It is not the first transmission source coming to mind, but if you come to think of it, it seems logical. Virus particles are not only present in respiratory droplets, but also in stool. Flushing a toilet can therefore be risky business.
Computational Fluid Dynamics simulations show that flushing gives an upward velocity of 5 m/s. 40% to 60% of the virus particles whirl up to well above the toilet seat, reaching a height of over 1 meter. The risk of exposure is therefore increased.
The authors give recommendations for improved toilet design. Meanwhile, a first practical recommendation is to put the toilet lid down before flushing. Some social distancing in time looks prudent too. Waiting queues might become longer though.