Transmission of COVID-19 by aerosol, implications for public health

This is a posting by experts explaining the transmission of Covid-19 by aerosol and an explanation of what that means.  Some European health ministers and prime ministers do not seem to understand the concept.  

We are a multidisciplinary and international group of experts who conclude that neglecting the transmission of COVID 19 by aerosol is the source of the difference between the countries which control or do not control the spread of the new coronavirus.

When a person carries a respiratory virus, his cough, sneezing, but also his talking or singing will produce a cloud of droplets from his mouth or nose.

The modes of contamination of respiratory viral diseases can be classified into three categories according to the size of these droplets:

  • For large droplets if you are at a short distance (less than the famous 6 feet) they can be directly projected on you (the healthy person).
  • These same droplets can fall on a surface and contaminate it. If you touch it and bring your hands to your face there is a high risk of being contaminated.
  • But for very small droplets, they stay airborne and propagate over distances well over 6 feet, and this infectious aerosol can be inhaled, leading to contamination.

The recommendations currently given to the population to slow down the epidemic are exclusively based on the first two modes of transmission of COVID-19 described above and exclude the third possibility.

What is an aerosol

An aerosol consists of microdroplets that remain suspended in the air. In fact, the smaller the drop, the greater the resistance force of the air relative to its weight and the more the droplets settles slowly, possibly at almost zero speed. In addition, these microdroplets can decrease in size by evaporation of water, which allows them to remain almost indefinitely in suspension.

The movement of droplets of different sizes produced during normal conversation, or a cough or sneeze, can be visualized by laser illumination as shown in the figure below, adapted from an article published by one of the authors of this column in the Journal of Fluid Mechanics in 2014. It is shown that the aerosol cloud can propagate much further than isolated droplets thanks to the interactions between the droplets and the gas phase of the cloud. And that a sneeze can send droplets up to 6 meters away.

A recent video made by Japanese scientists also shows very well how these microdroplets can remain in suspension in the air and travel very far, see -of-coronavirus-in-breakthrough-study

When a person is infected, whether symptomatic or not, microdroplets can contain virus that remains infectious for a long time (the viability of the virus is measured in hours), and they are therefore capable of infecting the people who breathe them.

In still air, the smallest particles will reach, after emission, distances of several meters over times of the order of a minute, but the overall movements (convection) of the air are likely to transport them over much longer distances. large in a very short time and much less than an hour.

Our social distancing measures are therefore not adequate to the reality of the mode of aerosol transmission of this new coronavirus.

The importance and usefulness of the different masks.

Besides the detection of infected persons associated with containment, the best way to prevent aerosol transmission at the level of the individual is well known, the use of personal protective equipment, in particular masks of the N95 or FFP2 types which effectively protect against aerosol transmission. Even though surgical masks are much less effective against aerosols than FFP2, they are nevertheless scientifically proven to be effective, both in limiting transmission by infected individuals and in limiting the contagion of uninfected people.

In fact, the cities and countries that have best controlled the spread of the virus are Taiwan, Singapore and Hong Kong (despite their proximity to the epicenter of the pandemic), followed by China and South Korea. They were able to flatten the curve and thereby attenuate and then control the spread of the virus. These countries recognize the importance of minimizing all modes of transmission, including droplet and aerosol transmission, where the role of a mask is critical to the protection of the individual.

Figures from China demonstrate the importance of aerosol transmission and the essential protective role of masks, N95/FFP2. In fact, more than 3,000 people in the medical profession were infected in Wuhan this year during the first wave of COVID-19 and all the images show us the staff with simple isolation masks (surgical type) and not FFP2. On the other hand, 42,000 health professionals were sent in for reinforcement, but equipped with FFP2, and not a single infection was deplored among them!

The wearing of masks by all citizens.

In Asian countries, wearing a surgical mask is the minimum requirement for citizens in public and the effectiveness of these masks in reducing the transmission of respiratory diseases is widely demonstrated. It is therefore particularly disturbing to hear public authorities in France, Belgium and the US repeat since the beginning of this crisis that masks are useless for the public and that they would give a false sense of security. These public authorities insist that aerosol transmission would not be an important element for COVID-19, but these assertions are without scientific foundation and distance us from the actions that we can all take to flatten the progression curve of the virus and to stop it. infection. Masks should play a key role in helping us out of the crisis.

These comments seem to be motivated by the shortage of FFP2 masks in particular, which must of course be reserved as a priority for nursing staff and other front-line staff, which all adults understand perfectly, and there is therefore no need to hide the reality of the situation. This only creates confusion and a feeling of helplessness among the general public. On the contrary, informing the public of the reality of the situation would allow them to adapt as best they can to the danger we face.

While the WHO continues in its absurd recommendation to say that masks are not necessary for the general public, France persists in the same direction, the CDC (Centers for Disease Control and Prevention) in the United States reconsider this position: “Up to 25% of people infected with the new coronavirus may not have symptoms,” warns Redfield, director of CDC. The agency had said on several occasions that ordinary citizens do not need to wear masks unless they feel sick. But knowing that people can be infected and contagious without feeling sick, Redfield said the advice is “being critically reviewed”.

