Flushing the toilet with the lid up creates a cloud of spray that can be breathed in and may spread infection, such as coronavirus, say researchers.
Chinese scientists calculate that flushing can propel a plume of spray up and out of the toilet bowl, reaching head height and beyond.
Droplets can travel up to 3ft – or 91cm – from ground level, according to the computer model used by the scientists from Yangzhou University.
Shutting the lid would avoid this.
The work is published in the journal Physics of Fluids.
Coronavirus is spread through airborne droplets from coughs and sneezes, or objects that are contaminated with them.
People who are infected can also have traces of the virus in their faeces, although it is not yet