Flushing the toilet can release aerosols and bioaerosols into the surrounding air, with some particles reaching heights corresponding to the breathing zone of adults, according to a systematic review by researchers at Flinders University in Australia.
Published in the scientific journal Science of the Total Environment, the study analyzed 22 works on aerosol production during toilet flushing and the factors that influence the amount of particles released and their dispersion in space.
“Flushing the toilet creates turbulence that can release aerosols into the surrounding air. Some studies detected these aerosols at the height of adults’ breathing, indicating a potential inhalation exposure pathway,” explains Lira Adiyani, the study’s lead author and a PhD student at Flinders University.
According to the researchers, the microorganisms present in these aerosols may originate from toilet contamination during use as well as from the water used in the flush. The higher the amount of microorganisms present in the toilet, the more bioaerosols may be released.
“The greater the number of microorganisms present in the toilet, the more bioaerosols can be released. Therefore, larger flush volumes generally produce more aerosols,” says Lira Adiyani.
Particles can remain in the air for at least 20 seconds
The concentration of aerosols varied considerably across the studies analyzed, depending on experimental conditions, toilet characteristics, and the methods used to collect samples.
In trials where microorganisms were deliberately introduced into the toilet water, higher concentrations of these organisms were associated with higher concentrations in the aerosols. Higher concentrations were also frequently recorded near the toilet, especially at the level of the seat.
Aerosols were also detected in the air for at least 20 seconds after flushing.
Although the review did not find a statistically significant association between ventilation or the position of the lid and the concentration of aerosols, the researchers consider that both factors may influence how particles disperse in the bathroom.
Adequate ventilation may help to disperse and remove suspended particles from the air. Meanwhile, using the lid may alter the direction of dispersion: rather than rising directly, aerosols may escape through the openings between the lid and the toilet seat.
Therefore, the researchers recommend closing the lid before flushing, when one exists, although they acknowledge that the measure does not completely prevent the release of aerosols.
“Based on the available evidence, I would recommend closing the toilet lid before flushing, when a lid exists, as a precaution to minimize potential inhalation exposure—recognizing, however, that this does not completely prevent aerosol leakage,” emphasizes Lira Adiyani.
Hygiene and lower-volume flushes can help
The authors argue that good toilet hygiene, including regular cleaning of the toilet itself and surrounding surfaces, as well as handwashing after use, could help reduce microbial contamination and exposure to microorganisms potentially causing diseases.
Flush volume was identified as another important factor in aerosol production. The researchers therefore suggest that using toilets designed for lower-volume flushes or, where appropriate, the option of lower volume on dual-flush toilets, could contribute to reducing the amount of aerosols generated.
The research also highlights the need to develop engineering solutions capable of reducing bioaerosol formation, especially in places where the risk of microorganism transmission is higher, such as hospitals.
“There is a need for more research to develop design and engineering interventions that reduce the bioaerosols generated by toilets, particularly in high-risk settings, such as hospitals,” says Harriet Whiley, co-author of the study and a professor at the College of Science and Engineering of Flinders University.
The researchers thus consider that proper bathroom ventilation and closing the lid before flushing can constitute simple precautionary measures. However, more studies are needed to determine with greater precision the effectiveness of these measures in reducing aerosol dispersion and inhalation exposure.
