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Aerosol Transmission of COVID-19 Virus: What Is, and Isn’t, up for Debate

Aerosol Transmission of COVID-19 Virus: What Is, and Isn’t, up for Debate

Representative image. Photo: ashishsharma/Flickr, CC BY 2.0.

As many countries have started to relax their respective lockdowns, some agencies, companies and facilities are beginning to turn a blind eye to a big concern: Can the novel coronavirus be transmitted through the air?

A major reason for this relaxed attitude is the absence of scientific consensus between two contending groups on the matter of aerosol transmission. One group says the virus is transmitted through mucous droplets exhaled by infected humans, and which fall down within a short distance (so the 2 m ‘safe distance’ rules). The other group claims the virus is viable even in much smaller droplets, known as aerosols, that can stay afloat for longer and travel larger distances.

These opposing viewpoints are often presented in the press in an oversimplified way, without the requisite nuance.

In this debate, as it stands, we know the following:

1. The virus can’t propel itself

2. The virus is inside mucous droplets exhaled by people, and the virus’s ‘range’ depends on how far these droplets can travel

3. How far these droplets can travel depends on conditions like air movement, humidity, ventilation and whether one is indoors or outdoors

4. The best way to avoid these droplets is to: i) ventilate your room as much as possible; ii) maintain a safe distance from people; and iii) wear a mask.

Based on what we know of other respiratory viruses, we expect the one that causes COVID-19 to have three main modes of transmission – close contact (droplets), fomites (touch surfaces) and airborne transmission.

Since the COVID-19 outbreak first began, the WHO has highlighted the first two modes but didn’t pay much attention to the third.

Now, why are scientists divided over something seemingly so simple? A part of the answer lies with labels and the history of medicine.

When we breathe, talk, cough, or sneeze, we exhale many small and large fluid particles. Most of them are 1-10 microns wide. (A human hair is between 50-80 microns wide.) Some droplets are larger as well – even as big as 1 mm. You might have noticed flecks of saliva escape people’s mouths when they’re talking. But these are rarer.

For historical reasons, epidemiologists and aerosol scientists have evolved different definitions of ‘droplet’ and ‘aerosol’, giving rise to confusion about what these terms refer to today among different experts. However, the epidemiologists’ definition is more outdated, and leads to problems in risk communications and in agreeing on the transmission modes of various diseases.

A particle of size 5 microns is too small to see with the naked eye. But if you’ve ever dusted your room with sunlight streaming in through a window, you might have noticed these particles floating around for a long time, and over a relatively large distance. According to a standard reference for aerosol engineers, a 50-micron particle can stay afloat in still air. And the volume of a 50-micron particle is 1,000-times larger than that of a 5-micron particle.

The WHO took a few weeks to begin recommending that everyone wear masks as a preventive measure. Now, however, both the WHO and the US Centres for Disease Control recommend the use of masks – but neither considers aerosol transmission to be likely. It took an unusually public outcry from over 200 experts for WHO to admit that aerosol transmission was even possible. One reason could be that the organisations didn’t wish to create panic. But from the authors’ point of view, it could be a healthy dose of panic to counteract the complacency among companies and governments as the latter proceeds to relax lockdowns.

One thing we must remember is this: the virus does not understand or follow our definitions. With reputed infectious disease experts like Dr Antony Fauci acknowledging the possibility of aerosolised transmission, we hope that progress towards a consensus will be quick. So while scientists work to determine the significance of aerosol transmission during the coronavirus pandemic, we must assume in the interest of safety that aerosol transmission is possible and viable.

This may sound scary. However, you can dispel this fear with a few simple, effective steps to protect ourselves from aerosolised transmission. We need to maintain a safe distance from other people because, as it travels through the air, the aerosol cloud exhaled by an infected person becomes gradually diluted. Even homemade cloth masks can help filter small droplets since masks don’t just work like a sieve. They help break up the ways in which the droplets are ejected into the air, disrupting normal flow. And it helps when everyone in the community wears masks. Note that mask fit is important.

An important but often ignored factor that influences transmission is ventilation. When you’re outdoors, a mask plus physical distancing can help considerably lower risks. The atmosphere is large enough to quickly dilute exhalations by individuals. Poorly ventilated and crowded indoor spaces – like call centres, cafés, and restaurants – can enhance the risk of exposure. To ventilate your room, simply open a window or few. Also, try not to use wall-mounted split air-conditioners; if you have to, do so with a window open. And if the ambient air quality isn’t very good, consider using cheap, DIY filters.

Panic is bad – but so is complacency and a false sense of security. Instead, an informed and balanced diet of fear, prepared with knowledge of what could go wrong, is good. Let us stay mindful of the different ways in which the virus can move around, and act accordingly. It’s as simple as using a good mask, maintaining distances and paying attention to ventilation.

Check out the figure below to get a sense of how safe you are. Start from the centre and move outward, towards the perimeter. Click to enlarge or view as a table.

Source: BMJ 2020;370:m3223/Authors provided

Asit Mishra is a researcher of healthy, green buildings with the University of California, Berkeley. Amit Mishra is a professor at the University of Cape Town, South Africa.

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