A COVID-19 outbreak in a residential building in Spain during the early months of the pandemic likely spread through shared bathroom ventilation ducts, according to a study published this week in PLOS One.
The outbreak occurred in June 2020 in a seven-story apartment building in the city of Santander in northern Spain, during a period when transmission in the city (population 172,000) had dropped to zero. Fifteen COVID cases were identified in four vertically stacked apartments connected by the same bathroom ventilation shaft. No cases were detected in surrounding apartments or elsewhere in the building.
Bathroom vents the likely culprit
A team led by researchers at the University of Colorado Boulder sequenced virus samples from infected residents to confirm that the cases were connected, then measured air flow and air pressure in the building. They also tested carbon dioxide (CO2) levels in a vacant apartment. Because CO2 is released into the air when people exhale, high levels of it wouldn’t be expected in a vacant apartment. But the empty apartment was full of it.
“It was like there was a ghost in the room,” David Higuera, an engineer who lived in the building and who first suspected the bathroom ventilation system as the culprit, said in a news release.
The building was constructed in the late 1960s, before new building code standards were introduced in 1970. Units had shared bathroom ducts that used natural convection to vent air out through the roof, a phenomenon known as the chimney or stack effect. That process is driven by temperature-induced differences in air density.
But factors like operating a kitchen fan, opening windows, or pressure changes in the building can reverse the flow of air in the ducts, causing it to go back into the apartments rather than out through the roof. The researchers concluded that infectious aerosols likely traveled through the building’s shared vertical bathroom duct system, allowing virus-laden air from one apartment to enter bathrooms in units above and below.
“The most plausible transmission route was the bathroom vertical ventilation duct system, which facilitated movement of infectious aerosol between vertically connected homes,” write the authors. Residents in units who had modified or blocked their bathroom ventilation systems before the outbreak did not experience infections.
This style of ventilation system is uncommon in the United States, but the outbreak shines a light on a broader concern. “Even if you are far from the source, if your air is connected, you can still get sick,” senior author Shelly Miller, PhD, professor emerita in the Department of Mechanical Engineering at the University of Colorado Boulder, said in the release. “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.”
Similar outbreak in multi-unit building in 2003
The findings mirror reports from a 2003 severe acute respiratory syndrome (SARS) outbreak at Hong Kong’s Amoy Gardens housing complex, in which virus-laden aerosols traveled through bathroom floor drains, infecting more than 300 people and killing more than 40. In 2022, a study in Hong Kong found that nearly 9% of those who lived in high-rise buildings contracted the Omicron COVID variant because of vertical building transmission.
The researchers say the findings highlight the importance of taking proactive measures to limit the transmission of virus-laden aerosols in multi-unit residential buildings. “By recognizing the role of shared infrastructure in disease propagation, authorities and building managers can implement timely measures to protect occupant health and prevent the spread of airborne pathogens,” they write.
