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Masks have been a common sight all over the world since SARS-CoV-2, the novel coronavirus, invaded our lives. We set out to investigate if they work. Our goal was to find out how the virus travels through the air in buildings so we could understand more about the risk of airborne infection – including whether masks can help to control the number of respiratory droplets in the air and therefore reduce transmission.

This is what we know so far.

As we talk, cough and breathe, a jet of air rushes out of our lungs through our mouth and nose - in the process, it gathers respiratory fluid from the lungs, throat, and mouth creating droplets which are thenÌýemitted into the air. High energy vocal activities, such as singing and coughing,Ìýincrease the amount of dropletsÌýand provide a greater force to propel theseÌýfurther into the spaceÌýaround us.

Most of the droplets produced are tiny at less than five microns (a micron is a thousandth of a millimetre) –Ìýwe call these aerosols. Anything larger than this is called a droplet and these can be as large asÌý100 microns.

Each breath, word or cough will produce many thousands or millions of aerosols and droplets over aÌýspectrum of sizes. Whatever their size, they are propelled forward in aÌýcloud of warm humid airÌýfrom our mouth towards other people in a shared space. The larger droplets will tend to fall to the ground quickly due to gravity but smaller ones can remain suspended in the air for many hours.

Over the past 18 months, SARS-CoV-2 has been detected in air samples in many different situations, most often in places like hospitals. Generally, PCR tests were used to assess whether SARS-CoV-2 RNA was present. The viral RNA molecules wereÌýfound in exhaled aerosols, in numbers varying from theÌý10sÌýto theÌý100,000sÌýper cubic metre of room air.

Infected people are the source

We now know that asymptomatic infected peopleÌýdo not necessarilyÌýhave a lower viral load than those displaying symptoms. In both cases the amount of virus can be as little as aÌýfew thousandÌýor as many asÌýhundreds of billionsÌýof viral genomes in a millilitre of saliva or nose sputum, a proportion of which will be live virus.

Therefore, as a micron is a thousandth of a millimetre, we can work out that at the time when they leave the body there will probably be few virus particles in most aerosols of five microns – but there might be from tens to tens of thousands of virus particles in a 100-micron droplet. And each breath, word or cough will produce many thousands or millions of aerosols and droplets over aÌýspectrum of sizes.

Once exposed to the drier air outside our body,Ìýfluid evaporates from the virus-laden larger dropletsÌýwhich then become aerosols – the number of virus particles remains the same, but they are concentrated into a much smaller and lighter aerosol. That means that they can stay suspended in the air for hours and pose an infection risk.

The risk of indoor settings

We know that in a given room the amount of virus someone inhales isÌýdirectly proportionalÌýto the amount of virus emitted into the air by an infected person. In simple terms, the more virus is breathed out into a room, the more other people in the room will breathe it in.

The exact amount of virus a susceptible person inhales depends on several factors includingÌýproximityÌýto the infected person (or people) and time spent in the enclosed setting. The virus is more concentrated closer to the infected person, whereas at distances greater than two metres, the virus in the exhaled air will dissipate and become diluted within the room volume. But – and this is where the real danger lies – in poorly ventilated spaces the virus quantity can build up. So, if you spend longer in a room with virus laden air, you willÌýinhale more virus.

Masks catch viral particles

It isÌýchallenging to exactly measureÌýthe net benefit of masks on a population because of the vastÌývariations in viral loadÌýbetween people, whether they are singing, shouting or talking, the size of the indoor space and time spent in it. Masks make a relatively small difference when people release just a few viruses every hour because they were never releasing enough virus to infect another person. Likewise at the other end of the spectrum, reducing the emission from super-emitters still often results in high overall emission of virus. Wearing a mask will reduce the amount of virus emitted, but how much it helps is dependent on how much virus is being emitted in the first instance.

Therefore, there isÌýno conclusive evidenceÌýon their efficacy because it’s so hard to adjust for all the other variables which affect the level of transmission.

But even without certainty over the exact number of cases prevented by wearing a mask, we do know that they willÌýdefinitely catch some of the virusÌýladen aerosols and droplets – and thatÌýwill haveÌýanÌýimpact on reducing the number of infections.

A mask is one of many weapons in our arsenal which also includes vaccination, social distancing, ventilation and hygiene. As theÌýomicron variantÌýrapidly spreads, the case numbers are very worrying. It is more important than ever that we use all the means we have available to reduce the spread of SARS-CoV-2.

This article originally appeared in on 17th December 2021.

Links

• Original article in
• Ïã¸ÛÁùºÏ²Ê Department of Civil, Environmental & Geomatic Engineering
• Ïã¸ÛÁùºÏ²Ê Faculty of Engineering