Air from your neighbor’s bathroom may be making an unwelcome visit to your place. A new study finds that airborne illnesses such as COVID-19 can travel between separate apartments through shared bathroom ventilation shafts, effectively turning supposedly sealed units into connected breathing spaces. Researchers traced a fast-moving outbreak in a seven-story apartment building in Santander, Spain, to airflow in a vertical bathroom duct, and they warn the finding has practical implications for older multifamily buildings in the U.S. and beyond.
How researchers reconstructed the case
The peer-reviewed paper, published Tuesday in PLOS ONE, combines genetic sequencing, on-site carbon-dioxide monitoring and airflow modeling to reconstruct how virus-laden aerosols moved through the building’s ducts. The research team, which included University of Colorado Boulder professor emerita Shelly Miller alongside collaborators in Spain and Canada, according to CU Boulder Today, concluded that the shared ventilation shaft was the “most plausible transmission route.”
The Santander outbreak
In June 2020, while the seven-story building was under lockdown, a single positive case on the third floor was soon followed by infections among 15 people across four vertically stacked apartments, as reported by Denver7. Investigators also recorded unusual CO2 spikes in an otherwise vacant unit and used genomic sequencing to link cases, findings summarized in coverage by MedicalXpress.
How the air moved
The building relied on a passive “stack effect” setup in which warm bathroom air rises through a shared vertical shaft and exhausts through the roof. Modeling in the study shows that shifts in weather, pressure or indoor temperatures can flip that flow. Running a kitchen exhaust hood can, in minutes, pull bathroom air from the shaft into a neighboring apartment, a mechanism the authors describe in PLOS ONE.
What it means here
That exact bathroom-vent design is uncommon in U.S. construction, but the underlying principle of connected air pathways and pressure-driven reversals still applies to hotels, older multifamily buildings and other properties with shared ducts. Federal health guidance emphasizes ventilation as a core defense: improving air flow, increasing filtration and aiming for multiple air changes per hour all help cut airborne transmission risk, according to ventilation guidance from the CDC.
Practical steps for tenants and landlords
Simple steps can cut risk. Run a functioning bathroom exhaust fan, install an outdoor-venting fan with a non-return flap, or add a portable HEPA air cleaner to reduce particle levels in occupied rooms. Building guidance suggests a minimum intermittent bathroom fan capacity around 50 cubic feet per minute; for retrofit details and fan recommendations, see the Building America solution guide at Building America. Tenants who notice odd drafts, persistent cooking smells or elevated CO2 should raise the issue with property management. In the Santander case, the engineer who lived in the building protected his family after he retrofitted an exhaust fan, as Denver7 reports.
Building codes and next steps
The authors urge regulators and building managers to prioritize retrofits in pre-1975 housing stock and to require measures such as forced exhaust and non-return flaps where vertical shafts remain, according to CU Boulder Today. Local code changes could be costly, but researchers say targeted fixes are often cheaper and faster than wholesale replacements for many older buildings.
For apartment dwellers, the takeaway is straightforward: air moves in unexpected ways, and a working exhaust fan or a small HEPA unit adds an extra layer of protection. The Santander case is a reminder that building design matters, and that when air is connected, so are residents’ risks.