However, the position of the Chinese government, represented by George Gao, the director general of the Chinese Center for Disease Control and Prevention, could not be clearer: Not wearing a mask to protect yourself from coronavirus is a “big mistake “

Wearing a mask is compulsory in public in several Asian countries in the current situation, countries which have the best control over the spread of the virus on their territory. In Europe, the city of Prague is the only one to have mandated the wearing of a mask or scarf in public since March 18, why?

The city of Prague, an example to follow.

Petr Ludwig, had produced an educational video on the importance of wearing masks, not as protection for oneself, but to protect others in case one is contagious without knowing it. The video cites a study by researchers at the University of Cambridge which concludes that surgical masks are 3 times more effective than homemade masks, however they recommend homemade masks as a last resort when surgical masks are not available.

As the shortage of masks supplied by the Czech government continued, hospitals used social media to ask if people could sew masks for them. In an unprecedented demonstration of support, many people started making masks, not only for hospitals but for everyone. The effort was both individual and organizational – theaters, non-profit organizations, small businesses and factories that normally produce clothing, linens and accessories have shifted their focus to full-time sewing . Local businesses sewed, supplied hospitals, homes for the aged, police and firefighters. The masks were delivered to hospitals or to friends and neighbors who often found them in their mailboxes. In some areas, people have created “mask trees” to make additional masks available to others.

Of course, the badly worn FFP2 and surgical masks do not offer perfect protection. But the stated objective is “to flatten the curve” and we must adopt all measures to reduce contamination, including homemade masks which offer significant protection when shopping. We must stop pretending that masks are not effective, even if they are imperfect; the best should not be the enemy of good.

Beyond the necessary flattening of the curve, it is the mass production of masks to N95/FFP2 standards for the public that will allow the fastest return to an almost normal economic activity.

Organizational implications of aerosol transmission

Confined spaces favor aerosol transmission and can be the site of episodes of super-contagion, as was the case in evangelical gatherings in France (Mulhouse) and in South Korea (Daegu). Indeed, the phenomenon can be exacerbated by an air heater or air conditioning system, where the air is recirculated inside the premises for the sake of saving on heating (or cooling). It is therefore recommended to stop any device of this type, which contributes to circulating the ambient air. On the other hand, the massive and frequent aeration by outside air in premises still frequented by the public, pharmacies, the post office, small food stores and supermarkets, contributes by dilution to a reduction in the number of infectious particles to which the public is exposed.

The space heater systems of many buildings obviously include a supply of fresh air to maintain indoor air quality, but they have only been designed for ordinary pollutants and maintaining an acceptable humidity level. Even more serious, autonomous air heater systems, essentially consisting of a simple heat exchanger fitted with a fan, are widely used for heating workshops, garages, food stores, etc. Their users (craftsmen, small industrialists, traders, etc.) must be aware of the danger they present and the need for immediate shutdown.

The safety distance.

Finally, the work carried out by researchers in Japan, and by a signatory from this article in the United States, shows that the safety distance of 6 feet is largely insufficient, the communication on “barrier measures” suggests to the population that this distance is sufficient to greatly reduce the risk of contamination. This is not the case in the absence of a mask and this communication causes the population to be confused and not to understand the value of social distancing!

The dangers of continuing non-essential economic activities

Containment measures in Italy, Belgium and other countries are showing their effects, with an increase in the doubling time visible after a certain delay since the implementation of containment. The flattening of the curve, combined with a hospitalization time shortened by the treatments available, may allow us to avoid saturation of hospitals, but we must avoid a second wave of propagation in the population that remains active.

This is why we recommend that all governmental and private organizations suspend all activities which are not absolutely essential or carried out by teleworking, because in the absence of FFP2 masks, the virus will continue to spread in our institutions and our companies. The larger the population, the larger the second wave, and the greater the risk of congestion in our hospitals.

We pleaded for the respect of the principle of reality, for the application of the necessary measures of public health and the principle of precaution in medicine, which enjoin us to act while protecting us by all possible means from all possible modes of transmission of COVID-19. Instead of covering our eyes so as not to see this reality, wrongly considered as anxiety-provoking, let us take our destiny in hand by covering our mouth and nose as effectively as possible.

In summary, let’s be adults and accept the reality of the risk of aerosol transmission, limit all non-essential activities, use all means of protection for our respiratory tract, from N95/FFP2 to the humble shawl, while waiting to develop the tools to return to full activity, namely a massive nucleic and serological screening capacity for this virus associated with confinement of infected persons, and a capacity for local production of N95/FFP2 masks for the entire population.


Marc Wathelet, Ph.D. (doctor of science) specialist in coronaviruses.

Giovanna Borsellino, M.D., Ph.D., Neuroimmunology Unit, Santa Lucia Foundation IRCCS, Rome, Italy.

Etienne Decroly, Ph.D., Directeur de recherche CNRS, Laboratoire AFMB, Membre du conseil scientifique de la société française de virologie.

Nathanael Goldman, Pediatrician, MSc Public Health, MSc Clinical Trials (LSHTM).

Philippe Koch, M.D., Ph.D., ophtalmologue, Professeur à l’Université Orsay-Paris Sud.

Bertrand Rowe, docteur ès sciences, Directeur de Recherche CNRS retraité, consultant, lauréat du prix Descartes 2000 de la recherche européenne.

Yang Zhao, Ph.D., Assistant Professor, Department of Agricultural and Biological Engineering, Mississippi State University

Nora Zekhnini, M.D., General practitioners

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