Knowing exactly how far our spittle flies is vital in modeling the spread of infectious diseases and protecting public health, and researchers have now carried out new experiments to get some clarity and precision on these measurements.
The study authors invited 23 volunteers into their lab in France to see how far spit droplets traveled when these participants were talking, coughing, and breathing normally, both with and without masks.
An approach called the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique was used for measurements, which essentially deploys a high-speed camera to capture the size and speed of droplets as they pass through laser light.
“Experimentally determining the size and velocity of these droplets, along with the properties of the exhaled air cloud, is crucial for predicting their behavior post-emission and developing effective strategies to mitigate infection transmission,” write the researchers in their soon-to-be-published paper.
“Despite the efforts of the scientific community, there is still a lack of comprehensive characterization of exhaled droplet size distribution, with different studies yielding significantly varied results.”
Speaking and coughing produced droplets between 2 and 60 micrometers (μm) in size, the researchers found, while for normal breathing the droplet sizes were between 2 and 8 μm. As you would expect, coughing expelled the fastest-moving droplets (by an order of magnitude), and droplets in the highest concentrations.
More droplets drifted up and down in the breathing exercises, whereas speaking and coughing produced a more narrow jet. Encouragingly, wearing tissue or surgical masks blocked between 74 and 86 percent of the droplets across all types of exhalations.
Also of note was the variation between the study participants, and even between different tests from a single volunteer. This backs up the idea of superspreaders: people who tend to spread infections more than others.
“Significant variability in both droplet size and velocity measurements was observed among volunteers, with slightly reduced variability when considering repeated tests by the same volunteer,” write the researchers.
“Understanding how within-volunteer variability relates to different volunteer or environmental conditions requires further analysis and testing.”
The carefully collected data here should prove useful for future studies looking at how infections can spread, and how we can stop them from spreading – a complex problem that continues to be a challenge for professionals.
In further research, the team is keen to get a broader range of volunteers to go through their ILIDS process, and to develop guidelines around precautions (such as mask wearing) that can make a significant difference to the spread of disease.
“Taking the measurements on a larger sample of volunteers would make it possible to assess the variability between them, which includes both variability related to the different emission itself and to the different shape of the face, and thus to the different adherence of the protective masks,” write the researchers.
The research has been published in Physical Review Fluids.
