Lasers Shed Light on Why You Need to Close the Lid Before You Flush

Toilets are designed to efficiently empty the contents inside the bowl through a downward motion into the drainpipe. In the flush cycle, water comes into forceful contact with the contents inside the bowl and creates a fine spray of particles suspended in air.

We found that a typical commercial toilet generates a strong upward jet of air with velocities exceeding 6.6 feet per second (2 meters per second), rapidly carrying these particles up to 5 feet (1.5 meters) above the bowl within eight seconds of the start of the flush.

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To visualize these plumes, we set up a typical lidless commercial toilet with a flushometer-style valve found throughout North America in our lab. Flushometer valves use pressure instead of gravity to direct water into the bowl.

We used special optics to create a thin vertical sheet of laser light that illuminated the region from the top of the bowl to the ceiling. After flushing the toilet with a remote electrical trigger, the aerosol particles scatter enough laser light to become visible, allowing us to use cameras to image the plume of particles.

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Even though we expected to see these particles, we were still surprised by the strength of the jet ejecting the particles from the bowl.

A related study used a computational model of an idealized toilet to predict the formation of aerosol plumes, with an upward transport of particles at speeds above the bowl approaching 3.3 feet per second (1 meter per second), which is about half of what we observed with a real toilet.

toilet flush cycle

Water streams forcefully into the toilet bowl during a flush cycle.

SouthHamsian/Wikimedia/(CC BY-SA 4.0)

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