Lidless toilets spread poop droplets – worse than you think

lidless toilet, physics of bathroom, particles of urine, faeces, covid

A never-before-seen look at bathroom physics.

Yeah my mom told me that Dr. Phil told her to put down the lid of the toilet when we flush. But do droplets of urine, faeces and infectious viruses like Covid spread with every flush?

According to scientists reporting in a new study at Nature, they do. The researchers used lasers to illuminate in unprecedented detail the aerosol cloud emitted by a flushing toilet of a common type in US public restrooms. Sort of in the way Laurie Tumer illuminated the spread of pesticides in her photographic artwork (see my Grist story from 2005).

So maybe Dr. Phil started it, but more than a decade now, researchers have been saying that lidless toilets do eject droplets of water containing things that might harm us. The droplets carry urine, faeces and, potentially, infectious diseases such as Covid-19 and norovirus. What wasn’t clear to the scientists is how quick and wide the particles spread after the flush.

toilet physics, flush laser and particles of poop flying in the air, green laser

The lidless toilet setup in the lab. A green laser catches droplets of toilet matter as it flies in the air

Within 8 seconds, these toilet drops shoot up to our breathing zone and no doubt we are putting them inside our body. Time for a toilet redesign at public restrooms which typically do not have lids? Lead researcher John Crimaldi thinks so; “Look at the video,” he says. “It couldn’t possibly be worse.”

How high poop particles soar when we flush 

In the study the researchers from University of Colorado positioned a laser to beam a laser of green light above the lidless bowl: If any normally invisible particles emerged as the toilet flushed they would scatter the light and appear as green dots. To record the physics of the toilet, the team rigged up a camera a few feet away.

Then … flush.

What happened silenced everyone: “We were all just stunned,” recalls Crimaldi. “We saw this incredibly energetic jet of particles shooting up towards the ceiling.”

A cloud of particles flew into the air and hovered. 

“People just started laughing. They were like, ‘Oh, my God, you’ve got to be kidding me. That’s what happens when you flush the toilet?’” he recalls.

Should we be worried about lidless toilets?

“The bowl water in a toilet contains elevated levels of pathogens for dozens of flushes after that contaminated feces was put in there,” Crimaldi says.

So probably, yeah.



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