Flushing a toilet releases an aerosol plume of breathable pathogens that can reach adult breathing height and remain suspended for at least 20 seconds. Flinders University researchers reviewing 22 studies recommend closing the lid as a precautionary measure, though experts warn bathroom contamination extends far beyond the toilet bowl.
When you flush a toilet, high-velocity water impacts surfaces and creates turbulence that projects droplets into the surrounding air as airborne aerosols. These particles can contain biological material capable of transmitting pathogens including legionella, influenza, and norovirus, according to a systematic review published in the journal Science of the Total Environment.
Researchers at Flinders University analyzed 22 studies examining toilet-generated bioaerosols. The investigation found that microbial concentrations in flush water directly influence the volume of hazardous particles released into the room. Larger flush volumes create greater turbulence, which in turn generates higher concentrations of suspended aerosols.
What Lira Adiyani and Flinders University Researchers Found About Lid Position
The debate over whether to leave toilet seats up or down has divided households for generations, but the new review offers some clarity on how closing the lid alters the physics of a flush. While the review found that lid position and ventilation were not statistically linked to overall aerosol quantity, it plays a distinct role in directional dispersion.
When the lid is open, particles disperse broadly into the room. When the lid is closed, aerosols are redirected rather than eliminated, escaping through the gaps between the lid, seat, and bowl rim rather than shooting straight upward.
“Based on the available evidence, I would recommend closing the toilet lid before flushing, where a lid is available, as a precautionary measure to minimise potential inhalation exposure – while recognising that it does not completely prevent aerosols from escaping.”
Lira Adiyani, PhD student and lead author at Flinders University
Some studies evaluated in the review detected these bioaerosols at heights corresponding to the breathing zone of adults. Once airborne, the particles can remain suspended for a minimum of 20 seconds. Because small aerosols penetrate deep into the human respiratory tract when inhaled, they establish a direct exposure pathway according to the research team.
Professor Harriet Whiley on High-Risk Vulnerabilities and Building Design
While everyday household environments pose minimal risk for healthy individuals, researchers emphasize that institutional settings require a different standard of engineering. Co-author Professor Harriet Whiley pointed out that water systems—particularly recycled or rainwater supplies—can harbor opportunistic pathogens.
“They don’t affect healthy people, but if you’re immunocompromised you can get very sick, and it can be fatal.”
Professor Harriet Whiley, Flinders University’s College of Science and Engineering
To combat these risks, the research team is focusing on protecting the most vulnerable through improved architectural planning in hospitals and aged-care facilities. Designing toilets that achieve an effective flush using lower water volumes and reduced turbulence remains a primary engineering priority for modern water security challenges.
Beyond the mechanics of the flush itself, experts note that public restrooms present multiple vectors for microbial exposure.
From door handles and floors to bathroom dryers and faucets, microbes blanket facility surfaces. Compounding the issue is the widespread use of personal electronics. Citing independent observations, Dr. Tajouri noted that between 50% and 80% of individuals—and nearly 100% of younger generations—bring mobile phones into restrooms, frequently transferring fecal microorganisms like salmonella and campylobacter directly to their fingers and devices.
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