Is the Covid-19 virus airborne?

Health guidance was clear: the virus can't spread through the air. But some scientists are urging a rethink: is the Covid-19 virus airborne?

Since the onset of the crisis, it has been oft intoned that the Covid-19 virus is not transmittable through the air, that it’s therefore not airborne and that, instead, the primary mode of infection is through the inhalation of larger, liquid droplets, temporarily suspended in the air through a cough or a sneeze. But scientific opinion is now mixed as to whether this is really the only mode of infection we should worry about. There are those who assert that we are underestimating the virus’ potential to spread itself through the air.

The WHO’s recommendations

The World Health Organisation’s (WHO) guidelines for the prevention of the spreading of the virus have focused on the transmission via the inhalation of droplets. These water or mucus droplets, emitted through coughing or sneezing by an infected party, are large enough for the virus that causes the novel Coronavirus, SARS-CoV-2, to attach itself to. If inhaled they can make it to the inner respiratory system, where they can multiply and eventually bring about a new case of the virus.

Luckily, the largeness and density of these liquid droplets means that they can’t stay airborne for any long amount of time. This is what underwrites our current social distancing practice: a water droplet coughed out won’t travel more than 2 metres on average before it falls to the ground. Keeping one’s distance from one another should therefore eliminate this route of transmission, and, as this is thought to be the predominant mode of transmission, should drastically reduce the spread of the virus.

And, fortunatelly, the adoption of such measures, as well as face masks which prevent such large molecules from being released into the open air, have been generally successful in hemming in and stemming the virus’ otherwise rapid spread.

And yet, despite the theory’s apparent success, it has been challenged in recent months. Some scientists have started to question whether droplet-transmission is the only mode of transmission we should be worrying about. Some argue that we should also be worrying about the air we breathe, as the Covid-19 virus could well be airborne.

An open question

Since the crisis began, the extent to which the Coronavirus spreads through the air has been a keenly debated, and never quite settled question. This is due to many factors, but not least among them is the dependence of the definition between respiratory droplets and airborne droplets on probabilities, rather than on absolutes.

“Is the Covid-19 virus airborne?” is not a question one can simply answer an unqualified “yes” or “no” to. It must always be answered within the context of the risk: the higher the probability of the Coronavirus spreading itself through the air, the more inclined we should be to answer yes. If, however, the Coronavirus could spread itself through the air, but real-life situations are such that it actually seldom does, then a “no” will be the correct answer. To make this all a bit more clear, we will have to go into the science of minute airborne particles, or aerosol physics.

We said earlier that respiratory droplets are considered to be ‘large’. In terms of the air we exhale when breathing out and speaking, this is literally their defining quality: a respiratory droplet is separated off from an ‘aerosol’, those smaller particles of liquid which we emit, purely based on its size. A liquid molecule of 5μm and less is an aerosol; above 5μm, a respiratory droplet.

They are of the same ‘stuff’, but differ in size. The distinction is grounded in the generally different behaviour of the two sizes of particles when released into the air.

Say both a 20μm respiratory droplet and a 4μm aerosol droplet are coughed into the air by a Covid-19 patient. Both carry the Covid-19 virus. Generally speaking, the 20μm droplet will have settled to the floor, before having travelled 2 metres, while the 4μm droplet remains floating in the air and can travel much further afield. Once it evaporates the Covid-19 matter will be left behind, suspended in the air as a “droplet nuclei”. Just as minute fine dust particles can stay airborne for minutes, hours and even days, so could, in theory, the aerosol droplets.

On the face of it, then, it seems that aerosol transmission should be just as viable a route of infection as droplet transmission. Here is where different cases around the world as well as statistics as to the virus’ contagiousness become put forward as evidence for one view or the other.

Scientists who are for the view that the Coronavirus can spread itself through the air (aerosol transmissions), reference certain instances of confirmed infections where aerosol transmission seems the most likely candidate means, such as a choir rehearsal in the U.S.A, , or the spread of the virus between three families in a Chinese restaurant who never came into close contact with one another.

