How can we stop Covid with ventilation?

Ventilation is hugely important to stop the spread of Covid, but what's the best way to ventilate our homes?...
30 July 2021

Interview with 

Abigail Hathway, University of Sheffield

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As the UK and other countries do begin to “open up” more again, and more public events and meetings are taking place, especially indoors, one thing we’re being urged to pay attention to in particular is ventilation: opening windows and doors, keeping fresh air flowing, and turning on air purifiers. And Sally Le Page has been finding out how this can help...

Sally - We all know that to protect each other from COVID it's hands, face, space. Wash your hands, cover your face with a mask, and keep two metres of space between people. But a while ago now, the government added a fourth part to the slogan -  hands, face, space, fresh air. We've known about the role of ventilation and fresh air in reducing the spread of this disease for a while. But for many of us, it's not obvious exactly how we should be using fresh air. And how important is ventilation really?

Abigail - Ventilation is massively important.

Sally - That's Abigail Hathway from Sheffield University, who co-wrote the guidelines on how we should be ventilating buildings during the pandemic

Abigail - If you're infected, you will be breathing out infectious aerosols. They're made up of saliva and then some virus as well if you're infected. And the saliva droplets will evaporate, so then you'll end up with something really tiny, and that means it can float around in the air for a long time

Sally - Are we talking minutes, hours, days?

Abigail - It could be hours. And that then can float around the air and someone else can breathe it in, and if you breathe it in, you could get infected. Now the one way to remove that from the space is by purging the space with lots of ventilation. The more fresh air you bring in, the more you can dilute anything that anyone was putting into the air. So your chance of getting infected is related to how much infectious material you inhale. So the more we can dilute the virus in indoor air, then the less chance other people have of being infected.

Sally - I live in a very normal flat. What should I do to improve the ventilation if I've, say, got someone coming over to watch telly with me?

Abigail - Make sure you open the windows about an hour before they came over. Because if you're infected, you want to make sure you're cleaning the air for your visitor. And equally, when they leave, I'd keep them open for, say, an hour afterward, just to make sure that if anything's lingered around you get plenty of air through. Depending on the kind of window design, if you have windows where you have low windows and high windows, it's much more effective to open a bit at the bottom and a bit at the top than just one of them with a big opening, you'll get a much more efficient air stream. If you've got windows on opposite walls then definitely open the windows on each side of the flat so that you get crossflow because you get much better movement of air by moving the air through the flat. And that might mean opening some internal doors to enable air to flow through the entire flat, rather than just being sort of stuck in one room with the door shut to that room.

Sally - What about fans? Because it's summer I've got a big fan that I have standing in the middle of my room just to keep me cool. Does that help if I've got all the doors and windows shut, but I've got a fan on, the air is circulating? Does that help with COVID?

Abigail - So if you've got everything shut and the fan on, you're just going to be moving infectious particles around. So if you're in a shared room with lots of people, you'll just be very rapidly moving that around everybody in the space. And if you were trying to distance in that space, there is absolutely no point of that because you've just turned on a very efficient way of moving your infectious aerosols to them. As soon as you start opening some windows alongside your fan, then yes, you've got a lot of airflow coming past you. That's going to move those virus particles, but equally, there's a lot of fresh air diluting them. So by the time it gets to someone else, it'll be well diluted so they won't be breathing in so much

Sally - Ever so occasionally I leave my flat and I go on public transport, and I go onto buses, which have those tiny little windows at the top that can tilt open a little bit. Or on to trains. Should I be going in and opening all the windows on trains and buses?

Abigail - Where you can open them? I'd open them. Yes. If they're shut.

Sally - And one thing I thought - when I'm sat next to, say, the window on a bus if it's moving very fast it's blowing a lot of air in at me. Does that mean that if I was infected, it would be blowing all of my COVID back into the bus and people behind me will be getting more infected than if the window is shut?

Abigail - So in that situation what you're breathing out will be blowing back. But crucially, there's also a lot of air mixing with those particles. So the person behind you might get a reasonable dose, but actually the further you get away from you, the more that's going to be mixed with fresh air. And of course, if you wear a mask at the same time, that would reduce how many aerosols you are breathing out. So that would again reduce the risk for people behind you.

Sally - How can I, when I'm going and shops and businesses, how can I work out if there's good ventilation or not?

Abigail - I think it's very difficult as a sort of member of the public. What I do when I go into any place is I have a look at, you know, is there anything in the ceiling that looks like it might be an air vent coming in, as well as looking at perhaps the windows - it's not just looking at open windows. And also, you know, think about how long you're going to be in a space. If you're just popping in to collect some groceries you're not there for very long. If you're going to spend three, four hours in an evening somewhere, that's a much longer period of time to be there with all the other people in the space, and that's a scenario I'd be more concerned about.

Sally - And if there's, say, a crowd of people outside in a park, is that overwhelming amount of fresh air when you're literally outside, is that enough to counteract so many people in a small space?

Abigail - Once you're outside, then you're not going to have a risk of aerosols building up like you would inside, but that's where the distance thing comes into play. And so you do need to be aware of distancing, but ultimately outside is just so much less risky than inside. One thing to be aware of is, are you outside or are you in a semi inside space?

Sally - So that's things like bus shelters, fancy gazebos...

Abigail - Fancy gazebos can be the big thing because you can often put a lot of walls down. So if you're trying to get away from the weather and get away from the breeze, then just be aware you're getting away from that fresh air as well.

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