Coronavirus: How infected air can flow from one apartment to another

As the pandemic has progressed, evidence that the coronavirus is airborne has become increasingly strong. So much so that there is growing consensus that this is the main channel of transmission, rather than via surfaces as originally believed. This changes the prevention strategy defended until now. Although hygiene is still very important, ventilation is even more so. An unventilated, confined home is riskier than walking in the open air. Even with no mixing between households, the air ducts of some bathrooms can spread the virus if certain conditions are met.

This is not a problem if the building (whether it contains residential units, hotel rooms, office space or senior care facilities) has bathrooms with a window to the outside or if it was built starting in 1977, when the 1975 code went into effect. But in buildings that do not meet this criteria, the air can circulate freely between units. The bathrooms are ventilated by a duct that is nothing more than a hole connecting these units, which are separated by just a simple grille.

More modern buildings have “an air outlet with a vertical forced draft duct.” In other words, each bathroom no longer gives directly onto the hole. Instead, there is a pipe behind the grille that channels the air into the communal duct. This does not insulate the units completely from each other, but it makes it much more difficult for air to circulate freely between them.

Regulations got even tougher in 2006, when it became mandatory to add an electrical exhaust fan on top of the vent. This way, air can only come out of the homes through the grille, but it cannot go in. But David Higuera, an industrial technical engineer, notes that the noise they make tends to bother the residents of top-floor apartments, and the fans are often turned off at night.

Before 1975

1975-2006

From 2006

Bathrooms

Grille

Individual

channels

Main air duct with individual channels for each home.

Natural air duct that connects all the bathrooms.

A mandatory exhaust fan is added to the previous model and connected 24/7.

Before 1975

1975-2006

From 2006

Natural air duct that connects all the bathrooms.

Main air duct with individual channels for each home.

A mandatory exhaust fan is added to the previous model and connected 24/7.

Mandatory

exhaust

fan

Bathrooms

Individual

channels

Grille

For the virus to be transmitted through air ducts, very specific conditions must be met. It is difficult to estimate how much transmission takes place by this route, but several experts consulted by this newspaper consider it to be very rare. Manuel Ruiz de Adana, an associate professor at the Department of Applied Thermodynamics of Córdoba University’s School of Engineering, and an expert in the study of the airborne transmission of the coronavirus, explains that beyond the fact that the ducts must be old to allow enough air flow, an apartment would have to have a high concentration of aerosols which could then pass from one apartment to another in sufficient amounts for there to be an infection. A high concentration of aerosols is more common in poorly ventilated homes, making air transfer between them less likely, but not impossible.

Different models of grilles without filters. In buildings constructed prior to 1975 that have not been renovated, it is common for there to be a communal air duct shared by all the bathrooms above and below one another.

There are cases in which all the evidence points to ventilation channels being responsible for transmission. The most widely reported case is the one in Seoul, where eight Covid-19 infections were recorded in five apartments that shared bathroom ventilation. Investigators concluded that there was “no other possible contact” between those infected other than this shared air duct.

In Spain, one-third of Covid cases have no known origin and the spread in buildings has hardly been studied. But there have been several properties registering infections where ventilation channels could provide an explanation. In Spain, data from the National Statistics Institute (INE) shows that there are 9.7 million buildings dating from before 1980, representing 37.7% of all real estate.

One possible example is an apartment block in the Basque city of Bilbao where 33 people (including contacts) in six different apartments were infected. One theory blamed the elevators. The other possible culprit was the air ducts, since there was no close contact between the affected neighbors.

And in the northern city of Santander, in the Cantabria region, a building with 97 residents had to be confined in June because of an outbreak that seemed to be spreading among the apartments. Seventeen people from four different apartments became infected. All the units had bathrooms connected by the same air duct.

Engineer David Higuera’s home is connected to this same vertical air duct, but none of his family members were infected. Aware of the possibility of air transmission between apartments and the existence of bad odors, he installed a valve that allows air to escape from his bathroom, but prevents it from entering. The installation of such a valve is one solution and it proves more effective if the bathroom has an exhaust fan. Another simpler alternative, says Higuera, would be to temporarily cover the grille, but this method carries the disadvantage of simultaneously limiting the bathroom’s ventilation.

In July, the Spanish Health Ministry published recommendations for ventilation and air conditioning systems in buildings to prevent the spread of SARS-CoV-2. The document recommends a 24-hour air extraction system. In the rare cases where air ducts and windows coexist in a bathroom, the ministry recommends keeping the windows closed, as the reverse air flow could quickly bring air from other apartments into the bathroom.

If there are only windows, the correct thing to do is to open them for ventilation. “Although this solution may generate some discomfort due to drafts or the chill factor, the benefit of air renewal by cross-ventilation is proven to lower the rates of pollutants in rooms,” states the document. “Even in buildings with mechanical ventilation, regular ventilation with windows is recommended.”

Toilet seat: better down

The document also mentions fecal-oral contamination. When the toilet is flushed, aerosols can be generated and remain in the environment. A simple and largely effective solution is to make sure the toilet seat is down when flushing. But there is an added problem with sanitation ducts. As Jaime Vilaplana, from the utilities specialist Aliaxis, explains, if the installations are old or not in good condition, they can also trigger air leaks. And these are ducts that are also connecting different apartments vertically.

Sewer pipe

U-bend

This trap is designed so that the water at the bottom blocks smells coming in.

If the pipe has no water at some point, contaminated air can get into the home.

Sewer pipe

U-bend

This trap is designed so that the water at the bottom blocks smells coming in.

If the pipe has no water at some point, contaminated air can get into the home.

Sewer pipe

U-bend

This trap is designed so that the water at the bottom blocks smells coming in.

If the pipe has no water at some point, contaminated air can get into the home.

Sewer pipe

U-bend

This trap is designed so that the water at the bottom blocks smells coming in.

If the pipe has no water at some point, contaminated air can get into the home.

The World Health Organization suggested in a report that SARS (the most direct predecessor of the current coronavirus) spread through a building in Hong Kong in this manner, generating more than 300 cases and 42 deaths. This can happen when U-traps, which hold water at the bottom that prevent air from circulating, run dry or when pressure changes cause gurgling that leads to air leaks. Again, this does not seem to be a means of mass transmission, but, according to Vilaplana, the risk from this source can be minimized in bathrooms by keeping sinks, bathtubs and bidets plugged when not in use, keeping the toilet seat down, and ensuring that U-traps do not dry out by pouring water down the drain regularly.

English version by Heather Galloway.