Scientists see the impact of flushing the toilet in a whole new light

Scientists shine a light on what comes up when you flush.


Flushed toilets emit aerosols that spread pathogens in feces, but little is known about the spatiotemporal evolution of these plumes or the velocity fields that transport them.

Using bright green lasers and camera equipment, a team of CU Boulder engineers determined how tiny water droplets, invisible to the naked eye, are rapidly ejected into the air when the toilet is flushed. They saw the impact of flushing the toilet in a whole new light.

Their study is the first that measures the speed and spread of the resulting aerosol plume by directly visualizing it.

John Crimaldi, lead author on the study and professor of civil, environmental, and architectural engineering, said, “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. By making dramatic visual images of this process, our study can play an important role in public health messaging.”

However, until recently, no one knew what these plumes looked like or how the particles got there. Previous research has employed scientific tools to identify the existence of these airborne particles above flushed toilets and showed that larger ones could drop on nearby surfaces.

To reduce exposure risk through ventilation and disinfection techniques, better toilet and flush design, or other exposure risk reduction measures, it is crucial to understand the trajectories and velocities of these particles, which can carry pathogens like E. coli, C. difficile, noroviruses, and adenoviruses. The COVID-19 (SARS-CoV-2) virus is prevalent in human faeces. However, there is currently insufficient proof that it may effectively transmit by toilet aerosols.

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

According to the study, these airborne particles travel swiftly, at a rate of 6.6 feet (2 meters) per second, and can be seen in the toilet bowl as high as 4.9 feet (1.5 meters) above the ground in just 8 seconds. On the other hand, the smallest aerosols (particles less than 5 microns, or one-millionth of a meter) can cling to surfaces for minutes or more. In contrast, the largest droplets often settle onto surfaces in a matter of seconds.

It’s not only their waste that bathroom patrons have to worry about. Many other studies have shown that pathogens can persist in the bowl for dozens of flushes, increasing potential exposure risk.

Crimaldi said, “The goal of the toilet is to remove waste from the bowl effectively, but it’s also doing the opposite, which is spraying a lot of contents upwards. Our lab has created a methodology that provides a foundation for improving and mitigating this problem.”

At the University of Colorado Boulder, Crimaldi is the Ecological Fluid Dynamics Lab director, which uses laser-based equipment, dyes, and enormous fluid tanks to examine everything from how chemicals travel in turbulent water to how aromas reach our nostrils. It was a matter of convenience, curiosity, and circumstance that led to using the lab’s technology to monitor what transpires in the air when a toilet is flushed.

Scientists 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 could measure their speed and direction. Meanwhile, two cameras took high-resolution images.

The toilet was the same kind commonly seen in North American public restrooms: a lid-less unit accompanied by a cylindrical flushing mechanism—whether manual or automatic—that sticks up from the back near the wall, known as a flushometer-style valve. The brand-new, clean toilet was filled only with tap water

They knew that this spur-of-the-moment experiment might be a waste of time, but instead, the research made a big splash.

Crimaldi said, “We had expected these aerosol particles would float up, but they came out like a rocket.”

“The energetic, airborne water particles headed mostly upwards and backward towards the rear wall, but their movement was unpredictable. The plume also rose to the lab’s ceiling and, with nowhere else to go, moved outward from the wall and spread into the room.”

“The experimental setup did not include solid waste or toilet paper in the bowl, and no stalls or people were moving around. These real-life variables could all exacerbate the problem.”

They also measured the airborne particles with an optical particle counter, a device that sucks a sample of air in through a small tube and shines a light on it, allowing it to count and measure the particles. Smaller particles not only float in the air for longer but can escape nose hairs and reach deeper into one’s lungs—making them more hazardous to human health—so knowing how many particles and what size they are was also important.

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

Journal Reference:

  1. Crimaldi, J.P., True, A.C., Linden, K.G. et al. Commercial toilets emit energetic and rapidly spreading aerosol plumes. Sci Rep 12, 20493 (2022). DOI: 10.1038/s41598-022-24686-5


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