Flushing a toilet generates "strong turbulence within the bowl" that can lead to "large-scale virus spread" if the lid isn't closed, worrying new COVID-19 research has found.
The study, conducted by researchers at Yangzhou University in China, was published in scientific journal Physics of Fluids earlier this month.
Researchers used a discrete phase model to simulate toilet flushing, observing a "massive upward transport of virus particles" that the study's authors have since described as "alarming".
"Forty to sixty percent of particles reached above the toilet seat, leading to large-scale virus spread," the study's abstract reads.
The research shows toilets can be a place where COVID-19 spreads easily - due to the obvious heightened risk of fecal-oral transmission - and suggests there is a simple way to curb this risk that the public at large doesn't know about.
"As can be seen from our daily experience, flushing a toilet can cause violent turbulence, which will aid large-scale spread of viruses present in the toilet bowl," the research reads.
"It has also been shown that flushing the toilet without putting the lid down is a bad habit. Health risks brought by the improper toilet flushing were also identified ... where aerosol particles carrying viruses could spread indoors.
"However, a clear and comprehensive explanation has not been given to the public, and therefore, there has not been widespread acceptance and implementation of this simple precaution."
Good hygiene practices have been touted by public health experts in New Zealand and around the world as one of the key ways to combat the spread of coronavirus, as the disease is transmitted most easily via droplets.
However the practice of closing the toilet lid before flushing has not been assigned anywhere near the same importance as hand-washing, wiping down common surfaces or the use of face masks.
Nearly 8 million people have been infected with coronavirus across the globe since the original outbreak in Wuhan, China late last year. The virus has caused the deaths of 435,000 people worldwide.