An investigation carried out by scientists from the University of Colorado Boulder (United States), the University of Cantabria, the University of Valencia, and Concordia University of Canada concludes that diseases can be transmitted between different apartments when they share ventilation systems.
The work, published in the journal "Plos One", examined a COVID-19 outbreak recorded in a building in Santander after the partial lockdown came into effect in Spain on March 14, 2020. At that time, infections in the city were almost non-existent, and the rules for mask use and social distancing were still in force. Nevertheless, within days, 15 residents of four vertically aligned apartments became infected, without direct contact between them.
The researchers determined that the air circulating through the bathroom ventilation ducts, shared between these apartments, exposed their occupants to SARS-CoV-2, even in a context of low community transmission. In contrast, apartments whose exhaust grilles were sealed or had individual extractors did not register infections, nor did those that did not have these common bathroom ducts.
The team combined real measurements and simulation models to analyze how infectious aerosols move within and between dwellings in this residential building in Santander. The results indicate that relying solely on natural convection, or chimney effect, in bathroom ventilation ducts does not prevent pollutants from moving from one floor to another.
In particular, people in enclosed spaces with insufficient ventilation can be exposed to airborne respiratory pathogens transmitted through interconnected ventilation systems.
Engineer David Higuera, co-author of the work, lived in the building with his partner and their two young children. From the beginning, he suspected that the ventilation ducts connecting the bathrooms were the source of the problem. "I knew that if what my wife and I suspected was true, it could have significant scientific implications for public health," explains Higuera, who recounts that he contacted regional health authorities and the media, although at first he encountered "very little interest."
"We tend to think that if we close the door of our apartment, we are safe and cannot get infected. But our study shows that, depending on the ventilation system installed, this may not be the case," said lead author Shelly Miller, professor emerita in the Department of Mechanical Engineering at the University of Colorado Boulder.
The analysis also showed that meteorological variations can modify the air pressure in the ventilation duct: higher temperatures can reverse the direction of flow and cause air to return indoors through the grilles. Furthermore, the use of the kitchen extractor hood could intensify the spread, by sucking almost all the air from a neighboring bathroom into another apartment in a few minutes.
"The most plausible transmission route for this outbreak was the vertical bathroom ventilation duct system," the authors conclude.