Scientists shine a light on toilet flushing

A powerful green laser helps visualise aerosol plumes from a toilet during flushing. Image: John Crimaldi.

The health impact of flushing the toilet is being seen in a whole new light by US scientists

We may not want to think about what comes back up when we flush, but using bright green lasers and camera equipment, the physicists and engineers at University of Colorado Boulder (UCB) have done just that. They ran an experiment to reveal how tiny water droplets, invisible to the naked eye, are rapidly ejected into the air.

These aerosol particles are known to transport pathogens and could pose an exposure risk to people using public lavatories. The researchers say this is the first study to directly visualise the resulting aerosol plume and measure the speed and spread of particles within it.

The good news is that this vivid visualisation of potential exposure to disease also provides a way to help reduce it.

“If it's something you can't see, it's easy to pretend it doesn't exist," says John Crimaldi, lead author of the study and professor of civil, environmental and architectural engineering at UCB, "but once you see these videos, you're never going to think about a toilet flush the same way again.

"By making dramatic visual images of this process, our study can play an important role in public health messaging.”

It has been known for over 60 years that when a toilet is flushed, solids and liquids go down as designed, but tiny, invisible particles are also released into the air. Previous studies have used scientific instruments to detect the presence of these airborne particles above flushed toilets and shown that larger ones can land on surrounding surfaces, but until now, no one understood what these plumes looked like or how the particles got there.

"Once you see these videos, you're never going to think about a toilet flush the same way again."

John Crimaldi, University of Colorado Boulder

Making the breakthrough involved using 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 the airborne particles were, while the pulsing laser could measure their speed and direction. Meanwhile, two cameras took high resolution images.

“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 ceiling of the laboratory, and with nowhere else to go, moved outward from the wall and spread forward into the room.

Two lasers were used in the experiment. Image: Patrick Campbell / University of Colorado Boulder.

Understanding the trajectories and velocities of these particle is important because they can transport pathogens such as E. coli, C. difficile, noroviruses and adenoviruses. Exposure to risk can be mitigated disinfection and ventilation strategies, or improved toilet and flush design.

While the virus that causes Covid-19 - SARS-CoV-2 - is present in human waste, there is not currently conclusive evidence that it spreads efficiently through toilet aerosols.

“People have known that toilets emit aerosols, but they haven't been able to see them,” said Crimaldi. “We show that this thing is a much more energetic and rapidly spreading plume than even the people who knew about this understood.”

The study found that these airborne particles shoot out quickly, at speeds of 2 metres per second, reaching 1.5 metres above the toilet within 8 seconds. While the largest droplets tend to settle onto surfaces within seconds, the smaller particles - aerosols less than 5 microns, or one-millionth of a meter - can remain suspended in the air for minutes or longer.

Aerosol plumes can be visualised for the first time. Image: Patrick Campbell / University of Colorado Boulder.

The toilet itself was a type commonly seen in pubic restrooms in North America - a lidless unit accompanied by a cylindrical flushing mechanism. A brand-new toilet was used and filled only with tap water.

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 variables could all exacerbate the problem, said Crimaldi.

“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. “Our lab has created a methodology that provides a foundation for improving and mitigating this problem.”

Laser focus

Crimaldi runs the ecological fluid dynamics lab at UCB, which specialises in using laser-based instrumentation, dyes and giant fluid tanks to study everything from how odours reach our nostrils to how chemicals move in turbulent bodies of water.

While these results may be disconcerting, the study, which is published in the journal Scientific Reports, provides experts in plumbing and public health with a consistent way to test plumbing design and disinfection and ventilation strategies to reduce exposure risk to pathogens in public restrooms.

“None of those improvements can be done effectively without knowing how the aerosol plume develops and how it's moving,” said Crimaldi. “Being able to see this invisible plume is a game-changer."