Coronavirus advice: Ventilate, ventilate, ventilate!

Mr. Prévôt, why is it suspected that aerosols play a role in the spread of Covid-19?

André Prévôt: There have been infections more often in recent months even though people have been maintaining a safety distance of 1.5 or 2 metres. So these people weren't infected through coughing or anything like that. They just spent more time in the same room as an infected person. There is a high probability that these infections took place via aerosols and not via droplet infection – that the pathogen got into the lungs through the air we breathe.

What exactly are aerosols, and why are they able to transmit diseases?

An aerosol is a liquid or solid particle that is suspended in the air. It also can be a virus. Specifically, the following happens: Millions of viruses can be emitted when a person is coughing, thousands of viruses when speaking, in droplets of different sizes. These liquid drops consist mainly of water and are typically around one to more than a hundred micrometres in size, that is, just a hundredth to a tenth of a millimetre. The water evaporates relatively quickly – this can happen in seconds to minutes – and the droplet shrinks. At some point, what remains is a mixture of the virus and some other semi-liquid organic components that do not evaporate so quickly, such as parts of saliva.

These particles remain in the air and disperse throughout the room. That, then, is similar to particulate matter – a term that may be more familiar.

How long do aerosols usually stay in the air?

It depends on the size. If the particle is large, it sinks relatively quickly due to gravity; when it is small – typically five micrometres and smaller – it moves with the air. Indoors, it takes a long time for such particles to land on the floor. That can take from hours to days.

What is the difference between aerosol and droplet infection?

In droplet infection, you're talking about large drops that can't be breathed into the lungs: 10, 50, or 100 micrometres in size. Droplets of spit, which are produced when someone coughs or sprays saliva while speaking, are at the upper end of these drop sizes. If drops are that big, they land on the ground relatively quickly, so a distance of 1.5 or 2 metres is sufficient. Infection occurs only through the direct release and intake of such drops.

In contrast, respirable particles – aerosols – start at five micrometres and smaller and sometimes remain in the air for a long time. For such particles, social distancing is insufficient; they are transported farther and disperse throughout the room, and other people can breathe them in and become infected.

How likely is it that someone could get infected through such floating virus-containing particles?

That depends on how many virus particles you have to take in, and how. For Covid-19, this is not yet known. The threshold is a bit different for every virus – and probably also for every person.

Does this mean that I would not automatically become infected if I am in a room with an infected person for a long time?

No, it depends on the concentration. The probability increases when there are more aerosols, that is, when a person emits more. Whether or not there is a superspreading event also depends on how well the room is ventilated and how exposed people are. It is certainly not the case that everyone is always infected.

What was the intention behind the publication?

The two lead authors, Lidia Morawska and Donald Milton from the Queensland University of Technology in Australia and the University of Maryland School of Public Health in the USA, realised early on that aerosol infection might play a role in Covid-19. Reason enough to say: You should also take this route into account. Because if infection via aerosols is possible, completely different measures are required than with a pure droplet infection. There are intensive discussions at the World Health Organization about formulating further recommendations on how to minimise infections via aerosols.

What measures are these?

For one thing, avoiding a lot of people in a room minimises the risk. Or you could just ventilate very well or even filter the air indoors to keep the potential concentration low.

So, ventilate whenever possible?

Yes exactly. I have the impression that many people are aware they have to keep their distance and wash their hands. But many do not see that ventilation is important because it has not been clearly communicated. Now is a good time to think about it, because more and more people are returning to their workplaces, and more people are together in an office again.

Ventilation can be a very simple measure; it's a shame if you don't use it when it's so simple. Especially as long as the temperature outside isn't below minus 20.

But then spending the night with other people in a room in an alpine hut – as many hikers in Switzerland do – is probably not a good idea, is it?

It always depends on how many people are in the room for how long. But yes, it is rather cold at 2,000 or 3,000 metres, so the risk is high that you won't want to leave the windows open all night. So a mountain hut, then, would be a place where the risk is elevated.

If I can't open the window to air out the room, can fans help?

Simply running a fan certainly does not give you an advantage. If you mix the air in the room well with a fan, it actually increases the chance that there will be infections. I would not recommend that. But you also can, for example, open the office door to the corridor, so that the air volume becomes higher.

But when it gets colder in the next few months, that's bad news, isn't it?

Yes, that's probably one of the reasons why the infection rates for viral infections are generally higher in winter.

Should we then give more thought to good ventilation in general, even after the coronavirus?

Yes, the lead authors also think ventilation systems should be checked to see how good they actually are. This also has an effect on other viral diseases – the normal flu and cold viruses could be minimised. It is certainly worth investigating. For example, would it help to run the ventilation faster? Or, how could you optimise the ventilation system in general so that the risk of infection would be reduced?

If Covid-19 can be transmitted via aerosols, do fabric masks help at all?

Yes, at the point where the virus droplets are produced – that is, when the mask is worn by the person exhaling the droplets. At five micrometres, they are still reasonably large, so a lot remains in the filter, that is, in the mask. Not all, but a considerable portion. Masks help less as protection when inhaling, because the particles are already much smaller. It is not nothing. But then, it is no longer complete protection. This requires a special type of mask that also traps respirable particles, that is, particles with a size of less than ten micrometres.

Are you yourself conducting research at PSI on the transmission of viruses via aerosols?

As an aerosol researcher, I was asked to sign the letter in Clinical Infectious Diseases because the lead authors wanted to have renowned scientists check to make sure everything in the letter was plausible. But I am not a virologist myself, and I am more interested in the general chemical composition of aerosols and their effects on lung diseases and the climate.

Could you envision a research cooperation on this in the future?

I could imagine the following: There have been reports that dogs can smell the disease, Covid-19. If that's true, then you can probably measure it too. For example, volatile compounds that are specific to the disease may be emitted. It would be exciting if that could be measured in the air we breathe. So the question is whether there are markers in the air we breathe that are indicative of the disease. We may want to pursue something in that direction.

Interview: Paul Scherrer Institute/Brigitte Osterath