COVID-19 in aerosols

Investigating the airborne spread of COVID-19
14 July 2020

Interview with 

Shelly Miller, University of Colorado


A woman sneezing into a tissue.


You might be forgiven for being slightly baffled this week with the news about airborne transmission of coronavirus and the efforts by over 200 scientists to get the World Health Organisation to change its advice. If you remember a few weeks ago we reminded you of the old phrase:  Coughs and sneezes spread diseases, the catchy phrase that was part of a British public health campaign dating back to the 1940s. And it's still very relevant today, especially with COVID wafting around. And with recent analysis from the Office for National Statistics (ONS) suggesting that more than half of cases of coronavirus infection may be asymptomatic, those 200 scientists say we need to raise our game. Because someone with no symptoms can still pass on coronavirus in small respiratory droplets that they breathe out. This is known as aerosol or airborne spread and that’s why the scientists are urging the WHO to put more emphasis on this route of transmission. Engineering professor Shelly Miller from the University of Colorado, Boulder is one of them, and she spoke to Chris Smith...

Shelly - We wrote an open letter to the World Health Organisation to get them to accept that this disease can be transmitted by inhalation of small airborne particles containing virus. And we wanted them to really consider and acknowledge this transmission route, because we have been struggling with incomplete messaging across the world.

Chris - Now when you say 'we' - how many people were involved in this?

Shelly - Initially there were 36 scientists who came together in March, and we wrote a letter to the WHO; and they read the letter and we had conference calls with them, and they said they would review the science, which they did, and they did not change their position. So then we wrote a paper which was published last month. And then finally we took our letter and got over 200 additional scientists to sign it and publish it openly to the world so that they would know what conversations we were trying to have with WHO.

Chris - What did the WHO say when they got your letter?

Shelly - Well they wanted to talk with us again, and we have a call scheduled with them again. They just put out a statement yesterday, that I have interpreted as meaning they are re-looking at the data and reconsidering our request. And then they want to also have a conversation to further the discussion. We were all thrilled and I was just moved to tears.

Chris - And so prior to your analysis, what was the view, then, about what the main route of transmission was and what the risks were?

Shelly - Well from the WHO, their main position was that you could only get it from an aerosol if you were in an aerosolisation procedure room in the hospital; otherwise, if somebody coughed or sneezed near you, you could be sprayed with the droplets; or the droplets would be transferred by touching different surfaces, and then you would touch the surface and get infected. From my read with the public health service in the UK, and also with CDC, they have more readily embraced the idea that you can also inhale small respiratory particles that are floating in the air for long periods of time. You don't have any idea where they came from - just somebody talking, laughing, or singing, expelled them - and now you inhale them and can get also sick.

Chris - Is it all part of a spectrum then? When I talk, I breathe, shout, sing, yell, all these sorts of things, I produce a range of particle sizes of droplets coming from my airways. And previously people were obsessing about the really big ones. But you're saying, because there's a spectrum of droplet sizes, there, there are some small ones, and the small ones play an equally important part in the transmission?

Shelly - Yes. That's a wonderful way of putting it. That's exactly right.

Chris - And why could this possibly have been neglected before then? I'm baffled!

Shelly - Yes, me too! We know many diseases are transmitted only by the aerosol route. That includes measles and chickenpox. Flu is transmitted this way as well. After SARS 1, when we had that outbreak in Hong Kong, we found lots of evidence that that was also spread via the airborne route. And so now we come to CoV-2, and we just are baffled by why that is not also a possibility.

Chris - At the moment we are very busy making workplaces so-called 'COVID-safe'. And this means keeping people a certain distance apart, lots of sticky tape - the people who make stripey black and yellow sticky tape are making a fortune at the moment as we decorate the floor with it - so we've got specific routes for people to take round buildings and so on. But if people are spraying out sprays of these fine particles that can linger in the air for absolutely ages, does that make a lot of these efforts amount to really nothing? Because if you've got a nice air con unit buzzing away in the corner of the room, circulating all the air, is that not just mixing all this up and we're going to get exposed anyway, regardless of how far we stand from each other?

Shelly - The actions that we have been taking are key, but we need to add a few more on top of that to address this additional risk. The farther away you get from a person that's the source of the infection, the more you reduce your possibility of exposure; but also, how do you make sure the air in your building is clean? If all you have is a recirculating air conditioning unit, then what you should also consider is adding an air purifier. That would be a great way to clean the air in the room from suspended particles that may linger.



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