As the return to school drew nearer, detailed hygiene plans were being finalised, with handwashing, masks and deep cleaning schedules taking centre stage. 

But did your school pay enough attention to the question of ventilation? 

Dr Chris Iddon of the Chartered Institution of Building Services Engineers talks us through the key concerns around keeping the air as safe as possible. 

How dangerous are stuffy rooms in terms of the virus?

We can safely say that stuffy rooms are more dangerous than well-ventilated rooms in terms of the virus. But they have always been...not dangerous exactly, but unacceptable because of the other indoor pollutants in the space.

And that’s why there’s always been a need to ventilate spaces, which is covered in both the building regulations and workplace regulations.

What is a safe level of ventilation?

We don’t know what a safe level of ventilation is for pathogens, although we do know that poorly ventilated spaces have been linked to transmission events.

Current guidance is to maximise the outdoor air as much as reasonably possible and at the very least the rooms should be ventilated as per the Department for Education’s BB 101 guidance on design for ventilation in schools. 

This document uses exhaled CO2 as a measure of ventilation and says that 1,000 parts per million is indicative of good ventilation. And 1,500 parts per million is acceptable for naturally ventilated spaces, on the basis that it’s sometimes more difficult to manage the flow rate through windows - teachers haven’t got time to be adjusting the openings and the flow rates through the windows are dependent on wind and indoor/outdoor temperature differences, which are variable.

The document says that the CO2 concentration shouldn’t exceed 2,000 parts per million for more than 20 consecutive minutes.

But I’ve seen measurements of classrooms that are ventilated with windows, where the guidance CO2 level is regularly exceeded in the wintertime, purely because occupants are not familiar with the need to ventilate enclosed spaces and instead, they are keeping windows and vents closed. 

How can you tell if your room is adequately ventilated?

You can often sense that a space is poorly ventilated when you enter it. Think of a meeting room with lots of people in; as soon as you walk in, there’s a stuffiness or it’s a little smelly. But your nose will quickly adjust to that.

In a classroom, you might also notice lethargy in pupils. I’ve spoken to teachers who say that on a wet day, when they’ve got the students trapped inside for breaks all day, the class is a lot more lethargic by the afternoon and that’s probably because ventilation hasn’t been particularly great. 

You can also use sensors. Typically that would be a non-dispersive infrared CO2 sensor, which many newer schools have anyway, but they are quite easy to get hold of. 

Lots of classrooms will have high windows that are hard to reach - is that an issue?

There should be an accessible means of opening high-level windows, and this is covered in the building regulations. 

But high-level windows are good! When you open them, the air falls because it’s cooler and mixes with the air that’s already in the room. So by the time it reaches the occupied zone, where people are, it’s tempered a lot more than it would have been if you’d opened a low-level window. 

And should doors be open?

Doors can be open (providing this doesn’t impact the fire regulations), but only if it doesn’t enable air to move from one zone to another (the current recommendation is to try and avoid that where possible). If a class is in a bubble, you want to try and maintain the air in that bubble as well. 

If air flows from your classroom through your door into the door of the classroom opposite, and out the windows, that might mean that it moves the virus from one space to another (although it would be in a more diluted form).

And how useful are masks in a classroom?

We’re dealing with a respiratory disease, the virus is released from noses and mouths, so masks have to be effective to a degree.

There are studies showing that even cloth coverings are effective (upwards of 70 per cent) at stopping droplets containing virus getting into the atmosphere.

Even if they’re only 1 per cent effective, that’s going to reduce the number of secondary transmissions across the whole population and that’s the whole idea. 

Face coverings are just another thing that we can do - on top of social distancing, ventilation, good hygiene, test and trace - to reduce that potential transmission and hopefully reduce the magic R value to below 1. A school is never going to be a zero-risk scenario, but it never has been. 

Should air conditioning be used?

In schools, we usually see systems that recirculate air within a single room and cool or heat it, and you can have those on, providing there’s a supply of outdoor air. That’s the critical thing.  

The effect of moving air around the space is also beneficial because it helps dilute any source. So if someone is spreading the virus it’s going to be at a high concentration near them - which is where we get social distancing from - and further afield it’s going to slowly mix with the space.

But if you can speed up that mixing and dilution of any airborne aerosols that’s a good thing, because it reduces the number of aerosols that somebody else might breathe in.

How about heating? Windows would normally be closed to save money in the winter...  

Buildings are designed with ventilation in them, and heating systems will have been designed to assume that ventilation is happening.

So closing windows to save money on heating, and therefore stopping ventilation, is the wrong thing to do, and it has always been the wrong thing to do. It’s not providing a very good indoor environment, and that can have a negative effect on students’ cognitive abilities.  

Some schools have alternative sources of ventilation when windows are closed - newer schools often have mechanical or hybrid ventilation systems, for example - and opening windows supplements this, providing higher ventilation rates.

What are the key things teachers and leaders should bear in mind around ventilation?

There’s a sliding scale of risk. Closed windows are very risky, but with the windows open a little bit that risk drops quite dramatically. Try not to sit people right next to an open window, because you’ll get occupant interaction that is undesirable, along the lines of “I’m cold, I’m closing this window” and we don’t want that. 

Ultimately, the important thing is to make sure your ventilation system is working, whether that’s opening a window, or ensuring a mechanical or hybrid system is functioning correctly. If it is comfortable to increase the ventilation rate and bring in as much outdoor air as reasonably possible, then that should be encouraged.

Dr Chris Iddon is chair of the Natural Ventilation Group at the Chartered Institution of Building Services Engineers