Engineers at the University of Colorado Boulder have confirmed what the germ-phobes among us have long suspected: A commercial toilet flush releases a Vesuvius-like cloud of tiny droplets and aerosol particles that reach more than 5 feet above the seat.
Although it’s not visible to the naked eye, when illuminated by green lasers, the column looks like a splash of microscopic confetti thrown at the world’s largest party, one made up of tiny droplets of water and whatever else might be in the bowl.
The research, published this month in the journal Scientific Reports, was just an exploration into fluid mechanics. The team only flushed toilets with clean water, and didn’t investigate how infectious any particles might be in the plume.
But their tools confirmed that each stream reaches much further than most of us would like to believe.
“We were all amazed,” said John Crimaldi, lead author of the study. “I said, ‘Oh, my God– This What happens?”
Crimaldi is a professor of hydrology who specializes in fluid mechanics—specifically, how air, water, and other materials move in flow with it. He looked at the ways ocean currents distribute sperm and eggs to fertilize coral reefs, and how odor molecules travel through the air to communicate information to the animals.
He turned his attention to toilets at the urging of fellow Boulder resident and co-author Carl Linden, an environmental engineer studying the disinfecting properties of UV light.
While brainstorming the perfect test case for a UV-based surface disinfectant, Linden’s mind first had to go to some dirty places.
“Where are we exposed to viruses, and where are we exposed to pathogens?” Linden said. And one of the thoughts I had was, ‘Well, what’s going on in the toilets?'” “
Linden was specifically depicting commercial toilets: the tankless toilets found in public toilet stalls. Most public toilets in North America are equipped with what is known as a flush meter style valve, which relies on pressure rather than gravity to force the water through the bowl.
The result is a high-powered flow that leaves a dazzling cloud of water vapor in its wake – a smaller, less cheery version of the mist that rises over each log as it finally sinks into Disneyland’s Splash Mountain.
Previous studies have confirmed that surfaces around public toilets are often hotbeds for faecal bacteria. Linden believed that UV light could be an effective disinfectant — but first, he needed to better understand how microscopic pathogens move throughout space.
He turned to Crimaldi, whose lab uses lasers to visualize fluid movements that would otherwise be imperceptible to the human eye. The Crimaldi Fluid Mechanics Laboratory has an annual summer tradition of taking a week to tackle a small-scale science challenge, with no funding or any pressure to publish. The toilet question was quite appropriate.
“We said, ‘Maybe nothing will come, or maybe we’ll get something really cool,'” Crimaldi said.
Instead of hauling their equipment to the nearest bathroom, the team installed a female lab worker’s toilet on top of a metal frame that could align their lasers. Then they calibrated the water pressure in the plumbing to match typical commercial water pressure.
They knew that their laser would make some aerosols visible. They weren’t prepared for the slight explosion that greeted them on the first flush.
“It’s like a volcano erupting,” said Crimaldi. Some of us were stunned in the silence. Some of us were just laughing out of disbelief, and also kind of like, “Oh, my God, we’re really onto something here.”
Then the team trained a pulsed laser and a pair of science cameras on the spray to measure the speed of individual water molecules. Toilet flushes don’t have the speed of a sneeze, which can send droplets flowing at up to 100 mph, or even a cough, which can travel droplets at 50 mph.
The study authors reported that the aerosols in the “surprisingly energetic and chaotic” plume had a maximum velocity of 2 meters per second, or just under 4.5 miles per hour. However, once they were up in the air, they took a while to settle down again. Roughly eight seconds after the flush, the particles still hovered more than 5 feet above the rim of the bowl—beyond nose level for most people. Many remained in the air for more than a minute.
“I’m definitely a lot more inclined after watching these videos to wear a mask in a public restroom than I would have before,” Crimaldi said.
Although the experiments were conducted with toilets devoid of all but clean water, Crimaldi strongly suspects that adding toilet paper and human excrement into the mix only results in more mess and energy to flow.
“I have an intuitive feeling that the presence of solids might exacerbate the problem because there are just additional things that the water can affect and create more opportunities for this active mixing of the liquids,” he said.
Linden hopes to use this experiment as a starting point for future research that tracks the distance that bacteria and other pathogens travel in these aerosol clouds, and how long they remain infectious. Based on these findings, “we can start to think, OK, what interventions can we use?” He said. “What are some toilet remodels we might want to consider?”
Advocates for alternatives to flush toilets said the study strengthens the case for better ways to dispose of human waste.
“This new study adds some exciting visual evidence of another flaw of traditional Western toilets and our strong desire to flush and forget,” said Brian Nelson, a microbiologist and author of Flush: The Remarkable Science of an Improbable Treasure. “Many composting toilets use a suction flush and little to no water, so this could be another reason to consider the benefits of these eco-friendly models.”
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