NIDDK test suggests speech-generated droplets raise COVID-19 risk

Simple speech—just talking without sneezing or coughing—can disperse thousands of potentially harmful droplets into the air, according to research from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) published in the New England Journal of Medicine on April 15 as a letter to the editor.

The dispersal of droplets returned to negligible levels when the speaker wore a damp washcloth over their mouth, underscoring the potential benefits of cloth face masks for controlling the spread of COVID-19.

The NIDDK researchers performed a laser light-scattering experiment in which a light sheet was directed through slits on the sides of a cardboard box with a black interior. When a person said "stay healthy" into the open end of the box, the size of the resulting droplets was estimated from the brightness of flashes that appeared as the droplets passed through the light sheet.

The number of flashes in a single frame of the video was highest when the “th” sound in the word “healthy” was pronounced (Figure 1A). Repetition of the same phrase three times, with short pauses in between the phrases, produced a similar pattern of generated particles, with peak numbers of flashes as high as 347 with the loudest speech and as low as 227 when the loudness was slightly decreased over the three trials.

When the experiment was repeated with a damp washcloth over the speaker's mouth, the flash count remained close to background levels, suggesting a decrease in the number of forward-moving droplets.

Read the letter: https://www.nejm.org/doi/full/10.1056/NEJMc2007800