How to prevent airborne transmission of the coronavirus indoors, according to the latest studies

How to prevent airborne transmission of the coronavirus indoors

Research published by Science has shed light on the mechanisms by which Sars-Cov-2 can be transmitted. Here's how to best manage an enclosed space: aerosol is the main vector rather than droplets

(photo: Simon Karemann / Unsplash) With the arrival of September and then of autumn, more and more people they abandon the summer habits of being outdoors and resume frequenting mainly closed spaces, most of the time shared with other people. From the office to the restaurant, passing through cinemas, shops, school classrooms, gyms, friends' houses and meeting rooms, there are many situations in which it will be necessary to continue to be careful to limit as much as possible the passage of the new coronavirus and its variants from person to person. Starting with airborne transmission, which as known since the beginning of the pandemic represents the main channel of viral spread.

The scientific community has never worked as in recent months - as well as on all other fronts linked to Covid -19, vaccines included - to better understand the dynamics of viral transmission by air, putting together everything we know so far and drawing some useful conclusions for practical purposes as well. An intercontinental research group with leading scientists from China, the United States and Israel put them in black and white, and the resulting scientific paper was published in the journal Science at the end of August.

Droplets are only relevant at a very short distance

That it is not only the small droplets of saliva that we have learned to call droplets, but also the aerosol of very fine suspensions, to represent a vector contagion of Sars-Cov-2 certainly does not sound like a novelty. But maybe the droplets have been overrated anyway. If we mean by this word those drops of size visible even to the naked eye (by convention, from a diameter of one tenth of a millimeter upwards) that can be projected outside the mouth and fall to the ground by gravity, then their relevance in terms of distance soon decreases.

Except in cases of sneezing, singing or particularly violent screams, droplets can have a range between 1.5 and 2 meters, but in fact they travel at a height that being able to infect those in front of you for just twenty centimeters. Except for very narrow contacts or surfaces contaminated by significant quantities of viral load, in short, these more macroscopic drops would not carry the coronavirus and infect.

The mask remains essential

This does not mean at all that the mask is useless, on the contrary: in addition to stopping the larger droplets, the tissue in front of the nose and mouth can also block many of the finer droplets, i.e. those that then remain in suspension in the air for a long time and more easily constitute the means of transport on which the Sars-Cov-2 travels.

However, the mask and the spacing in indoor spaces may not be enough, since particles large one thousandth of a millimeter (one micron) or even less they are able to infect even from a distance, both over time and for people who are far away but in the same closed space. Moreover, as the authors of the publication reiterated, the finest aerosol is also the one that on average penetrates the airways the most, representing a more than significant chance of contagion.

The droplet deposit area in the respiratory system according to their size (graph: Science)

What can be done individually

Beyond the usual precautions - wear a mask and maintain a minimum of physical distance - the scientists insisted on two fronts: type of mask and ways to wear it. On the typology, as obvious, the new data confirm that an Ffp2 is more protective than a surgical one, not so much for the droplets (which are well shielded in all cases) but above all for the fine particles of a few microns in diameter, adequately filtered by the Ffp2 but contained only 50% by surgical masks.

And if the choice between surgical and Ffp2 is sometimes also dictated by other non-strictly health issues, such as ease of use and cost, in all cases, however, the importance of wearing masks with great care was reiterated. Regarding the aerosol, in fact, the mask not only acts as a screen but also as a filter for the air we inhale (much more efficient for the Ffp2, as known), and to minimize the air flows that pass in the cracks and in spaces not well covered can make a difference.

Finally, the last precaution suggested on an individual level is to always reflect whether or not to stay in a certain closed space shared with other people, moving away if you think that the situation is not handled safely. More than the number per se or the density of people in the room (except in striking cases), the characteristics of ventilation and air recirculation should be the ones that deserve attention, because - as we will see below - these are the ones that science says to be really decisive.

What can be done, for those who manage a space

Restrict access to spaces allowing it only to those who have the green pass (where provided) and are asymptomatic, ensuring the correct use of the masks and the maintenance of physical distances are obviously the a priori indications, not even mentioned by the new Science study. On the other hand, a decisive aspect for which we have scientific evidence is the ventilation system. But how to quantify if the ventilation is adequate or not? According to the authors of the research, a more than satisfactory way is to exploit as an indirect measure that provided by the classic and already very widespread sensors for the concentration of carbon dioxide.

In fact, the quantity of CO 2 present in a room it is an indicator of how much there has been exhalation by the people inside, and therefore when the level of carbon dioxide rises it is very likely the concentration of aerosol, potentially vehicle of the virus, is also rising. If the CO 2 exceeds 2 thousand parts per million, it means that the ventilation is clearly inadequate, and the ideal level indicated by scientists is between 700 and 800 parts per million, which is a very low value and a sign of an excellent turnover. air.

For those who have systems capable of carrying out calculations of this kind, adequate ventilation of the room should correspond to at least 4 (and up to 6) complete changes of the room air every hour, i.e. a hourly air flow equal to 4-6 times the volume of the enclosed space in question. Of course, the effect is further increased by the presence of air filtration, purification and possibly disinfection systems. While on the other hand there are the recirculation systems: unless they are equipped with an efficient air cleaning system, the systems that simply re-introduce the same air into the environment are more of a problem than an aid, since they favor the diffusion of the aerosol over the whole space.

Type of ventilation and shape of the room

Although we do not yet know in detail the survival time of Sars-Cov-2 on aerosols and droplets of different sizes, in general in an inadequately ventilated space the aerosol will tend to accumulate the faster the more people are in the room, in proportion to the size of the environment itself. For this reason, the smallest spaces such as public transport or the most crowded ones are considered the most critical, and in general the minimum recommended air flow is 10 liters of air per second for each person inside.

However, these are indicative figures. Much depends, in fact, on the precise conformation of the closed space, on where the points of exchange of air with the outside are positioned and also on the type of ventilation that is used, that is, whether natural or mechanical. In fact, together with evaluating the values ​​of the flows themselves, it is important to establish whether there are parts of the environment in which the air tends to stagnate, and even if the dynamics of air circulation tend to accumulate aerosols and particles in suspension at specific points. Aspects not at all detailed, but which could indeed be as relevant as the mask for containing the spread of the virus in all closed environments.


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