Closing the toilet lid before flushing? Science still cannot say for sure [Huam-dong Paper Lab]

Closing the toilet lid before flushing is common hygiene advice. But a new analysis finds that the practice's effectiveness has never actually been scientifically confirmed. Although more than 20 studies have examined toilet flushing, almost none of them measured what happens when the lid is closed.

A research team led by Lira Adiyani and Harriet Whiley at Flinders University in Australia recently published an analysis in the international journal Science of the Total Environment, Volume 1048. The analysis pooled data from 22 existing studies on airborne microorganisms generated by toilet flushing. Rather than running new experiments, the team gathered scattered figures from prior research and re-examined them together.

Water droplets from a flush can fly as high as a person's head

When a toilet is flushed, the fast-moving water strikes the bowl and creates turbulence. Some of the smaller droplets thrown up in the process rise into the air, potentially carrying microorganisms from the water and waste.

Previous studies have detected airborne droplets as far as 150 centimeters horizontally from the toilet and as high as 163 centimeters above the floor. That is roughly the height at which a standing adult breathes, between 150 and 170 centimeters. This raises the possibility that droplets thrown up by a flush could enter a person's airway the moment they stand up after flushing.

The droplets stay airborne for at least 20 seconds after a flush. Most measure 3 micrometers or smaller, small enough to bypass the nose and throat and travel deep into the lungs.

The research team said these figures come from short, human-scale measurements, and that droplets could travel much farther than 150 centimeters if carried by air currents.

After pooling and statistically analyzing the 22 studies, the team found that airborne microbial concentration was driven mainly by two factors: the amount of microorganisms in the water and the force of the water flow.

When the microbial concentration in the water rose tenfold, the airborne concentration increased 2.6-fold. For every additional liter of water used per flush, airborne concentration rose 1.3-fold. A larger volume of water per flush also means stronger turbulence.

In practice, the household toilets used in the studies varied widely, using as little as 3 liters or as much as 13.3 liters per flush. Even among toilets of the same type, water use per flush varied more than fourfold depending on the model.

Factors that determine airborne microbial concentration when a toilet is flushed. Microbial concentration in the water and the volume of water per flush were both confirmed to raise airborne concentration, while the effects of ventilation and lid position remain inconclusive due to a lack of data. [Science of the Total Environment, Volume 1048]

The toilet lid may help, but not by a statistically meaningful margin

However, airborne microbial concentration was 23 times higher when the toilet lid was left open than when it was closed. That leaves room to argue that closing the lid reduces the amount of water thrown into the air, but the figure carries no statistical weight. Only a single study in the pooled data actually measured flushing with the lid closed, making any comparison effectively impossible.

The research team suggested that rather than reducing the amount of droplets produced, a closed lid may simply redirect where they end up. Even with the lid shut, droplets can still escape through the lid itself or through gaps around it. Still, the team could not draw firm conclusions about this ventilation effect either, citing insufficient related research.

Setting the statistical uncertainty aside, the researchers advised closing the lid before flushing, turning on a ventilation fan or opening a window, and using toilets that use less water per flush. They said such measures can reduce both the amount of airborne droplets and how long they remain suspended.

The team said the analysis has a limitation: because the 22 underlying studies each used different measurement methods, direct comparisons between them are difficult.

Reference paper

DOI: 10.1016/j.scitotenv.2026.182111

Citation: L. Adiyani, K. Ross, B. Van den Akker, and H. Whiley, "Aerosol and bioaerosol generation from toilet flushing: A systematic review and risk factor analysis," Science of the Total Environment 1048 (2026) 182111.

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