Covid: how can schools improve air quality to reduce transmission?

Pupils will return to school in England, Wales and Northern Ireland this week amid experts warning the return could fuel a surge in Covid cases. We take a look at measures that could improve air quality in schools and reduce transmission.

Carbon dioxide (CO2) monitors

Crowded rooms, such as classrooms, with poor ventilation create a higher concentration of the CO2 we breathe out. CO2 monitors do not change air quality but are a cheap, effective canary in the coalmine.

Experts have noted their limitations, particularly in spaces of low occupancy or large volume.

However, Sage member Catherine Noakes, professor of environmental engineering for buildings at the University of Leeds, speaking in a personal capacity, said classrooms were a good setting for CO2 monitors.

“Their spaces are about the right size, that they’ve got a regular occupancy,” she said.

Opening windows

“Many school classrooms will just have opening windows [for ventilation], although certainly over the last 10-15 years, there have been more schools with additional equipment as well,” said Dr Shaun Fitzgerald, an expert in ventilation from the University of Cambridge.

Noakes added that making sure windows could open was “probably the most viable short-term option” for schools.

Opening windows all the way can quickly make a room cold in winter, but the amount they need to be open to improve ventilation depends upon the windspeed and the difference in temperature outdoors.

“In colder weather opening the window a small amount can result in ventilation that is almost as effective as opening the window fully in the summer,” the Environmental Modelling Group (EMG) of Sage has written.

Noakes has tweeted a range of tricks for improving ventilation by opening windows.

“For winter ventilation it’s a good idea to open windows that are high level,” she said. “Because then the cold air comes in above everybody’s heads and has a chance to mix with the warmer air, rather than give you a really cold draft in the space where you’re sitting.”

If high-level windows are absent, then heat exchangers or fans to mix the incoming cold air with the warm room air may be needed, Fitzgerald added. Noakes said many CO2 monitors also measure temperature, which can tell teachers they can “afford to shut the windows” when “your CO2 is really low and your temperature is also really low”.

Ozone disinfecting machines

While some studies have suggested ozone is effective at inactivating the coronavirus, it is an environmental pollutant and can cause irritation and damage to airways. What’s more, people need to be trained for the machines to be used safely.

“The challenge with ozone disinfection is you cannot use it in an occupied space,” said Noakes, adding the process can take about two to five hours.

“To be honest, you’d probably only do that if you’ve got an outbreak, you wouldn’t do it routinely every day,” she said, noting that its benefits might prove limited since, even in poorly ventilated rooms, the virus will have disappeared from the air overnight.

“I’m a bit skeptical that they can replace standard cleaning,” Noakes added, echoing concerns from the Health and Safety Executive.

Air conditioning

Mechanical ventilation units are good, said Noakes, as they bring in fresh air. However, those that recirculate air may make things worse.

“People think that they’re comfortable so they think the room’s well ventilated, and it’s not,” she said.

Air cleaners

There are many air cleaning products on the market, although Noakes said evidence on their effectiveness can be patchy.

One option is an ultraviolet cleaning device, although the best can be complicated to design and install.

Another option is an air filter device – basically a box containing a fan and very fine HEPA filters. Air is drawn through the device, and the coronavirus is removed.

However they can be noisy, it is important to pick the right size and they cost about £200- £500 a unit.

“You probably need at least two per classroom,” said Noakes, adding it is better to first identify where ventilation might need improvement but windows could not be opened.