Aerosolised particles shoot out of a lid-less toilet when it is flushed: study

The particles can carry pathogens and potentially expose users of public bathrooms to disease.

A powerful green laser helps visualise aerosol plumes from a flushed toilet. | Photo Credit: Patrick Campbell/CU Boulder

RESEARCHERS have known for over 60 years that when a toilet is flushed, solids and liquids go down as designed, but tiny particles, invisible to the eye, are also released into the air. Studies have detected the presence of aerosolised particles above flushed toilets and shown that larger ones can land on surrounding surfaces, but until now, no one knew what these plumes looked like or how the particles got there.

Now, using bright green lasers and camera equipment, a team of engineers at the University of Colorado, Boulder, US, have demonstrated what happens when a lid-less, public restroom toilet is flushed. This study, the first to directly visualise the resulting aerosol plume and measure the speed and spread of particles ejected, was published in Scientific Reports.

It found that particles shoot out quickly, at speeds of 2 m/s, reaching 1.5 m above the toilet within 8 seconds. While the largest droplets tend to settle onto surfaces within seconds, the smaller particles (less than 5 microns) can remain suspended in the air for minutes or longer. These aerosolised particles are known to transport pathogens and pose an exposure risk to users of public bathrooms. However, this experimental demonstration could also enable the development of ways to reduce the potential exposure to disease, such as disinfection and ventilation strategies, or improved toilet and flush design.

“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you see these videos, you’re never going to think about a toilet flush the same way again,” said John Crimaldi, lead author of the study. Moreover, it is not only their own waste that toilet users have to worry about. Studies have shown that pathogens can persist in the bowl for dozens of flushes, increasing potential exposure risk. “The goal of the toilet is to effectively remove waste from the bowl, but it’s also doing the opposite, which is spraying a lot of contents upwards,” said Crimaldi.

The researchers used two lasers: one shone continuously on and above the toilet, while the other sent out fast pulses of light over the same area. The constant laser revealed where in space the airborne particles were, while the pulsing laser measured their speed and direction. Two cameras took high-resolution images.

The toilet used for the experiment was the kind commonly seen in North American public restrooms: a lid-less unit accompanied by a cylindrical flushing mechanism that sticks up from the back near the wall. A brand-new, clean toilet was filled only with tap water.

“We had expected these aerosol particles would just sort of float up, but they came out like a rocket,” said Crimaldi. The energetic, airborne water particles headed mostly upwards and backwards towards the rear wall, but their movement was unpredictable. The plume also rose to the laboratory’s ceiling and moved outwards from the wall and spread into the room.

The researchers also measured the airborne particles with an optical particle counter. Smaller particles were found to not only float in the air for longer but were also able to escape nose hairs and reach deeper into the lungs. The experiment also yielded the spectrum of particles and their sizes. The experimental setup did not include any solid waste or toilet paper in the bowl, and there were no stalls or people moving around. These real-life situations could exacerbate the risk, said Crimaldi.