DENVER — Airborne diseases like COVID-19 can easily spread between apartments in multi-family housing buildings using a common bathroom ventilation system, according to a new study from researchers at the University of Colorado Boulder and several other institutions.
Shelly Miller, a University of Colorado Boulder professor emerita of mechanical engineering and the study's senior author, said the research was inspired by an incident in Spain.
“I lived in Barcelona, Spain, for one year, 2021 to 2022, and many of the homes are multifamily housing units, you know, tall buildings, lots of apartments. And I was really intrigued by the design and by how they're built. And they're all connected by patios and, many of them, by bathroom ventilation systems. And because I lived there, I heard through some colleagues I was working with in Spain that there had been an outbreak in Santander,” Miller said.
Miller said her colleagues connected her with an engineer named David Higuerra, who experienced an outbreak in his apartment building, even though the building was on lockdown and residents had no contact with anyone outside their household.
Together, they began investigating how the disease was transmitted.
“We found that there were many scenarios in which the transmission could happen between apartments, and the reason that this could happen was that the building had airflow moving between bathrooms and vented to the rooftop through natural convection,” Miller said. “When there's a change in weather like the pressure, like a low-pressure system or a high-pressure system, or a change in outdoor or indoor temperatures, then the airflow within the building can change, and so then it can take an aerosol from one bathroom or from one apartment and transmit it through the ducting into another apartment.”
Miller said previously that Higuerra retrofitted his bathroom ventilation system with an exhaust fan to prevent his next-door neighbor’s cooking smells from entering his apartment, which turned out to be the key to keeping his family healthy.
“In the U.S., most of our homes have exhaust fans in our bathrooms, but they don't have them in that building in Spain, so he put one in, and he runs it all the time. And anyway, as a result, his family did not experience transmission of COVID. And there were a couple of other families who had also done this retrofit, and they were also protected,” Miller said.
But Miller said they have noticed similar transmissions in the U.S. in hotels.
“So we don't often build buildings in the same structure in the same way here in the U.S., but there are situations that are analogous. So, for example, we have seen transmission happen between rooms in a hotel because, for example, one room would have a sick person, and then if the airflow shifted because of pressure temperature differences, the aerosol could be transmitted through the building in a different way than it was originally expected or designed,” Miller said.
Miller said she worked with colleagues in Spain and Canada on the study.
She said it’s important to be aware, especially during outbreaks of airborne illnesses, that changes in airflow can impact transmission.