Diseases can spread between apartments via shared ventilation, study shows

Airborne diseases like measles, influenza and COVID-19 can easily spread between units in multi-family buildings via a type of bathroom ventilation system commonly used around the world, new research suggests.

The study, conducted inside an older high-rise in Spain early in the coronavirus pandemic, adds to a growing body of evidence that airborne viruses can spread between separated indoor spaces, transmitting disease without face-to-face contact.

“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,” said senior author Shelly Miller, professor emerita in the Department of Mechanical Engineering at the University of Colorado Boulder.

Published in the journal PLOS One on May 12, the research was a collaboration between CU Boulder, the University of Valencia and the University of Cantabria in Spain, and Concordia University in Canada.

“The ghost in the room” 

The project got its start in Santander, Spain, in June of 2020, just as residents began to emerge from lockdown. Cases had dropped to zero, and people were still masking and social distancing, when residents of a seven-story building got a call from public health officials. Someone on the third floor had tested positive for the SARS CoV2-virus.

The building was locked down again. But soon 15 people across four vertically stacked apartments had contracted COVID-19.

Engineer David Higuera lived in the building with his wife and two small children. They had a hunch that the ventilation shafts connecting bathrooms were to blame.

“I knew that if what my wife and I suspected was happening, it could have significant scientific implications for public health,” said Higuera, a co-author on the study.

Higuera contacted regional health authorities and the press, but he said he was initially met with “very little interest.”

So he partnered with Miller, a world expert in airborne particles, who was on sabbatical in Spain.

They worked with epidemiologists to genetically sequence samples from infected residents, confirming that they likely got it from one another. Then they brought in specialized equipment, shipped from CU Boulder, to measure airflow and air pressure in the building. 

At one point, Higuera placed a plastic bag over the bathroom duct and filmed as it filled with air coming in from other units. In another experiment, the team remotely measured carbon dioxide (CO₂) levels throughout the day in a vacant apartment.

CO₂ is emitted through human breath and wouldn’t be expected to exist at high levels in a vacant space. Yet the apartment was filled with it.

“It was like there was a ghost in the room,” said Higuera. 

Team engineers developed a computer simulation to model how virus-laden particles, emitted from breath or flushed toilets, could flow throughout the building under different circumstances.

Ventilation ducts in the bathroom of the Santander building. Courtesy David Higuera.

As with many older buildings across Spain and around the world, the bathrooms weren’t designed to have windows or fans. Instead, they use the “stack effect” in which shared vertical ventilation shafts move air out through vents in the wall, exhausting it through the roof via natural convection. 

The study found that shifts in weather could impact air pressure in the shaft, with hotter temperatures pushing air flow back through the vents. Switching on the kitchen exhaust hood could exacerbate the spread, sucking almost all the air from an adjacent bathroom into a nearby apartment within minutes.

“The most plausible transmission route for this outbreak was the bathroom vertical ventilation duct system,” the authors conclude.

Could it happen elsewhere?

The study is not the first to document disease spreading through ventilation systems.

In 2003, during an outbreak of severe acute respiratory syndrome (SARS) in a Hong Kong high-rise, virus-laden particles made their way into a shared ventilation system via a bathroom floor drain, infecting 321 and killing 42 people.

The Santander building was built in 1969, and that type of ventilation system was phased out in Spain in 1975 with new building codes. But roughly one-third of the country’s buildings were built before then.

This type of ventilation is uncommon in the U.S. today, but similar systems still exist worldwide in older buildings. 

“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.” 

Notably, Higuera installed a fan in his own bathroom during the pandemic, with a flap that prevented airflow from coming in. No one in his family got sick.

The authors are now calling on officials in Spain to update building standards to assure that older buildings take similar steps to mitigate disease transmission risk. They say architects everywhere should do more to address indoor air quality when designing new buildings.

For individuals wanting to reduce airborne contaminants in their own work or living space, Miller recommends using a high-quality air purifier. 

Alberto Garcia, a recent mechanical engineering PhD graduate from CU Boulder, contributed to this study.