lesson in which no purifiers were used.

He and his colleagues found a reduction of more than 90 per cent of particles when the mobile purifiers were used.

“The operation of mobile air purifiers in classrooms seems feasible as a practical measure that can quickly be implemented during an epidemic,” the researchers summed up in the paper published in March.

Speaking to Tes, Professor Curtius added that the results underline that, given the circumstances schools will be in by September, air purifiers will be a key weapon against Covid transmission.

“Air purifiers work very well and should be a key priority especially as a) children are not vaccinated, b) the variants are more infectious, c) mask wearing might be stopped, and d) many people get together in closed rooms,” he says.

So buy a few air purifiers, stick them in the classroom and away you go? Not entirely. As Professor Curtius explains, the key variable that schools have to think about for purifiers is the clean air delivery rate (CADR).

“For a school room of [about] 200m3 volume, we recommend a total CADR of 1000 m3/h [metres cubed per hour] - about five times the room volume or about 40m3 per person per hour when a school room is occupied by 25 students.”

This CADR could be achieved with a single powerful unit or several small ones spaced around the classroom. This second approach was what the researchers did in their tests, with four units dotted around the room used in the experiment.

“We used four in our original test as we used household appliance devices that delivered about 250 m3/h each,” he says.

“An important advantage of several small devices is that they can be distributed over the room and the air purification is very evenly distributed without any poorly ventilated areas.”

Masking the issue 

This is persuasive stuff.

Adding further weight to these findings is a similar study by the Centre for Disease Control and Prevention in the US, which also found that using high efficiency particulate air (HEPA) cleaners can drastically reduce infection risks.

In a test environment, HEPAs were placed in a room that featured a “simulated infected participant” to monitor the airborne distribution of the particles. The research showed the devices, “significantly reduced the exposure of nearby participants and a speaker to airborne particles”.

What’s more, a notable additional finding came from a variation in the test, where those in the room with the HEPAs wore masks, leading to an even greater reduction in transmission: “The combination of HEPA air cleaners and universal masking was more effective than was either intervention alone.”

Furthermore, the tests actually showed that masks were more effective on their own than HEPAs.

“The use of masks without air cleaners reduced the aerosol exposure of the receivers by 72 per cent, and the use of air cleaners without masks reduced the exposure by up to 65 per cent. When used together, the HEPA air cleaners and masks reduced exposure to respiratory aerosols by up to 90 per cent.”

So, masks and filters both together are better, but, of the two, masks are the most effective.

This may add more frustration to those who have questioned why the government was so quick to remove the requirement to wear face coverings in school - and gives more credence to schools intending on keeping this requirement.

It also may make those unable to afford to buy and instal air purifiers more confident that a mask-first approach should keep students and staff safe.

Real-world deployments 

However, given the evidence on how air purifiers can reduce particle transmission on their own, many schools may still prefer this option as it would remove the need to manage mask wearing and all the issues that can cause.

What, then, is the reality of having a large object whirring away around the classroom all day long?

Matt Payne, head of lower school at Nord Anglia International School New York, is one teacher who knows. His school made the move to instal HEPA filters last year, around the time public schools were being provided with the devices as part of a city-wide move to improve ventilation.

“We could only see the upside to having them; they improve the air quality [and] peace of mind for adults and parents,” he says.

However, it’s clear that such units are not exactly discreet, with Payne saying they are “fairly big” at around “2ft high and wide and 1ft in depth”.

“When we ordered them, we didn’t know what the regulations would be so we erred on the side of caution to make sure we didn’t need to buy a whole new set again in case they weren’t powerful enough. We went a bit overboard and probably got something more powerful than we needed,” he adds.

Noisy purifiers 

Because of this, he says the units can be “a little noisy” and this can be hard if you are showing a video or someone is in the lesson on Zoom, although, overall, they do just become “background noise”.

Nonetheless, the issue of noise is one that Professor Curtius says schools need to think about - and shows why using several small units may be better than one large one.

“The noise levels are the key issue for practical implementation. It is very important to make sure, before the devices are bought, that the noise levels are low enough [but can still reach] the total CADR of 1000 m3/h.”

This can be done, it seems. None of the 26 students who took part in the German study reported any issues with the noise of the purifiers while only one teacher of six surveyed was “strongly disturbed” by the noise and two “somewhat disturbed”.

Of course, this was only one study done for around a week. Use them over a whole term and the white noise of a purifier throughout every lesson might start to grate and disrupt learning.

This is something that Professor Mon-Williams says is a crucial element of his research and why it will take a while for results to filter through.

“We can’t just say ‘Here is a system that works in one setting, let’s roll it out on a national basis’ when we don’t know if there may be unwanted consequences of doing this,” he says.

“If [air filters] turn out to be too noisy then you have invested huge sums of money with an unwanted consequence and [the technology] cannot have a meaningful impact.”

Worrying about noise levels of multiple purifiers is perhaps premature when, thus far, there has been no word that schools will be given any extra cash for this purpose - especially as a single basic air purifier unit costs from a minimum of £80 up to £500.

Where to splash to cash?

