A research team, including specialists from the National Institute of Advanced Industrial Science and Technology (AIST), set out to resolve lingering questions about whether closing the toilet lid before flushing enhances hygiene. Prompted by increased hygiene measures and differing messages during the COVID-19 pandemic, the study examined droplet and aerosol behavior when a typical siphon-type toilet was flushed, a model that efficiently removes waste with a strong water flow using 6 liters of water per flush.
The experiment was conducted in a sealed environment that mimicked a private restroom. Researchers first observed that when the toilet was flushed with the lid open, droplets traveled up to 40 to 50 centimeters before falling, and the resulting aerosols remained airborne, some for several tens of minutes. Using a particle counter, the team noted that particles spread out over a broad area, reaching 40 centimeters above the bowl and 15 centimeters in front, exposing the user to these tiny particulate matters.
When the toilet was flushed with the lid closed, the spread of aerosols was significantly reduced in the upper part of the bowl although some leakage occurred toward the front due to air being forced out through small gaps between the lid and the bowl. The research suggested that if the gap is kept at least 15 centimeters away from the water flow, the risk of aerosol spread can be minimized. Additionally, tests with simulated viruses showed that roughly one-third of these particles adhered to the walls and the underside of the toilet seat, highlighting the importance of cleaning these areas despite the overall minimal virus count.
Takashi Fukuda, a senior researcher at AIST’s Sensing System Research Center, reassured that even if a toilet is flushed with the lid open or lacks a lid entirely, Japanese hygiene standards typically make such concerns minor. The researchers hope that these findings will contribute to improved practices in bathroom hygiene and increased public awareness about proper toilet management.