October 26, 2020

Five key questions about the possible airborne transmission of coronavirus

At the end of last week the Centers for Disease Control and Prevention (CDC) indicated on its website that the coronavirus could be transmitted, in addition to droplets of saliva that we launch at a short distance, through other particles much smaller called aerosols, which stay in the air longer and travel beyond six feet (1.83 meters), until now considered a safe distance.

But this Monday, after reporting on it the CNN, that update was removed from the web and the version that there was, where it is pointed out that the main route of infection are respiratory drops produced when an infected person coughs, sneezes or speaks and their secretions with the virus reach another located nearby. Officials from the CDC 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 own CNN, to whom a federal official said there was no political pressure behind that change.

In any case, the incident has brought to the fore the debate held by the scientific community about the importance (or not) of the so-called airway, aerosols, in the spread of a disease that has already killed about a million people all over the world.

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

1 What transmission mechanisms of the coronavirus are contemplated?

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

The large droplets, also called ballistics, are particles of saliva or respiratory fluid expelled by infected people when coughing, sneezing and, to a lesser extent, when 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, in the Anglo-Saxon world).

The aerosol airway is the one that is debated. They are also particles of saliva or respiratory fluid, but smaller in size, sometimes called droplet nuclei. For this reason, they can stay in the air longer – from tens of seconds to hours – and travel longer distances, depending on their size. After being emitted when speaking or singing, for example, they infect 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

Regarding surfaces, also known as via fomite (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 putting your hands to your 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 at 5 µm (microns or micrometers): if it is larger, it is salivary or respiratory droplet 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.

For their part, a group of independent researchers, including the Spanish José Luis Jiménez 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 actual boundary between ballistic droplets and aerosols is in the 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.

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

3 Which route of transmission is the most important?

There is also no consensus. The CDC state 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.

According to the who, 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 in the New York Times, this organization also admitted that airborne transmission is possible of coronavirus by aerosols in poorly ventilated closed environments, but that more research is needed to confirm this.

The group of ‘dissident’ scientists disagrees. They think that, according to their evidence (summarized in a Web page), the aerosol pathway is at least as important as the other two, if not more so. In fact some, like Jimenez, consider that it may be the main one, with some contribution from fomites and less from ballistic droplets, clarifying that COVID-19 can be transmitted by aerosols without having to be at a rate as high as 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 defending the ‘aerosol pathway‘of contagion.

In the article signed by experts in July on New York Times It is also noted 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 floating in tiny droplets inside.

5 In any case, do we have to continue with the current protection measures?

If he interpersonal distancing, he handwashing and the use of masks they remain essential to prevent the spread of the disease. In addition, with regard to masks, we must insist on their good 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.

Also, the most recent evidence indicate that it is important limit the social circle of people with whom we maintain close contact, ensure that the encounters with these people are in outdoor places and ventilate frequently closed spaces, where we must be strict with the use of masks.


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