It’s well-known by now that the coronavirus can spread from person to person via respiratory droplets.
But droplets aren’t the only bodily fluid that the virus can travel in: multiple studies have found traces in infected patients’ poop.
A new study from the American Institute of Physics evaluated how far these potentially viral poop particles can spread when you flush a toilet. It found that a toilet’s flush could spew tiny droplets from the toilet — and the material inside — up to three feet from the toilet, which could land on other surfaces around the bathroom.
It also found that the turbulence from a flush generated such small particles that they could float in the air around the toilet for up to a minute, where they could be inhaled by another bathroom user. Shared bathrooms can be risky for this reason.
“One can foresee that the velocity will be even higher when a toilet is used frequently, such as in the case of a family toilet during a busy time or a public toilet serving a densely populated area,” co-author Ji-Xiang Wang, who studies fluid dynamics at Yangzhou University, said in the study’s press release.
It’s unclear if the amount of virus that would be in these particles is enough to infect another person, but you should still lower the the lid before you flush.
The study helps highlight the risks that could be posed by shared bathrooms as the US and other countries reopen. In general, four main factors raise your risk of catching the virus: enclosed spaces, crowds, close contact with others, and difficulty social distancing.
A small enclosed space like a bathroom presents a high risk, particularly if many people are sharing it.
Toilet flushes create a ‘vortex’ of droplets above the bowl
The researchers used a fluid dynamics model to track the movement of the droplets in a toilet bowl after a flush.
When a toilet flushes, water from the tank above the bowl is pushed down into the water in the bowl — creating turbulence and changes in airflow.
The researchers studied two common types of siphon toilets. One has a single toilet inlet valve for flushing water. The other has two inlet valves, which creates a rotating flow.
These valves determine the amount of pressure that the water used for flushing applies to the raw waste in the bowl. That means different amounts of the wastewater in the bowl will be spewed out.
For both types of toilets, as the water pours into the toilet bowl from one side, it splashes the opposite side, creating a vortex near the far wall.
The vortex continues upwards in the air above the bowl due to inertia.
“Therefore, an airflow vortex also appears in the air zone above the toilet seat,” the researchers wrote. The droplets in this vortex are carried to a height of up to three feet. The droplets are so small that they can float there for up to one minute.
A two-valved toilet creates an even faster vortex, forcing about 60% of these small particles into the air even more quickly, the simulation shows.
If there is infected fecal matter in the toilet, the clouds will contain them.
Still, it’s unclear if these viral poop clouds can get you sick
It’s unknown whether these small particles can get you sick, because scientists are still not sure how much of the coronavirus you need to be exposed to in order to get infected.
The particles that are spewed from a toilet are tiny — they’re known as aerosols, which are smaller than the droplets that the virus prefers to travel in.
Scientists agree that the virus is primarily transmitted through respiratory droplets — particles larger than 5 micrometers — when an infected person coughs, sneezes, or speaks.
A clear solution to this dangerous problem is to close the lid before flushing. But in many countries, including the US, toilets in public restrooms don’t typically have lids.
The researchers suggest that a new toilet design could help prevent infectious disease transmission. A toilet with a lid that closes automatically before flushing, for example, could avoid the issue.