For many couples, one of the most irritating aspects of their entire relationship revolves around the humble toilet seat. Women want it down, guys eventually concede but then ‘forget’ all about it by the next flush, and fighting ensues.

Forget about the seat for a second. Here’s some toilet etiquette that may appeal to both egos and solve the debate once and for all: everyone has to close the lid. This way, everyone in the household does an equal amount of up-and-down lifting. However, maybe the best reason we should all keep the lid down after using the toilet has to do with hygiene.

A new study in South Korea found that an open toilet during flushing can spread at least twice as much bacteria in its vicinity than one with the lid closed.

What comes out when you flush

When we flush a toilet, it’s more than just waste being whisked away. It’s a launching pad that sends microscopic particles into the air, some potentially laden with disease-causing pathogens. In 2022, researchers at the University of Colorado Boulder embarked on a quest to shed light — literally — on this unseen menace.

They used green laser beams to illuminate the aerosol plume produced by a flushing toilet, revealing how particles from the toilet bowl can become airborne and potentially pose a health risk to individuals in public restrooms. The findings are startling: particles ejected during a flush can reach speeds of up to 6.6 feet per second and ascend to nearly 5 feet above the ground in just seconds.

Furthermore, particles smaller than 5 microns can linger in the air for more than a minute, traveling in many directions, including upwards and along walls, potentially spreading pathogens far from their point of origin. These findings were mirrored by an earlier 2020 study from China that used a computer model to show how water from a flushed toilet can spray particles as high as three feet.

“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you see these videos, you’re never going to think about a toilet flush the same way again,” said John Crimaldi, lead author of the study and professor of civil, environmental, and architectural engineering at the University of Colorado Boulder.

“The goal of the toilet is to effectively remove waste from the bowl, but it’s also doing the opposite, which is spraying a lot of contents upwards,” he added.

An automated lid-down toilet

Researchers at the Asan Medical Center, a large hospital in Seoul, South Korea, developed an automated toilet flushing system that operates only when the toilet lid is closed. This was out of concern that droplets from flushing systems — containing urine, feces, and infectious microorganisms — could endanger the patients using the public restrooms of the hospital. Human feces can harbor disease-causing bacteria like E. coli, Staphylococcus, and Campylobacter, and hospital toilets are particularly vulnerable points for the transmission of these pathogens.

To evaluate the effectiveness of this device in reducing surface contamination in bathrooms, the researchers conducted a comparative analysis between toilets equipped with the device and those without it. The study involved eight patient bathrooms — four with and four without the automatic flushing device.

To measure contamination, the researchers placed eight blood agar plates around the toilet in each bathroom. These plates were strategically positioned all around the toilet to capture bacterial particles spread by flushing. After flushing the toilets once without any feces or urine, airborne particles were allowed to settle on the plates for 90 minutes. They were then collected and incubated for two days to allow for the growth of visible bacterial colonies.

The benefits of closed lids

The results were compelling, showing a significant reduction in bacterial contamination on surfaces surrounding toilets equipped with the automatic flushing device. On average, surfaces around these toilets had less than half the number of bacterial colonies compared to those without the device (6 vs 14 colonies).

“Our study underscores the importance of closing the lid before flushing and fitting automatic flushing devices to reduce the contamination of bathroom surfaces”, says lead author Jihye Park from the Asan Medical Center.

“We are now going to install automatic flushing devices in all hospital bathrooms along with automatic UV disinfection to reduce the spread of infections.”

The study is yet to be published in a peer-reviewed journal. It was recently presented at this year’s European Congress of Clinical Microbiology and Infectious Diseases (ECCMID 2024) in Barcelona, Spain.