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Five key questions about the possible airborne transmission of the coronavirus

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Kuldeep Singh
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Scientists are clear that COVID-19 is transmitted through ‘large’ droplets of saliva, but there is no consensus on whether aerosols, the tiny particles that we emit when we breathe and stay in the air for a while, also play an important role. A retraction from the U.S. Centers for Disease Control and Prevention has put this debate back on the table.

Late last week, the Centers for Disease Control and Prevention (CDC) indicated on their website that coronavirus could be transmitted, in addition to the droplets of saliva we launched at close range, through other much smaller particles called aerosols, that stay longer in the air and travel beyond six feet (1.83 meters), considered so far as a safety distance.

But on Monday, after CNN reported this, that update was removed from the web and the version that was available was repositioned, where it is noted that the main route of contagion is the respiratory drops produced when one infected person coughs, sneezes or speaks and their secretions with the virus reach another located nearby. Officials from the CDC officials admitted that a mistake had been made when publishing a draft that has not yet been reviewed by technicians, according to the Washington Post and CNN itself, to whom a federal official said there was no political pressure following that change.

In any case, the incident has once again highlighted the scientific community’s debate about the (or not) importance of the so-called airway, aerosols, in the spread of a disease that has already killed nearly one million people worldwide.

Here we raise five key questions to contextualize and understand this issue.

1. What coronavirus transmission mechanisms are contemplated?

In principle, there are three possible routes of contagion, although the border between the first two is diffuse: ‘large’ respiratory or saliva droplets, others much smaller (aerosols) and through contaminated surfaces.

Large droplets, also called ballistic droplets, are particles of saliva or respiratory fluid expelled by infected people by coughing, sneezing and, to a lesser extent, by speaking. They fly like a projectile and hit the mouth, nostrils or eyes. If they don’t hit anyone, they quickly fall to the ground at a meter or two (less than 6 feet).

It is the aerosol that is being debated. They are also particles of saliva or respiratory fluid, but smaller in size sometimes called droplet nuclei. For this reason, they can stay longer in the air – from tens of seconds to hours – and travel at longer distances, depending on their size. After being emitted when talking or singing, for example, they become infected when inhaled through the nose or mouth, or deposited in the eyes (less likely).

Time for particles of different sizes to settle on the ground in calm air, from a person's height. CDC Presentation
Time for particles of different sizes to settle on the ground in calm air, from a person’s height. CDC Presentation

Regarding the surfaces, also known as via fommite (any object contaminated with the pathogen), it can occur by touching, for example, a door handle, a switch or anything else where the virus has been deposited and then taking the hands to mouth, nostrils or eyes.

2. What is the difference in size between a respiratory droplet and the aerosol?

There is no consensus. Traditionally, the limit is 5 µm (microns or micrometers): if it is larger, it is salivary or respiratory droplets and if it is smaller, aerosol. This is how the World Health Organization (WHO) and the CDC consider it in their documents. However, experts from the CDC itself admit that 10-micron particles can remain in the environment for more than eight minutes, therefore they do not fall quickly.

Schematic representation of the routes of infection by ballistic droplets (blue) and aerosol (green, yellow and red) for a respiratory disease.
Schematic representation of the routes of infection by ballistic droplets (blue) and aerosol (green, yellow and red) for a respiratory disease. Both travel through the air from the infected person to the susceptible person, but ballistic droplets infect on impact and aerosols infect on inhalation. Two situations are shown without and with the use of a mask. / DKMilton / Journal of the Pediatric Infectious Diseases Society

For their part, a group of independent researchers from the University of Colorado (USA), have long considered that it is a “great mistake” to establish the border at 5 microns. According to these experts, the real limit between ballistic droplets and aerosols is 100 μm, taking into account the capacity of the particles to remain or not in the air for a long period and the accessibility to the respirable fraction of the lung.

3. Which route of transmission is the most important?

There is also no consensus. The CDC states that respiratory droplets are the main mechanism of spread, indicating contagion through surfaces as possible. Without counting what they published by mistake, they rule out that aerosols are a relevant route outside of hospital settings, where close contact with patients occurs, for example when applying intubation techniques.

Regarding the WHO, it agrees that salivary or respiratory droplets and fomites are the dominant pathways, and establishes the distance of one meter to avoid contagion. In July, after hundreds of experts lobbied through a letter to the New York Times, this organization also admitted that airborne transmission of the coronavirus by aerosols in poorly ventilated closed environments is possible, but more research is needed to confirm this.

The group of ‘dissident’ scientists disagrees. They think that based on their evidence, the aerosol pathway is at least as important as the other two, if not more. In fact, some consider that it may be the main one, with some contribution from the fomites and less from the ballistic droplets, clarifying that COVID-19 can be transmitted by aerosols without having to be at a rate as high as the measles, for example.

4. What are the consequences of confirming that aerosols are an important route of transmission?

If the particles with pathogens can remain in the air longer than is thought and go further, the importance of exhaustively ventilating the interior spaces should be emphasized much more, moving as many activities as possible to the outside, according to the scientists who defend the ‘aerosol’ route of contagion.

The New York Times article signed by experts in July also notes that masks may be needed indoors, even in socially distant settings. Healthcare workers may need advanced masks (like N95s) that filter even the smallest respiratory droplets while caring for coronavirus patients.

Ventilation systems in schools, nursing homes, residences and businesses may need to minimize air recirculation and add powerful new filters. Ultraviolet light may even be needed to kill viral particles that float in tiny droplets inside.

5. In any case, should we continue with the current protection measures?

Yes, social distancing, hand washing and the use of masks are still essential to prevent the spread of the disease. In addition, with regard to masks, we must insist on their proper placement (never below the chin or without covering the nose) and on the correct fit to the face to avoid ‘leaks’ of the air that we expel when exhaling. Experiments carried out at the Delf University of Technology (the Netherlands) have evaluated the effectiveness of different types of mask and found the risk that a person behind another who wears it poorly adjusted can run.

In addition, the most recent evidence indicates that it is important to limit the social circle of people with whom we maintain close contact, ensure that the encounters with these people are in the open air and frequently ventilate closed spaces, where we must be strict with the use of masks.

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