The government guidance for full school opening stresses that it is important to “ensure good ventilation” in classrooms and to maximise the amount of natural air flow by “for example, opening windows”.
That’s all very well in warmer weather, but do classroom windows still need to stay open once temperatures start to plummet?
How does ventilation help?
“The main issue [with closing windows] is the opportunity for the virus to concentrate in the air,” says Jack Gilbert, professor of paediatrics at the University of California, San Diego, and a specialist in microbes and the immune system.
“Every human is emitting 38 million bacterial cells, 7 million fungal agents and likely equivalent viral particles per hour. So, if you have [even just] 15 children and a teacher in an enclosed classroom, the math looks bad! Hence the importance of diluting the concentration effect by constantly exchanging air.”
Theoretically, if everyone in the room is free of coronavirus, the concentration of your emitted particles wouldn’t necessarily be an issue. But with the possibility of symptomless carriers, there is really no way to know just how many people could be spreading it around the room.
“If there is an infector in an indoor environment they will be releasing virus encapsulated in small respiratory droplets (aerosols) through respiratory activities such as breathing and talking, which can remain airborne for several hours,” explains Chris Iddon of the Chartered Institution of Building Services Engineers. “If the room is unventilated, the concentration of virus-laden aerosols will continue to increase within the room and could reach a level at which other susceptible people sharing the same room could breathe in an infective dose.
“Bringing in outside air helps dilute any aerosols in a room that may contain the virus.”
Masks are likely to help here, as they contribute to reducing the amount of virus getting into the air in the first place, but keeping the windows open is still important, even when masks are worn.
So, does that mean that teachers are going to be left with no choice but to keep the windows open, no matter how cold it gets?
Well, it isn’t quite that simple, because there will also be health impacts to think about if the temperature in the room gets too low.
“Cold air can make the mucosal membrane in the respiratory tract more likely to be susceptible to infection by rhinoviruses (common cold), flu and coronaviruses,” says Gilbert. “We do not yet know if this is true for SARS-CoV-2 specifically. However, it makes sense to ensure that the air in the school room is not too cold, due to the potential combination of cold air and enhanced concentrations of the virus.”
In terms of what “too cold” would feel like, Gilbert suggests that anything below 8C would probably be cause for concern (although he stresses that this is a best guess, not based on specific evidence).
“When the temperature gets down to less than 8C in the classroom, I would get worried that the cost would outweigh the benefit,” he says.
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Where is the balance, then? According to Iddon, any ventilation is better than none, so keeping the windows open a crack will help to reduce the concentration of virus in the air, while hopefully preventing the temperature in the room from falling too far beneath comfortable levels.
“Have the windows open ajar,” he suggests. “There should be at least sufficient outside air provided to meet current standards and guidance. For a naturally ventilated classroom, this is equivalent to about five litres [of air] per second per person, which can be achieved in cold weather without having to open all the windows fully.”
This is because the temperature difference between the air outside and the air inside, combined with wind, is what drives outside airflow through natural ventilation openings such as windows.
“So in cooler weather, the windows don't have to be open as wide as they would be in warmer weather to provide the same flow of outdoor air. [However], when it is comfortable to do so, the ventilation should be increased beyond this," Iddon says.
While having windows open even slightly could still draw complaints from the students sitting nearest to them, this is probably the best compromise that teachers can hope to achieve.
Beyond that, the risk of aerosol transmission can be reduced further by limiting activities that have been shown to increase aerosol generation, such as aerobic exercise, singing and talking loudly, Iddon adds.
“High-aerosol-generating activities should be discouraged in smaller classrooms,” he says.
What can schools do?
Inevitably, though, all these solutions place the burden of keeping classrooms safe on to teachers themselves, when many would argue that we should be looking for whole-school solutions instead. What advice do the experts have here?
According to Gilbert, the ideal scenario in the long term would be to retrofit all schools with more effective air cleaning systems, of the kind you might find on an aeroplane.
“It is possible to use an air exchange system to 'clean' the air, but that does mean retrofitting school air conditioning systems with HEPA filtration and potential UV decontamination units to ensure that air is cleaned before being returned to a room,” he says.
With school budgets already stretched, it is unlikely that we will see cabin-style filtration systems widely installed in classrooms any time soon.
And Iddon points out that while air-cleaning systems are effective at removing pathogen contaminants, they don't supply outdoor air, which is needed to dilute other pollutants and is still a requirement to meet building regulations.
"What should be considered are alternative ways to bring in outdoor air that is tempered (warmed)," he says. "There are already many ways to do this and a number of new schools do have such solutions already. This may be mechanical ventilation with heat recovery or a mixing box system. Both of these will warm the outdoor air before it enters the room, removing discomfort draughts yet still supplying adequate outdoor air to dilute pollutants."
Iddon also suggests that a more cost-effective short-term solution would be for schools to buy a non-dispersive infrared (NDIR) carbon dioxide sensor, which can be placed in a classroom for a few hours to monitor the concentration of carbon dioxide.
“In a well-occupied space, like a classroom, a CO2 sensor can be helpful in indicating what the ventilation provision is like,” he explains. “Values in excess of 1500ppm (which represents about 5l/s/p at steady state) are indicative of poor ventilation and further action is required to increase the flow of outdoor air into the space.”
Moving the sensor around the school to check each classroom should give an indication of any areas where ventilation may need to be increased, Iddon adds.
However, he also points out that a low level of carbon dioxide “cannot necessarily be used as an indicator that ventilation is sufficient to mitigate transmission risks” in a large volume space (such as a school hall) or a space where there is low occupancy.
Ultimately, then, unless you know that your school already has a mechanical ventilation system bringing in adequate amounts of tempered outside air, the advice remains to keep the windows open as much as you can, even if that’s only a crack. Other than that, the best thing teachers can do is continue to follow the guidance they are already following.
“Wear masks, keep classes at low density, and ensure that students and teachers wash their hands or use sanitiser regularly during the day,” says Gilbert.