Health AIST study: close the toilet lid before flushing

A team of researchers at the National Institute of Advanced Industrial Science and Technology (AIST) has conducted a study to determine the most hygienic way to use a toilet, particularly in light of increased hygiene concerns following the COVID-19 pandemic. The study focused on the practice of closing the toilet lid before flushing, a measure that gained popularity during the pandemic, often accompanied by various unverified claims.

Researchers installed a siphon-type toilet bowl in a sealed booth simulating a private bathroom to investigate the spread of droplets and viruses during flushing. By analyzing the behaviour of droplets and potential contaminants, they aimed to provide scientifically sound guidelines for toilet hygiene.

The method used 6 liters of water for each wash. The scientists studied the droplets and aerosols (the smallest particulates among droplets) when the toilet was flushed with the lid open. The droplets flew up to 40-50 centimeters and fell, while aerosols floated in the air. Smaller aerosols could drift in a private room for several tens of minutes, according to the researchers. Next, the researchers measured the aerosols with a device called a particle counter. When the toilet was flushed with the lid open, the particles spread over a wide area, including 40 cm above the toilet bowl and 15 cm in front of it on the user's side. When the toilet was flushed with the lid closed, the aerosols did not leak to the upper part of the toilet, and the amount decreased significantly. On the other hand, aerosols leaked out toward the front of the toilet bowl. This is believed to be due to air being pushed out of the gap between the lid and the toilet bowl by the water flow. When the toilet is flushed with the lid closed, the aerosol effect may be reduced if the toilet is at least 15 cm away from the lid.

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The researchers concluded that closing the toilet lid before flushing is more hygienic. To further investigate, they conducted a test using simulated viruses to track their spread. The results showed that about a third of the viruses adhered to the walls and underside of the toilet seat. As a result, the researchers recommend cleaning the toilet bowl and the surrounding surfaces. However, they emphasized that the amount of virus spread is minimal, and not all toilets have lids. A senior researcher at AIST, Takashi Fukuda, reassured that Japanese toilets are generally well-maintained for hygiene, so there's no need for excessive worry, even when flushing without a lid.



Earlier studies reached similar conclusions: