A series of grim experiments have revealed how toilet water sprays when we flush with the lid open, spreading potentially deadly germs into the air.
Scientists from the University of Colorado say an "invisible plume", which contains microscopic particles of pee or poop, is ejected from the toilet after flushing and can remain in the air for minutes.
Since the particles are invisible to the naked eye, researchers used bright green lasers and cameras to reveal how they are rapidly spread from a lidless public cubicle.
Lead author Professor John Crimaldi 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."
The airborne droplets shoot out at speeds of 6.6 feet per second, reaching 4.9 feet above the toilet within eight seconds.
The largest settle onto surfaces., while smaller ones less than five microns, or one-millionth of a metre, linger suspended for many minutes.
They can transport E. coli, C. difficile, noroviruses and adenoviruses. The virus that causes Covid-19, called SARS-CoV-2, has also been detected in human waste.
There is not currently conclusive evidence that it spreads efficiently through toilet aerosols.
Prof Crimaldi said: "People have known 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 even the people who knew about this understood."
Scientists warned that the pathogens can persist in the bowl for dozens of flushes - increasing potential exposure risk.
Prof Crimaldi said: "The goal of the toilet is to effectively remove waste from the bowl, 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."
The University of Colorado at Boulder team used two lasers. One shone continuously on and above a 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 itself was the same kind commonly seen in public lavatories in the US - a lidless unit accompanied by a cylindrical flushing mechanism.
Whether manual or automatic, a flushometre-style valve sticks up from the back near the wall. The brand-new, clean toilet was filled only with tap water.
Prof Crimaldi said: "We had expected these aerosol particles would just sort of float up, but they came out like a rocket."
The energetic, airborne water particles headed mostly upwards and backwards 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 forward, into the room.
The experimental set-up did not include any solid waste or toilet paper in the bowl, and there were no stalls or people moving around.
These real-life variables could all exacerbate the problem, said Prof Crimaldi.
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 dangerous to humans.
The disconcerting results shed fresh light on just how many particles end up in the air - and what size they are.
But they provide experts in plumbing and public health with a consistent way to test improved design, disinfection and ventilation strategies to reduce exposure risk.
Prof Crimaldi added: "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."
Researchers have known for over 60 years that when a toilet is flushed, solids and liquids go down as designed, but tiny, invisible particles are also released into the air.
Previous studies have used scientific instruments to detect the presence of these airborne particles above flushed toilets.
But no one understood what the plumes looked like - or how they dispersed.
The study was published in the journal Scientific Reports.
