A new study suggests airborne viruses can move between some apartments through shared bathroom ventilation systems.
Researchers investigated a COVID-19 outbreak in a Spanish apartment building and found ventilation ducts were the most likely transmission route.
The findings highlight the importance of building ventilation and indoor air quality in multifamily housing.
A study published in PLOS One examined a COVID-19 outbreak that occurred in a seven-story apartment building in Santander, Spain, during the early months of the pandemic. Researchers found evidence suggesting the virus likely spread between apartments through a shared bathroom ventilation system rather than through direct contact between residents.
The findings add to growing research showing that airborne viruses can move through connected indoor spaces. While the study focused on one specific building design, it offers new insight into how ventilation systems may influence the spread of infectious diseases in multifamily housing.
“We tend to think that if we shut the door in our apartment, we are safe and can’t get infected. But our study shows that, depending on the ventilation system in place, that may not be the case,” senior author Shelly Miller said in a news release.
The study
The outbreak occurred in June 2020 when 15 residents living in four vertically stacked apartments became infected with COVID-19.
To understand how the virus spread, researchers analyzed epidemiological data, genetically sequenced virus samples, measured airflow and air pressure within the building, and used computer simulations to model the movement of airborne particles.
The building used a shared vertical bathroom ventilation shaft, a design common in many older buildings in Spain. Unlike modern systems that rely on mechanical fans, these ducts use natural airflow to move air upward and out of the building.
The findings
Researchers discovered that weather conditions could sometimes reverse airflow within the shaft, pushing air back into apartments. They also found that running a kitchen exhaust hood could increase air movement between units.
In one experiment, elevated carbon dioxide levels were detected inside a vacant apartment, suggesting that air from occupied units was entering the space.
Based on the combined evidence, the researchers concluded that the shared bathroom ventilation duct system was the most likely pathway for transmission during the outbreak.
What this means for consumers
The study does not suggest that all apartment buildings pose the same risk. In fact, the researchers note that the ventilation design examined is uncommon in newer buildings and is relatively rare in the United States today.
Still, the findings underscore the importance of ventilation and indoor air quality in multifamily housing. According to the researchers, connected air spaces can sometimes allow contaminants to move farther than people might expect.
“While this is a special building design more common in Spain, it illustrates a broader concern — that even if you are far from the source, if your air is connected, you can still get sick,” said Miller. “This can happen in a multifamily apartment building through the ducts, in a hotel between the hallway and rooms off the hallway, in office buildings between offices, or on a cruise ship.”
The authors say building designers, managers, and policymakers should pay closer attention to ventilation systems, particularly in older buildings. They also point to measures such as properly designed exhaust fans with backflow prevention features as potential ways to reduce risk.
For renters and homeowners, the study serves as a reminder that healthy indoor air depends not only on what happens inside a single apartment, but also on how an entire building is designed and ventilated.