< Transmission

Is the SARS-CoV-2 coronavirus transmitted by aerosols?

Text updated on 2020-08-26


Yes, by breathing and talking, each person emits an aerosol of micro-droplets that stay in the air for a long time and move around like cigarette smoke. This aerosol contains less virus than the droplets but can be a cause of contamination, especially in closed places. Ventilation, aeration, and the wearing of masks strongly limit the transmission of COVID-19 by aerosols.

Aerosols and drops (or 'sprays') are two modes of aerial transmission of the SARS-CoV-2 coronavirus that causes COVID-19.

Postillate droplets are droplets larger than 2.5 µm in diameter (or 5 µm according to the authors), which quickly fall down within 1-2 meters of a talking or singing transmitter. They travel further when you cough, and even further when you sneeze (3 to 5 meters). These drops contaminate hands and objects, which increases the spread of the virus. These droplets behave like a spray (e.g., windshield washer spray). They contain more than 95% of the viruses released according to a simulation study with influenza virus.

An aerosol is a cloud of microdroplets typically less than 2.5 µm in diameter (maximum 1-5 µm according to the authors), and generally between 0.25 and 1 µm. These microdroplets can remain in the air for several hours. While breathing and talking, each one emits an aerosol. The aerosol behaves a bit like cigarette smoke: it is carried by air currents and accumulates in closed rooms. The aerosol is very light: it can spread far from the person who emits it (up to 10 m in a current of air) and penetrates very deeply into the lungs.

An aerosol can contain viruses if the transmitter is infected, and these viruses remain infectious for more than 16 hours. Because the aerosol is much lighter than sprays, it carries a smaller amount of virus than sprays. According to a simulation study with influenza virus, less than 5% of the viruses emitted are aerosolized. In hospitals, invasive procedures in Intensive Care Units (intubation, ventilation, bronchoscopy, etc.) produce particularly concentrated contaminating aerosols. Finally, in the open air and in well-ventilated or aerated places, the aerosol dissipates and dilutes very quickly, so that no virus can be detected in the air except for exceptions linked to a high carrier density of the virus or exceptional weather conditions, such as cold air that is stationary or during a pollution peak.

The relative share of each mode of transmission in COVID-19 is not precisely known, although both modes of transmission are well demonstrated, and have been since March 2020. Under strong pressure from scientists, the WHO recognized in July 2020 that aerosols can play an important role in enclosed spaces.

How to protect yourself from aerosols? The longer you stay in the rain, and the harder it rains, the wetter you will be. Similarly, the more droplets you are exposed to and the longer you stay in this environment, the greater the risk of exposure. Mitigating the risk is therefore based on reducing both the concentration levels of aerosols and the length of time you are exposed. If possible, open the doors and windows to increase the flow of fresh air. Reduce time spent in poorly ventilated and cluttered areas. In confined spaces, avoid the concentration of people, potential sources of virus emissions, and ensure that masks are worn at all times.


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Sources

An article showing that viral particles of the SARS-CoV-2 coronavirus agent from COVID-19 present in aerosols are infectious on cells. These particles are infectious up to 16 hours after aerosolization.

Fears, A. C., Klimstra, W. B., Duprex, P., Hartman, A., Weaver, S. C., Plante, K. S., ... & Nalca, A. (2020). Persistence of Severe Acute Respiratory Syndrome Coronavirus 2 in Aerosol Suspensions. Emerging infectious diseases, 26(9).

Major, early and detailed study of viral RNA contaminations in two hospitals in the city of Wuhan (China) showing that aerosols may contain SARS-CoV-2 coronavirus RNA. The authors also show the effect of ventilation to eliminate air contamination inside the hospital and the exceptional presence of viruses in the outside air.

Liu, Y., Ning, Z., Chen, Y., Guo, M., Liu, Y., Gali, N. K., ... & Liu, X. (2020). Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature, 582(7813), 557-560.

A good synthesis on the aerial transmission of SARS-CoV-2, published in June 2020, strongly advises to wear masks on the basis of scientific evidence. Incidentally, it draws an analogy between the diffusion of cigarette smoke and that of viral aerosols.

Prather, K. A., Wang, C. C., & Schooley, R. T. (2020). Reducing transmission of SARS-CoV-2. Science.

This experimental study models the passage of the influenza virus in particles of different sizes, with and without a visor, at various distances.

Lindsley, W. G., Noti, J. D., Blachere, F. M., Szalajda, J. V., & Beezhold, D. H. (2014). Efficacy of face shields against cough aerosol droplets from a cough simulator. Journal of occupational and environmental hygiene, 11(8), 509-518.

Emergency physicians describe the procedures to be performed to minimize their risk of being infected by COVID-19.

Odor, P. M., Neun, M., Bampoe, S., Clark, S., Heaton, D., Hoogenboom, E. M., ... & Kamming, D. (2020). Anaesthesia and COVID-19: infection control. British Journal of Anaesthesia.

At the NCID hospital in Singapore, which was well ventilated, no virus was found in the air in the room of a non-intubated, coughing, and strongly SARS-Cov-2 positive patient, but the air intake was contaminated.

Ong, S. W. X., Tan, Y. K., Chia, P. Y., Lee, T. H., Ng, O. T., Wong, M. S. Y., & Marimuthu, K. (2020). Air, surface environmental, and personal protective equipment contamination by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) from a symptomatic patient. Jama, 323(16), 1610-1612.

In Bergamo, Italy, in a heavily polluted industrial area with a very high incidence of COVID-19, the RNA of the SARS-CoV-2 coronavirus was detected in more than half of the outdoor air samples. During the study, atmospheric conditions were particularly stable with a significant peak of fine particle pollution (PM10).

Setti, L., Passarini, F., De Gennaro, G., Barbieri, P., Perrone, M. G., Borelli, M., ... & Clemente, L. (2020). SARS-Cov-2 RNA Found on Particulate Matter of Bergamo in Northern Italy: First Evidence. Environmental Research, 109754.

An excellent scientific synthesis published in June 2020 illustrates the existence and importance of aerosols in the transmission of COVID-19.

Morawska, L., & Cao, J. (2020). Airborne transmission of SARS-CoV-2: The world should face the reality. Environment International, 105730.

In this "letter-comment" co-signed by 239 scientists, Morawska and Milton argue that the WHO should recognize the role of aerosols in the transmission of COVID-19 and recommends ventilation and extraction of indoor air.

Morawska, L., & Milton, D. K. (2020). It is time to address airborne transmission of COVID-19. Clin Infect Dis, 6, ciaa939.

The authors of this opinion piece argue that aerosols are highly contaminating, comparing the slope of the epidemic in countries that have or have not imposed the wearing of masks in addition to distancing measures. They conclude that the use of masks is the most effective measure to limit the epidemic.

Zhang, R., Li, Y., Zhang, A. L., Wang, Y., & Molina, M. J. (2020). Identifying airborne transmission as the dominant route for the spread of COVID-19. Proceedings of the National Academy of Sciences.

A popular article, also available in French, which describes the turn taken by the WHO in July 2020 in the recognition of the "aerosol" pathway.

Erath B., Ferro, A. & Ahmadi, G. Aerosols are a bigger coronavirus threat than WHO guidelines suggest - here's what you need to know. The Conversation, 10 July 2020.

Further reading

How does she get COVID-19 ?

How do you put on and take off your mask?

How to untangle the truth from the false about the COVID-19 pandemic in the media?

What is close contact?

Can a person without symptoms infect others?

Do I have to wear a mask if I don't have any symptoms?