Japan researchers ask: Does flushing toilet with lid closed help prevent disease spread?

TOKYO — With the arrival of the season marked by heightened concerns about influenza and other infectious diseases, hygiene habits developed during the COVID-19 pandemic remain in focus. One such practice is closing the toilet lid before flushing, but how effective is it in preventing illnesses from spreading?

While a U.S. study previously claimed that closing the lid did not significantly reduce contamination inside the toilet, toilets vary significantly across regions. To address this question using Japan’s standard toilet designs, a research team led by Takashi Fukuda at the National Institute of Advanced Industrial Science and Technology (AIST) conducted an experiment to visualize aerosol movement during flushing and analyze potential virus dispersion.

Using a water-saving Japanese toilet (6 liters per flush) in a simulated restroom, researchers tracked the quantity and spread of aerosols using laser-based measurement tools.

The findings revealed that closing the lid reduced aerosol particle generation to about a quarter of what occurred when the lid was left open, suggesting some hygienic benefit. However, while aerosols did not spread upward when the lid was closed, particles escaped about 15 centimeters in front of the toilet. This occurred because the water flow is thought to have pushed air out through gaps between the lid, seat, and bowl.

To measure potential viral spread, simulated virus samples were placed in the bowl before flushing with the lid closed. Viruses were detected on various surfaces, with 33% on the walls, 29% on the underside of the seat, 15% on the top of the seat, 11% on outside the bowl, and 10% on the underside of the lid, among other places.

Despite this, the total virus quantity that adhered to external surfaces was less than 1/100,000th of what was inside the bowl. Fukuda noted, “Whether the lid is open or closed, the risk of infection from touching surfaces like the toilet seat or walls in a typical Japanese restroom is minimal.”

The research team advises closing the lid, standing about 15 cm away from the toilet when flushing, and including nearby walls in regular cleaning routines. Fukuda emphasized, “We hope these findings will help drive further innovation in Japan’s world-leading toilet designs, supporting the development of more hygienic systems for everyday use.”

(Japanese original by Yoshimi Nakamura, Lifestyle, Science & Environment News Department)