Scientists are urging people not to use the call of nature as an excuse to take a break from wearing their face mask.
Flushing loos and urinals can fire virus particles into the air, Chinese physicists have found.
In a new study, they ran simulations which revealed "disturbing" findings.
Flushing a typical loo sent more than half the virus particles inside it into the air. Urinals were worse, potentially dangerous virus particles showing a "more violent climbing tendency" - flying higher and faster.
"From our work, it can be inferred that urinal flushing indeed promotes the spread of bacteria and viruses," said Xiangdong Liu of Yangzhou University.
"Wearing a mask should be mandatory within public restrooms during the pandemic, and anti-diffusion improvements are urgently needed to prevent the spread of COVID-19."
Since June, the World Health Organization has urged people to wear masks when out of the home, with increasing evidence the virus behind the COVID-19 pandemic lingers in the air, aiding its spread.
Prior research has also found the virus can be detected in patients' faeces and urine.
"Two of the COVID-19 reemerging confirmed cases in Beijing have been reported to be infected from a public toilet, which practically proves the danger from the public restroom," the study, published in journal Physics of Fluids, reads.
The researchers called the new findings a "serious public health challenge".