But scientists who oppose this view argue that the virus’ spread would be far more rapid, and its attack rate (the percentage of people who contract the virus having come into close contact with an infected person), much higher, if aerosol transmission was indeed a viable route of infection. They explain away the pro-aerosol side’s cited cases either through reference to the potential for other means of transmission being responsible, or from allowing this as a rare exception to the general rule of predominantly droplet transmission.

But they are left with an explanatory burden. If the obsevable effects of the Coronavirus don’t point to its transmissability through the air, why not? If our description of the behaviour of microscopic particles was correct, the aerosols should mean the virus is transmissible through the air.

In answer to this they reference another vital element of the puzzle: the viral load of the tiny aerosol particles. It may well be, they argue, that they are either too small to carry enough Covid-19 material, or that the Covid-19 material they do carry is no longer functionally infectious.

If the latter turns out to be correct, then aerosol transmission can be safely forgotten about, and the prevention of droplet transmission should continue to be our prime focus. But the former option leaves room for the viability of the aerosol transmission route, in certain conditions.

Airborne dust and aerosol transmission

Accepting the anti-aerosol transmission side’s former contention, that the aerosols do not contain enough Covid-19 material to bring about an infection, even when inhaled, there remains certain conditions where the aerosol transmission becomes viable.

These conditions are those where air does not circulate freely, people congregate and crowd each other, and the air is of such a quality as to promote long-term suspension of aerosols. Here, given a few infected persons present, the Covid-19 viral load could reach infectious airborne proportions simply through the proliferation of more and more virus-laden aerosols in the air. Eventually, a gulp of a heap of tiny Covid-19 matter will represent the first step towards a new infection.

Given that the infection does have such a low attack rate, and doesn’t seem to spread with as consumate ease as a disease like measles, for instance, we can conclude that the above mentioned scenario is indeed one that doesn’t occur too regularly. But, nevertheless, we should endeavour to avoid the creation of the conditions which would allow this situation to occur. This is as the WHO now recommends.

So, while the jury is still out as to whether the virus actually can spread through the air, it is best to take precautionary measures to prevent the arisal of such conditions.

One of these conditions will be the local particulate matter concentrations. The greater the amount of particulate matter in the air, the greater the potential for the Covid-19 aerosols to remain suspended; it has been hypothesised, as we reported here, that the particulate matter could act as a dusty float for any biological material in the air. The aerosol, with a lower density than the dust, would readily remain afloat in it, increasing chances of inhalation and dangerous levels of proliferation.

This makes it key to address the issues of high particulate matter pollution. The more microscopic particles in our air, the more likely Covid-19 aerosols will remain suspended among them.

References

Bourouiba L., (2020) Turbulent Gas Clouds and Respiratory Pathogen Emissions: Potential Implications for Reducing Transmission of COVID-19. JAMA. 323, (18). 1837–1838. doi:10.1001/jama.2020.4756

Hamner L, Dubbel P, Capron I, et al., (2020). High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice — Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020 69, 606–610. [Viewed online]. Available from: http://dx.doi.org/10.15585/mmwr.mm6919e6external icon.

Klompas M, Baker MA, Rhee C., et al., (2020). Airborne Transmission of SARS-CoV-2: Theoretical Considerations and Available Evidence. JAMA. 324 (5). 441–442. [Viewed online]. Available from: doi:10.1001/jama.2020.12458

Lednicky J., et al., (2020). Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. medRxiv. [Viewed online]. Available from: doi: https://doi.org/10.1101/2020.08.03.20167395

Morawska L., Milton K.D, (2020). It is Time to Address Airborne Transmission of COVID-19, Clinical Infectious Diseases. [Viewed online]. Available from: https://doi.org/10.1093/cid/ciaa939

Yuguo L, Hua Q, et al., (2020). Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly ventilated restaurant. medRxiv. [Viewed online]. Available from: https://doi.org/10.1101/2020.04.16.20067728

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