The spread and transmission of Covid-19 through coughing and sneezing are well known. Although the virus can be spread via infected surfaces or objects, it is mainly transmitted through nasal secretions, sputum and droplets generated when one coughs or sneezes. But not only. A new study proves that the microdroplets emitted when speaking or even breathing remain suspended in the air and can transmit the coronavirus. This research is available on the scientific prepublication site MedRixv.
Airborne microdroplets, vectors of the virus?
To make this discovery, researchers from the University of Nebraska (United States) conducted an experiment with bedridden patients. Speaking, these produced microdroplets which remained suspended in the air for several hours. This is called an aerosol.
In the patients’ rooms, the scientists took these microdroplets present in the air, before placing them in culture and finding that these samples were able to replicate. According to the study’s authors, this phenomenon represents evidence that microdroplets expelled when speaking or breathing can transmit Covid-19. The virus “replicates in cell culture and is therefore infectious”, said Joshua Santarpia, co-author of the study.
However, the authors point out that these are preliminary conclusions: their study has not yet been validated by their peers or published in a scientific journal.
Speaking loudly would produce thousands of contaminated droplets per second
A previous study published in May 2020 in the American scientific journal PNAS, already revealed that speech could be a probable mode of transmission of Covid-19.
Scientists believed that speaking could be dangerous, as could coughing or sneezing, because they discovered that the microdroplets emitted by speech can harbor various respiratory pathogens. “High viral loads of severe acute respiratory syndrome (SARS-CoV-2) have been detected in oral fluids of patients with coronavirus, including asymptomatic patients. However, the possible role of small droplets generated by speech, that could remain airborne for long periods of time has not been widely recognized “, says the study. Loud speech could emit thousands of contaminated droplets per second
To carry out this study, scientists from the National Institute of Diabetes and Digestive and Kidney Diseases performed an experiment. They made a person speak in a closed space. “We used intense laser light to visualize the droplets produced by speech when repeating sentences. We obtained quantitative estimates for both the number and size of the droplets that remain airborne.”, emphasizes the researchers.
During the experiment, the person repeated, with a loud voice, “Stay healthy”, that is to say “do you feel well” in French, for 25 seconds. This phrase was chosen because the “th” in the word “healthy” has been shown to be an efficient droplet generator. Observations by laser light scattering have revealed that loud speech can emit thousands of contaminated droplets per second.
The droplets would remain suspended in the air for 8 to 14 minutes
In a closed environment, the microdroplets are able to stay suspended in the air for 8 to 14 minutes, “which corresponds to droplets of about 4 micrometers in diameter, or 12 to 21 micrometers before dehydration”. “The biggest and brightest droplets fall to the ground at faster speeds than the smallest”, details the study. Conclusion: the smaller the droplets, the longer they stay suspended in the air.
“This direct visualization demonstrates how normal speech generates droplets that can stay in suspension for tens of minutes or more and are eminently capable of transmitting disease in confined spaces.”, say scientists. These observations thus confirm that there is a probability that speech is a main vector of virus transmission in confined environments. The study therefore recalls the importance of barrier gestures, of wearing a mask and respecting social distancing.