Public toilets eject energetic aerosol plumes
For nearly half a century, researchers have known that flushing a toilet releases droplets into the air. The largest of those droplets settle out in seconds, but smaller ones—aerosols with radii smaller than 5 µm—can linger suspended in air for several minutes (see the Quick Study by Stephane Poulain and Lydia Bourouiba, Physics Today, May 2019, page 70 ). Although the droplets are effectively invisible, they can expose people to infectious diseases, including COVID-19 and influenza, and little is known about the spatial and temporal evolution of the plumes.
Researchers led by John Crimaldi at the University of Colorado Boulder have now quantified the kinematics of those flush-generated plumes. To visualize them over the commercial 1.6-gallons-per-flush toilet typically found in North America, Crimaldi and colleagues used continuous and pulsed green lasers to create a thin sheet of light that illuminates a vertical plane on the toilet centerline. In their experiments, when the toilet was flushed, the resulting plume scattered the continuous laser light, and its structure was rendered visible for capture by a camera. The accompanying video reveals a complete time sequence following a flush. To obtain data on the location, speed, and direction of the particles, the researchers used the pulsed laser and a pair of cameras. Previous work has shown that the number of aerosol particles correlate with pathogen levels in the plume, so the researchers expect that the intensity of the scattered light should correlate with them as well.
The results demonstrate a surprisingly energetic and rapid growth of the plume: Each flush ejects particles upward at speeds up to 2 m/s, and the plume consistently rises to a height of more than 1.3 meters in 7.5 seconds. With a method in place to reveal the droplets’ dynamics, experts in plumbing and ventilation are in a better position to mitigate the risk of exposure to pathogens in public restrooms. (J. P. Crimaldi et al., Sci. Rep. 12, 20493, 2022 .)