Let’s imagine, though, that funding did arrive. It would undoubtedly be welcomed by schools, which would then want to get buying to help keep staff and students safe. The first question would, of course, be: “Which devices do we buy?”

This is not a question with a simple answer, as Graeme Fox, head of technical for the Building Engineering Services Association, explains.

“There is no silver bullet, one-stop-shop solution,” he says, going on to explain that different types of air purifiers would be required depending on various factors, from the size of classrooms and ceiling heights to whether there is a road nearby with the additional pollution this brings.

“Ideally, you’d want a proper ventilation engineer who knows what they are talking about to tell you what method is best employed for which situations,” he adds.

Fox acknowledges that many schools will not have money to spend on hiring ventilation experts and so would have to make any purchase decisions themselves.

He says schools should be fairly confident that anything they buy from a reputable supplier will aid ventilation, although it cannot be guaranteed. This is where those CADR figures may come in handy.

Clearly none of this is simple - and comes with the tacit warning that you could end up with kit that doesn’t really help out, and this may not encourage those with budget control to approve any spending.

The hope may be that the Bradford research offers more insights on the best locations in an average classroom to place air purifiers.

But as the unions’ open letter makes clear, waiting for those insights is not ideal when the start of term is just days away.

CO2 monitors to the rescue

What, then, can schools do in the short-term that won’t break the bank but could help keep people safe? The answer could be carbon dioxide monitors.

“CO2 monitoring is the most important thing,” says Fox.

These devices don’t improve ventilation themselves but monitor CO2 levels in a room and warn you if levels are too high. They are much smaller than air purifiers and can cost from as little as £20.

Ian Rix, headteacher of Ashley Junior School, is one teacher to have already installed CO2 monitors and says they have proved a big success.

“It is a devilishly simple but effective way of ensuring classrooms are an even safer place to be,” he says. “It has provided a genuine sense of security and assurance to adults and children alike.”

He explains that they work on a green, amber and red light system that allows a teacher to know if a room needs ventilation.

“On several occasions, the amber light has shown and windows and doors in hallways, as well as the classroom, have been opened and the monitor quickly changes to green,” he says.

“On four occasions so far, the monitor has gone to red and the classroom has been evacuated to enable fresh air to circulate and make the room safe.”

He admits that the costs were high, at £90 per device, but says their portable nature means they can be used across the school: “[With] year groups in the dining hall for lunch or meeting for assemblies, we can simply borrow one from a classroom for the event,” he adds.

The Health and Safety Executive also says CO2 monitors can be particularly useful in areas of poor ventilation - noting that they should be set at head height and away from windows, doors or air supply openings, and at least 50cm away from any one person to avoid giving a “misleadingly high reading”.

Unexpected window errors

This certainly sounds a workable system for some schools - cheaper, smaller, portable pieces of kit that alert you to an issue, and then you to just open a window and get some air flowing.

But talking of windows brings up its own issues - and not just that opening the windows in the middle of winter is hardly conducive to a good learning environment.

On a more basic level, many schools do not always have windows that can be easily opened, as secondary history teacher Rose Lawson knows from experience.

“Many schools wouldn’t open windows as it could be a health and safety concern if children were not under control. I’ve been in settings where they would not open windows as students lean out of them to try to get better phone signal.” 

Fox notes, too, that, in many older schools, windows are often almost impossible to open.

“If schools are going to have to rely on windows opening then maintenance staff have to make sure they can open - particularly in older schools, where they have possibly been painted over and sealed shut, or hinges don’t work properly, and so on.”

Or, if they do open, they don’t open enough to create any meaningful airflow, as a teacher in London notes: “The windows open but only a small portion so hardly any air comes in.”

Open the door

So even this is not simple.

But there is one solution that even the most dilapidated school classrooms should be able to rely on - opening the door to allow air to circulate.

“If you open the door to a corridor or the outside then the likely temperature difference will create convection currents that draw the warm air of the room from above and cooler air in from below, helping bring down the CO2 levels,” says Fox.

It may even be sensible to leave the classroom while this is happening to ensure students are removed from the saturated air that the CO2 monitor has flagged.

Of course, this would come with another set of challenges - from having to interrupt a key teaching moment to the behaviour challenge of decamping an entire classroom in a corridor for a few minutes.

However, given the importance of ensuring that schools keep staff and students safe, this may be a small price to pay to ensure there is healthy air for everyone to breathe.

“We are, like most schools, I should think, several thousand pounds out of pocket because we have ensured the safety of children and staff,” notes Rix.

Long-term benefits 

What about those who may say that, with vaccination rates rising and children next on the rollout, large outlays on CO2 monitors and purifiers will only a short period of use before being redundant - potentially wasting millions that could be better spent in schools?

Professor Mon-Williams says a longer-term view is needed, as improving air quality in schools could have a long-lasting and transformational effect on education.

“Even before Covid, we had huge numbers of pupils absent from school with winter bugs and so on,” he says.

“We know that any absence from school can have a major long-term negative effect on life outcomes, so anything we can do to reduce that could be incredibly important.”

This is certainly a positive view. But, for now, it seems clear that until funding arrives to help schools improve ventilation and monitor air quality, the government’s rhetoric around doing everything it can to help keep school’s safe will seem like hot air to many.