Airborne diseases like measles, influenza and COVID-19 can spread between apartments through a common type of bathroom ventilation system still found in older buildings around the world, according to new research.
The study investigated a puzzling outbreak in a seven-story residential building in Santander, Spain, during the early summer of 2020. At the time, COVID-19 cases in the city had dropped to nearly zero. People were still masking and social distancing. Yet 15 people across four vertically stacked apartments became infected.
‘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.
The outbreak came to light when a resident and engineer named David Higuera received a call from public health officials. Someone on the third floor had tested positive for SARS-CoV-2 using a PCR test, the gold-standard method that detects even tiny amounts of the virus by amplifying its genetic material. The building was locked down, but cases continued to appear in apartments directly above and below the original infection.
Higuera had a hunch. The building, constructed in 1969, predated modern Spanish building codes. Each apartment had a single interior bathroom with no window. Instead of fans or windows, the bathrooms relied on a shared vertical duct, which measured just 70cm x 20cm, that was supposed to carry air upward and out through the roof using natural convection, a phenomenon known as the stack effect.
He suspected this duct was carrying virus-laden air between apartments.
To test this theory, an international team of engineers and epidemiologists from the University of Colorado Boulder, Concordia University, the University of Valencia and the University of Cantabria conducted field measurements in one of the building’s apartments.
They measured air pressure differences between the bathroom and the shared duct. Normally, air should flow out of the bathroom and up through the duct. But when the pressure inside the duct became higher than the pressure inside the bathroom, the airflow reversed direction. Air pushed backward into the bathroom instead of leaving it.
The team also measured CO₂. In a vacant apartment with no residents, CO₂ levels should remain low. Yet the empty bathroom showed elevated CO₂ levels, indicating that air (and anything floating in it) was coming from other occupied apartments through the shared duct.
Higuera said: ‘It was like there was a ghost in the room,’
The team then performed genetic sequencing on virus samples taken from three infected residents. The level of similarity proved that all three residents caught the virus from the same shared source, not from separate community infections.
To understand exactly how the virus might have traveled, the researchers built two types of computer models. The first, called computational fluid dynamics, created a virtual model of the bathrooms and duct to calculate how air currents would flow. The second, called multi-zone airflow modeling, divided the building into separate zones – each bathroom, room and hallway – and calculated how air and particles moved between them.
Both models confirmed the same finding. Infectious aerosols could ride the reverse airflow up through the shared duct and enter bathrooms on higher floors. The models also showed that operating a kitchen exhaust fan worsened the effect, pulling almost all the air from a lower bathroom into an apartment above within minutes.
This 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. Similar systems still exist in older buildings around the world.
The study notes a famous parallel: during a 2003 SARS outbreak in a Hong Kong high-rise, virus-laden aerosols traveled through a shared ventilation system via a bathroom floor drain, infecting 321 people and killing 42.
Millar said: ‘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.
‘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.’
Being an engineer, and obviously lacking the access to jigsaws enjoyed by the rest of us, as the pandemic gathered momentum, Higuera installed a fan in his own bathroom, equipped with a non-return flap that prevented air from flowing back into the apartment when the fan was not running. No one in his family got sick.
The research team now recommends this as a simple, low-cost solution for both new construction and older buildings being retrofitted. They also suggest that building inspection standards be updated to address airborne disease transmission through shared ventilation systems, drainage pipes and other architectural cavities.
Miller concluded: ‘This was a situation where the physical design of the building determined who got sick and who didn’t. We need to start paying attention to that.’
The full research can be read here.
Photo: Duncan Kidd
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