Photo/Illutration A more hygienic way to flush? (Asahi Shimbun file photo)

A team of researchers has tackled a riddle that surfaced after the COVID-19 outbreak: Is it more hygienic to close the toilet lid before flushing?

The protocol to close the lids appeared in more signs at public restrooms during the pandemic, when awareness of hygiene management increased and various information circulated, some of which had no known scientific basis.

Researchers from the National Institute of Advanced Industrial Science and Technology (AIST) and others conducted an experiment to scientifically clarify precautions for toilet use and hygiene management.

Specifically, they examined the trends of droplets and viruses when a toilet is flushed.

In the experiment, a toilet bowl was installed in a sealed booth that resembled a private toilet room.

The experiment used the most common washing method for the bowl, called the siphon type, in which water and excrement are effectively washed away by strong suction.

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.

The researchers found that it seemed more hygienic to close the lid before flushing.

The researchers then examined where the leaked viruses would adhere to when the lid was closed, using a sample of simulated viruses.

The results showed that about one-third of the viruses adhered to the walls and the underside of the toilet seat.

The researchers recommend wiping off the walls as well as the toilet bowl when cleaning.

However, the amount of viruses that adhered to the walls and the underside of the toilet seat was very small, the researchers said.

And not all toilets have lids.

Takashi Fukuda, a senior researcher at the AIST’s Sensing System Research Center, said that even if the toilet is flushed without a lid or with the lid open, there is no need to be overly nervous because Japanese toilets are basically well managed for hygiene.

“We hope the results of our research will be useful for more hygienic toilet management and use, and for improving hygiene awareness,” he said.

The results of this study were presented at an international conference in November.

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