The coronavirus may have spread through bathroom drains 12 stories apart in a high-rise, raising concerns about fecal transmission

• The coronavirus may have spread through bathroom drains in a high-rise building in Guangzhou, China, a new study found.
• Researchers have detected the virus in human feces before.
• Now scientists think virus-laden feces particles could travel upward through an apartment building's plumbing after a person flushes a toilet.
• But only certain conditions make this scenario possible, like poor ventilation.

In February, three families in a high-rise building in Guangzhou, China, tested positive for the coronavirus. They didn't know each other, didn't live on the same floor, and hadn't come into contact.

So scientists started to suspect that the virus had spread through the building's plumbing.

Researchers know live coronavirus can be found in human feces and that the virus can spread via small airborne particles called aerosols. So it's not surprising that flushing a toilet can release "bioaerosols" — coronavirus-laden poop particles — that could infect other people. What's more surprising is that these particles seemed to travel across a 12-story gap between apartments in Guangzhou.

In a study published Tuesday, scientists traced the Guangzhou outbreak to a five-person family living on the 15th floor. Four of the family members had traveled to Wuhan, China, in January, while the coronavirus was spreading across the city.

Shortly after the family returned home, two middle-aged couples — one on the 25th floor and another on the 27th — started feeling sick as well. None of the other high-rise residents tested positive for the virus.

Footage showed that the families hadn't shared an elevator while anyone was infectious, and there was no trace of the virus on the elevator button or in air ducts.

The scientists did find positive virus samples in the five-person family's apartment, however — specifically in the master bathroom. So the researchers released ethane gas down the family's toilet, then checked for that gas in the apartments above. All of the families were under quarantine at the time, so there was no opportunity for the virus to spread through close physical contact.

Sure enough, the researchers found ethane in the middle-aged couples' apartments, as well as in two other apartments on the 16th and 21st floors. That's evidence that bioaerosols might have traveled through the plumbing system. Residents on the higher floors may have inhaled the particles directly or touched surfaces where the particles had landed in their bathrooms.

Certain apartments were more susceptible than others

The coronavirus typically spreads via respiratory droplets after an infected person coughs, sneezes, or talks. Researchers aren't sure about the degree to which the virus gets transmitted via aerosols, since the tiny particles are extremely hard to trap and study without killing the virus.

But a 2018 study showed that bioaerosols generally could linger in the air for 30 minutes or more after a toilet flush.

Researchers think certain environments favor airborne transmission, specifically poorly ventilated rooms. The risk goes down when a space is well-ventilated: According to a May study, the amount of coronavirus aerosol was cut in half after 30 seconds in a well-ventilated room, compared with five minutes in a room with no ventilation.

In the Guangzhou building, the middle-aged couples said they never opened the windows to their master bathrooms. That may have allowed exhaust fans to recirculate contaminated air throughout those rooms. Poor ventilation may have also helped the virus survive in high concentrations in drainage pipes as it traveled between floors.

The researchers think it was no coincidence that the middle-aged couples who got sick lived on a higher floor than the five-person family, since warm air rises in buildings during winter months. Scientists call it the "chimney effect."

To make matters worse, the middle-aged couples said they didn't regularly use their bathtubs, which might have caused the water seals in their tubs to dry out. That could make it easier for bioaerosols to seep through.

A striking resemblance to a SARS outbreak in Hong Kong

The Guangzhou outbreak isn't the only documented instance of the coronavirus appearing to travel through plumbing pipes.

In March, a couple living on the 32nd floor of a public-housing development in Hong Kong seemed to pass the virus to a 59-year-old man living two floors above them. Then in June, a 34-year-old woman, also in Hong Kong public housing, appeared to spread the virus to four other apartments that shared the same vertical drainage pipes.

Scientists often point to one past example to show how bioaerosol transmission is possible: In 2003, a patient infected with SARS used the toilet in a housing complex in Hong Kong. The virus then traveled to more than 300 other residents in the complex. Researchers later concluded that virus-laden bioaerosols were to blame.

According to a 2004 study, the SARS patient had "extremely high concentrations" of the virus in their feces and urine. After that person flushed the toilet, bioaerosols may have entered the building's air shaft because the exhaust fan was running and the bathroom door was closed.

From there, a plume may have traveled to the building's upper apartments. An investigative team from the World Health Organization found that the seals in the floor drains had dried out in many of those apartments.

But, like the new Guangzhou research, conclusions about the SARS outbreak in Hong Kong were based on circumstantial evidence: The epidemiologists can't prove bioaerosol transmission occurred. Instead, they deemed it the most plausible explanation after other theories were ruled out.

As scientists continue to study coronavirus aerosols, public-health experts have suggested that people in apartments open their bathroom windows, run the tub more frequently, and close their toilet lids.