Science

Toilet flushing can create infectious aerosols even when lid is closed

Toilet flushing can create inf...
A study has tracked aerosols generated by flushing toilets in public restrooms
A study has tracked aerosols generated by flushing toilets in public restrooms
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A study has tracked aerosols generated by flushing toilets in public restrooms
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A study has tracked aerosols generated by flushing toilets in public restrooms

An experimental study has quantified the volume of aerosol particles generated by flushing toilets in a public restroom. The real-world research suggests even with lids closed flushing toilets can increase levels of ambient aerosol particles, heightening the risk of airborne disease transmission in poorly ventilated public spaces.

In 2003, during the peak of Hong Kong’s SARS outbreak, 321 cases rapidly appeared in a single housing estate. Comprehensive investigations of the outbreak ultimately homed in on the primary case, a man visiting one of the apartment blocks.

On the day the man was visiting he had diarrhea and used a toilet in one of the apartments. Tracking subsequent cases, the investigation discovered the most likely route of viral transmission to other residents was via aerosolized particles discharged into sewage pipes.

Early in 2020, as the COVID-19 pandemic spread around the world, case studies resembling the earlier Hong Kong SARS cluster began to appear. One early cluster in a Chinese apartment building, for example, was widely analyzed and a recently published study concluded fecal aerosol transmission was possibly the cause.

It certainly is not news to suggest a large volume of micro-organisms is present in feces. But what happens to those microbes when they are struck by the water pressure of a flushing toilet?

A simulation study published last year used computational fluid dynamics to model the generation and spread of aerosol particles during toilet flushing. The researchers described their results as “alarming,” finding the flushing process creates and propels significant numbers of aerosol particles.

A new study, from a team of researchers at Florida Atlantic University, set out to quantify the volume of aerosols produced by flushing toilets in real-world conditions. To do this the researchers turned a medium-sized restroom on their university campus into an experimental laboratory.

Closed-lid toilet flushing, open-lid toilet flushing and urinal flushing were all investigated. Aerosol particles were detected for at least 20 seconds up to 5 ft (1.5 m) above the toilet after flushing.

While more particles were found to be generated by urinal and open-lid toilet flushes, the researchers still detected notable volumes of aerosols in the air after a toilet was flushed with a closed lid. As many public restrooms do not have lids on toilet seats it is suggested installing lids may help reduce aerosol dispersion but smaller particles can still escape through gaps in the lid.

Perhaps more concerning were the overall levels of ambient aerosols that were found to accumulate in the space after three hours of flush experiments.

"After about three hours of tests involving more than 100 flushes, we found a substantial increase in the measured aerosol levels in the ambient environment, with the total number of droplets generated in each flushing test ranging up to the tens of thousands," says co-author on the new study, Siddhartha Verma.

The researchers tracked aerosol particles smaller than three micrometers in size, which can remain suspended in the air for several hours depending on a room’s airflow. The restroom was fitted with two working air vents, however, this was not effective at dispersing the accumulating particles.

"The significant accumulation of flush-generated aerosolized droplets over time suggests that the ventilation system was not effective in removing them from the enclosed space even though there was no perceptible lack of airflow within the restroom," notes Masoud Jahandar Lashaki, another co-author the study. "Over the long-term, these aerosols could rise up with updrafts created by the ventilation system or by people moving around in the restroom.”

Research Footage of Study Exploring Toilet Flushing Power to Test Risk of COVID-19 Transmission

The new study solely focuses on the potential for biomatter to be aerosolized by a flushing toilet and does not prove infectious diseases can be spread through the air in public restrooms. Nor does it suggest this is proof COVID-19 is spread this way.

The researchers do, however, cite a number of well-reported case studies describing viral gastroenteritis, norovirus and other pathogens being spread by aerosolized particles present in public toilets. Traces of SARS-CoV-2 have also been found in fecal samples, suggesting this is a hypothetically plausible mode of transmission.

"The study suggests that incorporation of adequate ventilation in the design and operation of public spaces would help prevent aerosol accumulation in high occupancy areas such as public restrooms," explains co-author Manhar Dhanak. "The good news is that it may not always be necessary to overhaul the entire system, since most buildings are designed to certain codes. It might just be a matter of redirecting the airflow based on the restroom's layout."

The new research was published in the journal Physics of Fluids.

Source: Florida Atlantic University

9 comments
Daishi
This is scary but useful research. Despite myths, urine is not sterile either and most urinals are very poorly designed and create splashing that leaves particles everywhere. Usually my the floor under the urinals in my workplace bathroom is wet just a couple hours into Monday morning. If you stand too far away there is dropping that missed the urinal, if you stand too close the "water pressure" is too high and there is backsplash. Outside of redesigning urinals to better channel the flow there are after market solutions for this in the form of anti splash mats for the urinal itself but I have only ever seen once place that uses them. When I return to the office I'm debating just buying anti-splash mats for the urinals myself. The fact that my workplace bathroom has to be closed for cleaning already by 10 or 11 am on a Monday morning is gross at best and at worst a valid health concern.
michael_dowling
Why is it a surprise that you get spray with the lid down? My toilet has a 1/4" gap between the bowl and seat. The toilets in my office don't have lids!
Bob809
Perhaps the people that conducted these experiments will pass on all the information to the manufacturers of all No 1 and No 2 disposal devices worldwide, for young and old alike, for free, and liase with said comapnies after seeing their new designs. Otherwise, what is the point of this kind of research.
solas
Many different toilets have come out over the last hundred or so years since Thomas Crapper, but the US is stuck on that one, and just can't let it go. Some designs almost certainly solve this problem.
BlueOak
So... now we will be deluged with toilet accessory products that spray disinfectants in the bowl as it is flushed...

Who will be the next pandemic new product millionaire?
ARF!
... . . . uhhhh Mythbusters already proved years ago that you brush your teeth with poop particles even if you put a cap on your toothbrush and flush with the lid down if you keep your toothbrush in the same room as your toilet.
Bob Flint
What doesn't kill you, makes you stronger....natures cleansing itself.
Signguy
Bobflint...exactly; this is more fearmongering...
Jinpa
Busy turnpike restrooms are prime candidates for this problem. Out of the cars, into the toilets, then to the fast-food places and tables or cars to eat. No wonder we suspect the food, but that may not be the problem source after all.