A study from Chinese researchers at Yangzhou University on Wednesday found that the flushing action from both toilets and urinals can release clouds of virus-teeming aerosols, making public restrooms a potentially risky place for transmission. The researchers’ advice: If you’re going to use a public restroom, wear a mask.

“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,” the researchers warned.

Flushing a toilet or urinal has always posed some kind of hygiene risk, as the action creates an interaction between gas and air. In the paper, titled Virus Transmission from Urinals, published in the journal Physics of Fluids, the researchers found that a urinal was potentially even more risky. The research used a model to track aerosol particles released during flushing, and found that in a urinal, those particles could spread up to a typical man’s thigh level in about five and a half seconds, versus around half a minute for a flushing toilet.

As a result, flushing from urinals produces a “more violent climbing tendency,” the researchers said, adding it’s something they had noticed outside of their research.

This latest study builds on previous evidence that COVID-19 can spread through restrooms. As early as February, Chinese researchers flagged that the virus could spread through feces, and one of the virus routes of transmission could be via the “fecal-oral” route—a particular worry on cruise ships.

Queasiness aside, the research highlights just how difficult it is to track and understand the many sources of virus transmission—a struggle that is frustrating even the most diligent governments the world over, including New Zealand, which previously announced it had contained the virus.

And it appears to underscore a simple message to life in the COVID-19 era: If you must use a public toilet, bring your mask.