Green lasers show that the toilet lid should be closed before flushing

Engineers at the University of Colorado Boulder have confirmed what the germ-phobes among us have long suspected: flushing a commercial toilet releases a Vesuvius-like cloud of tiny droplets and aerosol particles that reaches more than 5 feet above the seat.

Though invisible to the naked eye, when illuminated by green lasers, the cloud appears like a burst of microscopic confetti being thrown at the world’s biggest party, a party made of tiny drops of water and everything else is still in the bowl.

The research, published this month in the journal Scientific Reports, was entirely an exploration of fluid mechanics. The team flushed toilets containing only clean water and did not examine the infectivity of any particles that might have been in the plume.

But their tools confirmed that every flush extends far beyond the bowl than most of us would like to believe.

“We were all amazed,” said John Crimaldi, the study’s lead author. “You said, ‘Oh my God— that’s it What happened?”

Crimaldi is a professor of hydrology specializing in fluid mechanics – specifically how air and water transport other entrained materials. He has studied how ocean currents disperse sperm and eggs to fertilize corals and how odor particles travel through the air to relay information to animals.

He turned his attention to toilets at the urging of his Bouldering colleague and co-author Karl Linden, an environmental engineer who studies the sanitizing properties of ultraviolet light.

When brainstorming the ideal test case for a UV-based surface sanitizer, Linden first had to go to some dirty places.

“Where are we exposed to viruses, and where are we exposed to pathogens?” said Linda. “And one of the thoughts I had was, ‘Well, what happens in toilets?'”

Linden specifically envisioned commercial toilets: the tankless, lidless workhorses found in public toilet stalls. Most public restrooms in North America are equipped with what is known as a flushometer valve, which relies on pressure rather than gravity to force water through the bowl.

The result is a powerful purge that leaves a fine cloud of water vapor behind—a smaller, less cheery version of the mist that rises above each tree trunk as it makes its final plunge into Disneyland’s Splash Mountain.

Previous studies have confirmed that surfaces around public toilets are often breeding grounds for fecal bacteria. Ultraviolet light could be an effective disinfectant, Linden thought—but first he needed a better understanding of how microscopic pathogens move in space.

He turned to Crimaldi, whose lab uses lasers to visualize fluid motion otherwise imperceptible to the human eye. Crimaldi’s Fluid Mechanics Laboratory has an annual summer tradition of taking a week to tackle a bite-sized scientific challenge, without funding or publication pressure. The toilet question fitted perfectly.

“We said, ‘Maybe nothing will come of this, or maybe we’ll get something really cool,'” Crimaldi said.

Researchers Aaron True (left) and John Crimaldi in a lab

Researchers Aaron True (left) and John Crimaldi stand in the lab studying aerosols emitted from a flush toilet.

(Patrick Campbell / University of Colorado Boulder)

Instead of carting their gear to the nearest bathroom, the team installed a working toilet in the lab on a metal frame that could be aimed with their lasers. They then calibrated the plumbing’s water pressure to match that of a typical commercial toilet.

They knew their laser would make some aerosols visible. They weren’t prepared for the little explosion that greeted them on the first flush.

“It’s like a volcanic eruption,” Crimaldi said. “Some of us were stunned into silence. Some of us just laughed in disbelief and also kind of said, ‘Oh my god, we’re really on to something here.’”

The team then pointed a pulsed laser and a pair of scientific cameras at the spray to measure the speed of its individual water particles. A toilet flush doesn’t have the speed of a sneeze, where droplets can squirt at up to 100 miles per hour, or even a cough, whose droplets can fly at up to 50 miles per hour.

Aerosols in the “surprisingly energetic and chaotic” cloud reached a maximum speed of 2 meters per second, or just under 4.5 miles per hour, the study authors reported. But once in the air, it took them a while to settle down. Almost eight seconds after the flush, particles were still floating more than 5 feet above the rim of the bowl — well above nose-level for most people. Many stayed in the air for more than a minute.

“Having seen these videos, I’m certainly much more apt to wear a mask in a public restroom than I might have been before,” Crimaldi said.

Although the experiments were conducted using toilets without clean water, Crimaldi strongly suspects that adding toilet paper and human waste to the mix will only add more chaos and energy to a flush.

“I have an intuitive feeling that the presence of solids might make the problem worse because it’s just extra things for the water to hit and to create more opportunities for this energetic liquid mixing,” he said.

Linden hopes to use this experiment as a starting point for future research tracking how far bacteria and other pathogens travel in these aerosol clouds and how long they remain infectious. Depending on these insights, “can we start thinking about, well, what interventions can we use?” he said. “What toilet redesigns could we consider?”

Proponents of alternatives to flushing toilets said the study strengthened the case for finding better ways of disposing of human waste.

“This new study provides some dramatic visual evidence of another downside of traditional Western toilets and our strong desire to flush and forget,” said Bryn Nelson, microbiologist and author of Flush: The Remarkable Science of an Intraly Treasure. “Many composting toilets use vacuum flushing and little to no water, so this could be another reason to consider the merits of these eco-friendly models.”

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