They never had contact with one another, they did not live on the same floor, they did not know one another: the three families from a high-rise building in Guangzhou, China, who all tested positive for the corona virus in February. Scientists now suspect that the virus could have spread through the sanitary facilities of the building, as they write in the journal “Annals of Internal Medicine”.
It is already known that human feces can contain the virus and that it can spread through small airborne particles called aerosols. So it’s not particularly surprising that human excrement can release contagious bio-aerosols as soon as people flush the toilet. What is very surprising, however, is that these particles can apparently spread over twelve floors in different apartments, as it probably happened in Guangzhou.
Infected people had never shared an elevator before
Scientists have now understood how a family from Guangzhou was most likely infected with the corona virus. The family lives on the 15th floor of the skyscraper. Four of the five members had been in Wuhan in January – the Chinese city where the virus was spreading rapidly at the time.
Shortly after the family returned to their home, two middle-aged couples also started feeling sick. One of the couples lives on the 25th floor, the second on the 27th floor of the high-rise building in Guangzhou. No other resident who lived that high up later tested positive. Video recordings also showed that the affected families had never used an elevator together during their infection. And there were no traces of the corona virus on the buttons in the elevator or in the ventilation shafts.
In the apartment of the family of five, however, the researchers found corona viruses – especially in the largest of the bedrooms. The scientists then released ethane gas in the family’s toilet and then checked whether the gas had spread to the apartments above. All families were in quarantine during this attempt. So there was no risk that the virus could have spread through too close physical contact.
Bio-aerosols can stay in the air for a long time
The researchers were able to detect ethane gas in the apartments of the two infected couples, as well as in two other apartments on the 16th and 21st floors. This strongly suggests that the bio-aerosols have spread through the sanitary facilities of the house. Residents on the higher floors may have inhaled the particles. They may also have touched surfaces in their bathrooms where the virus landed.
Typically, the coronavirus spreads through respiratory droplets when an infected person coughs, sneezes or speaks. How much the coronavirus spreads via so-called aerosols is not yet entirely clear. It is extremely difficult to capture and examine the tiny particles. But: A study from 2018 has already shown that bio-aerosols can stay in the air for half an hour after flushing a toilet, sometimes even longer.
Researchers largely agree that certain environments can promote virus transmission through the air. This particularly includes rooms that are poorly ventilated. If a room is well ventilated, on the other hand, the risk decreases: According to a study from May, the amount of coronavirus aerosols in a well-ventilated room halved after 30 seconds – in a room that was not ventilated after five minutes.
“Chimney effect” exacerbated the problem
The two infected couples from the Guangzhou skyscraper said they never opened the windows of their large bedroom. This could have made it easier for fans to circulate contaminated air into these rooms. It is also quite possible that the poor ventilation ensured that the virus was able to survive in high concentrations in the sewer pipes as it flowed between floors.
Scientists believe it was no coincidence that the two couples who fell ill lived on a higher floor than the family of five. Warm air finally rises in the winter months – researchers call this that “Chimney effect”, so the “chimney effect”.
What made things even worse: The two couples also said they didn’t use their bathtubs regularly. This could have caused the seals in their tubs to dry out – making it easier for bio-aerosols to seep through.
Previous cases of bio-aerosol contamination
The Guangzhou outbreak isn’t the first documented case where the coronavirus appears to have spread through sewer pipes. In March, a couple from Hong Kong living on the 32nd floor reportedly passed the virus on to a 59-year-old who lived two floors above them. And in June – also in a publicly funded housing project in Hong Kong – a 34-year-old woman probably transmitted the coronavirus to the residents in four other apartments with whom she shared the same horizontal sewer pipe.
To make it clear how transmission via bio-aerosols is possible, scientists often use an example: A SARS patient who infected 300 other residents of the facility after visiting a toilet in the Hong Kong apartment complex “Amoy Gardens” . Researchers then came to the conclusion that bio-aerosols contaminated with the virus were responsible for this transmission.
According to a study from 2004, the SARS patient had “extremely high concentrations” of the virus in their excrement. After this person flushed their toilet, bio-aerosols could have got into the air shaft of the building – because the fan was off turned on and the bathroom door was closed.
From there, a kind of cloud has probably spread into the apartments on the higher floors. In many of the apartments, the floor seals had dried out by this time, as an investigation team from the World Health Organization (WHO) later found out.
With this SARS transmission in Hong Kong and in the case of the high-rise in Guangzhou, the researchers had to rely on circumstantial evidence. The epidemiologists cannot prove that bio-aerosol transmission took place. Rather, they declared this transmission route to be the most plausible explanation of what happened after ruling out other theories.
Scientists continue to research coronavirus aerosols. Health experts are now suggesting that everyone should leave their bathroom windows open, use the bathtub more often than usual, and keep the toilet lid closed.
This article has been translated from the original English.