Even simply talking in a normal tone of voice leads to the spread of tiny droplets of oral fluid, just as coughing or sneezing, according to a study by the National Institutes of Health in Maryland, US, published in the prestigious New England Journal of Medicine.
The spread of droplets composed of saliva and mucus from the mouth and nose is crucial to the spread of the coronavirus (Covid-10), and lies at the basis of the debate over the wearing of face masks, as well as the rules on social distancing.
The issue is also a hot topic in the world of medical research. One recent study showed that the 1.5m social distance advised in Belgium may not be enough to protect against the cloud of droplets ejected by the force of a cough or sneeze – droplets which carry the virus and can travel up to 8m.
Now, a study from the National Institutes of Health demonstrates that the same droplets are expelled – albeit with much less force – when simply speaking.
The test subjects, all healthy volunteers, were asked to say the words “stay healthy” in a normal speaking voice, into a box. The droplets then passed through a film of laser light, and were photographed and measured.
The research found that the simple statement produced “numerous droplets” measuring between 20 and 500 micrometres – one micrometre is one-thousandth of a millimetre. Speaking more loudly produced more droplets, and the “th” sound in the word “healthy” produced the most.
The droplets themselves were smaller than those produced by a sneeze, but about the size which some studies have shown are produced by a cough. Because of the way the experiment was set up, it was not possible to measure the distance travelled by the droplets.
The most important result of the experiment, apart from demonstrating the existence of droplets produced during normal speech, was the effectiveness of a face mask.
Many if not most people think the face mask they wear when they go outside is to protect them from infection by the virus, despite the fact that it can enter through the eyes.
Medical professionals know, and have stressed since the beginning, that a mask is more effective in protecting others, and this study supports that view.
“When the same phrase was uttered three times through a slightly damp washcloth over the speaker’s mouth, the flash count remained close to the background level,” the paper says. “This showed a decrease in the number of forward-moving droplets.”
In other words, a simple barrier like a face mask reduces the number of droplets produced by the speaker substantially.
The paper concludes with one important caveat: the experiment’s results say nothing about whether the droplets in question are even able to contain the virus in an infected person.
“We did not assess the relative roles of droplets generated during speech, droplet nuclei, and aerosols in the transmission of viruses. Our aim was to provide visual evidence of speech-generated droplets and to qualitatively describe the effect of a damp cloth cover over the mouth to curb the emission of droplets.”