Flushing a toilet releases a cloud of tiny particles that can carry microorganisms and remain suspended in the air at the breathing height of an adult for at least 20 seconds, according to a systematic review published in the journal Science of the Total Environment by researchers at Flinders University and the University of Adelaide in Australia.
The team, led by doctoral candidate Lira Adiyani and environmental microbiologist Harriet Whiley, examined 22 existing studies on the aerosols and bioaerosols generated when a toilet is flushed. Some of those studies detected aerosols at adult respiratory height, pointing to a possible inhalation exposure pathway that most people never consider during an ordinary bathroom visit.
«Flushing the toilet creates turbulence that can release aerosols into the surrounding air. Some studies detected these aerosols at the breathing height of an adult, indicating a possible route of exposure through inhalation», Adiyani said.
The review also identified several factors that shape how many particles are released and how they spread. One of its clearest findings is that when the concentration of microorganisms in the toilet water rises, so does the concentration of biological particles released during the flush. The highest concentrations of bioaerosols were found on the toilet seat, a result that underscores the importance of cleaning not only the bowl but also the surfaces around it.
Earlier experimental work cited in the review offers a sense of the scale involved. Siddhartha Verma, a professor of mechanical engineering at Florida Atlantic University, described tests involving more than 100 flushes over roughly three hours. «We found a substantial increase in the aerosol levels measured in the environment, with the total number of droplets generated in each flush test reaching tens of thousands», he said.
Aerosols rose as high as 109 centimetres above toilets and up to 69 centimetres above urinals, and some particles stayed airborne for as long as 20 seconds, matching observations from previous studies. Particle measurements showed increases of 69.5 per cent for particles between 0.3 and 0.5 micrometres, 209 per cent for those between 0.5 and 1 micrometre, and 50 per cent for particles between 1 and 3 micrometres.
Closing the lid before flushing helps, but the evidence suggests it does not solve the problem entirely. A lowered lid can alter the path of the particles, yet it does not block them completely. Under certain conditions, aerosols can escape through the gaps between the lid and the seat and drift into other parts of the bathroom. Even so, Adiyani recommends lowering the lid as a precaution.
«Based on the available evidence, I would recommend closing the toilet lid before flushing, if a lid is available, as a precautionary measure to minimise possible exposure through inhalation, although I acknowledge that this does not completely prevent the release of aerosols», she said.
Ventilation is another factor worth attention. The reviewed studies did not establish a sufficiently clear relationship between ventilation and the quantity of aerosols produced, but air currents can influence how particles travel and disperse. Maintaining adequate ventilation therefore remains a useful measure for improving air quality in enclosed spaces.
Turbulence lies at the heart of the phenomenon. When a toilet is flushed, water enters at high speed, strikes the porcelain walls and mixes with air. This agitation breaks the water surface into numerous particles and produces a column of fine droplets that physicists call a plume. Several years ago, engineers at the University of Colorado Boulder visualised the process using lasers and cameras installed in a public restroom equipped with a flushometer toilet, the lidless model common in many American bathrooms. Their measurements recorded a flow exceeding 2 metres per second, capable of lifting aerosols up to 1.5 metres in less than 10 seconds.
For households and public facilities alike, the findings point to practical habits: close the lid where one exists, keep bathrooms ventilated, and clean the seat and surrounding surfaces regularly, since those areas carry the highest concentrations of bioaerosols after a flush.