Can the Novel Coronavirus Spread Through Speaking?
There are conflicting opinions on whether the novel coronavirus, the virus that causes COVID-19, can be spread via aerosol. Past studies have pointed to the possibility that the virus may remain suspended in the air under special circumstances, such as during medical procedures like intubation. In one study, the novel coronavirus stayed viable in airborne aerosols throughout the duration of the 3-hour experiment.
A laser light scattering experiment conducted by scientists from the University of Pennsylvania Perelman School of Medicine and the National Institutes of Health (NIH) concluded that speaking at a normal volume can produce small, aerosol-like droplets that hang in the air long enough to enter the airways of other people.
“Aerosols from infected persons may therefore pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation,” says Harvard University biologist Matthew Meselson, commenting on the study.
The experiment, published in the New England Journal of Medicine, generated droplets and visualized their trajectories using laser light scattering.
The scientists used a cardboard box with a black interior, slits along the side, and a HEPA filter on top to eliminate ambient dust. A 532-nanometer green laser operating at 2.5W optical power and transformed into a sheet of light 1 millimeter thick and 150 millimeters tall was directed through the slit and into the box’s interior.
An iPhone 11 Pro camera was aimed at the light sheet through a hole on the opposite side of the box. It recorded sound and video of light-scattering events at 60 frames per second. The scientists recorded ultrahigh-resolution video clips of the experiment while a participant spoke.
The volunteer subject first spoke into the box at a normal volume, saying “stay healthy.” Droplets (20–50 microns in size) generated while speaking this short phrase traveled 50 to 75 millimeters before they hit the light sheet.
Flashes were produced when the droplets passed through the sheet of light. The brightness of the flashes reflected the size of the particles and the fraction of time they were present in a 16.7 millisecond frame of the video. The number of flashes in a single frame of video was highest with the “th” sound in “stay healthy.”
The participant repeated the phrase three times at various volumes, with pauses in between. The number of flashes was highest with the loudest speech. Additionally, the flash count during the pauses between the phrases was above the background level, indicating that aerosolized droplets lingered in the air.
The experiment was repeated with the participant’s mouth covered with a damp cloth. The flash count remained similar to the background level observed before the start of the first trial, indicating a decreased number of forward-moving droplets.
Thus, this study provides evidence of speech-generated droplets and qualitatively describes the effect of a barrier over the mouth to curb droplet emission.
Sneezes can emit large infectious particles, which remain airborne for a short time before quickly settling to the ground. Infection through these larger particles occurs when people touch the surface where they have settled, then touch their eyes, mouth, or nose. The virus can then enter their body through their upper respiratory tract, where it might be flushed by nasal secretions or swallowed before an infection can begin.
The infectious particles produced by talking, however, behave differently. Since they are much smaller, they can evaporate into droplet nuclei. Droplet nuclei behave like an aerosol; in still air, a 10-micron particle can remain aloft for nine minutes. The tiny droplets can be carried by mild air currents caused by people moving thorough a room or natural or artificial ventilation. Inhaled viral particles can settle in the lungs after being inhaled, beginning the infection deeper in the body.
The UPenn and NIH study provides evidence that person-to-person transmission of the novel coronavirus, and other similar viruses, can occur through aerosolized droplets produced while speaking. “Aerosols from infected persons may therefore pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation,” said Harvard University geneticist and molecular biologist Matthew Meselson, in a review of the study.
This study also qualitatively describes the effect of a mouth covering in curbing droplet emission. According to Meselson, it is advisable to wear “a suitable mask whenever it is thought that infected persons may be nearby” as well as to provide “adequate ventilation of enclosed spaces where such persons are known to be or may recently have been.”
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