Scientists say they have found an answer to the long-running household debate over whether to close the toilet lid before flushing, according to a study published in the journal Science of The Total Environment. The research points to fine water droplets released into the air during flushing as the source of the problem.
The study explains that when a toilet is flushed, fast-moving water strikes surfaces and creates turbulence, producing droplets that can rise into the air and remain suspended as aerosols. These droplets can carry microorganisms from both the water and the user.
Why airborne particles matter
The researchers noted that airborne particles from flushing can potentially carry microbes capable of causing illness in people, a risk that came into focus during the COVID pandemic but has also been identified as a transmission route for viruses such as influenza and norovirus. Some studies have detected these aerosols at the height of an adult's breathing zone, according to Lyra Adiyani, a PhD candidate at Flinders University in Australia, who said this points to a potential route of inhalation exposure.
Adiyani and her co-authors carried out a systematic review of 22 previously published studies on aerosols generated by toilets to examine the question in more depth.
Lid position changes direction, not amount
The researchers found that the available evidence suggests the position of the lid is more likely to affect how aerosols disperse than how much is produced overall. In other words, people who leave the lid open are not entirely wrong, since closing it does not appear to significantly cut the volume of aerosols released.
What closing the lid does appear to do, the researchers said, is change the direction those particles travel. With the lid open, aerosols spread more widely into the surrounding environment, whereas with the lid closed, particles can be redirected through the gaps between the lid, the seat and the rim of the bowl.
Aerosols can still escape, but the likelihood of inhaling them is reduced. The researchers said the size of the gap and the design of the toilet seat could theoretically be adjusted to cut down further on the aerosols that get out.
Other ways to reduce exposure
Better bathroom ventilation could also help, though the authors said their results on this point were not statistically conclusive. For toilets with a dual-flush system, choosing the smaller flush volume can also help minimize aerosol formation, the study found. Regular cleaning and maintenance of bathrooms were also identified as ways to limit microbial contamination.
Based on the available data, Adiyani said she would recommend closing the toilet lid before flushing, where one is fitted, as a precautionary step to minimize potential exposure through inhalation, while acknowledging that doing so does not stop aerosols from being released entirely.
