Bathroom Vents May Have Spread COVID Between Apartments, Study Suggests

During the COVID-19 pandemic, people scrubbed their groceries, masked up in hallways, and avoided neighbors in pursuit of safety. But a cluster of cases in a Spanish apartment building suggests the real danger may have been lurking in a far less suspected place: the shared air shaft behind the bathroom wall.

In the summer of 2020, fifteen people across four apartments in a seven-story building in Santander, Spain, tested positive for COVID-19, during a stretch when the city of roughly 172,000 people had recorded nearly zero new cases in the wider community. No direct contact between residents of different households was identified. Surface samples from common areas like elevators and hallways came back negative. Yet the virus spread anyway, in a specific pattern: every infected apartment was stacked vertically, one on top of another, all connected by the same shared bathroom ventilation duct running from the basement to the roof.

A study published in PLOS One pieced together what happened, using air pressure measurements, computer simulations, genetic analysis of the virus, and real-world data from the building. Researchers concluded the most plausible transmission route was the building’s own ventilation system, specifically a shared bathroom exhaust duct that may have allowed infectious air particles to travel between floors and into neighboring apartments.

A 1969 Building’s Outdated Bathroom Duct Design May Have Sealed Residents’ Fate

The investigation might never have happened if not for one resident, David Higuera, an engineer living in the building and a co-author of the study, who noticed the cases on his floor weren’t random. When a second apartment directly above the first tested positive, the vertical pattern became impossible to ignore. What followed was an international research effort involving professors and students from four universities: the University of Colorado Boulder, Concordia University in Montreal, the University of Valencia, and the University of Cantabria in Spain.

Constructed in 1969, the building predates modern building codes in Spain. Each apartment had a single interior bathroom with no windows, ventilated by a small opening in the wall near the ceiling. That opening connected to a shared vertical duct running through the building to the roof, shared by pairs of apartments. Originally, the design relied on warm air naturally rising to carry bathroom odors and humidity upward and out. That system has a flaw: under certain conditions, air can reverse direction entirely, flowing back down into apartments rather than up and out.

Cooking Dinner May Have Drawn Viral Particles from a Sick Neighbor’s Apartment

Researchers conducted five rounds of measurements inside a fourth-floor apartment between May and December 2022, tracking air pressure, airflow speed, carbon dioxide levels, temperature, and humidity. Carbon dioxide, exhaled with every breath, served as a stand-in for where air was actually going. In a properly functioning exhaust system, carbon dioxide levels in an empty apartment should stay low. Researchers repeatedly detected elevated carbon dioxide in the empty bathroom, a sign that air from other apartments was flowing backward through the duct.

Conditions worsened when a kitchen exhaust fan was running. When a kitchen hood pulls air out at high speed, it creates a pressure imbalance that can draw air in from wherever it can find it, including through the bathroom duct. Measurements showed running the kitchen fan on its highest setting dramatically increased the amount of air reversing into the bathroom.

Computer models confirmed the pattern. One simulation tracked airflow and carbon dioxide movement between two vertically connected bathrooms. When the kitchen exhaust fan was running, airflow into the upper bathroom was significantly stronger than when a window was simply left open. A larger model simulated the entire building across multiple floors, tracking how infectious particles moved under different conditions, including varying wind direction, window placement, bathroom fan use, and kitchen hood operation.

In scenarios where a fourth-floor resident ran their kitchen exhaust fan, the model showed it pulled air upward from the third floor, where the index cases lived. When a third-floor bathroom fan ran simultaneously, particles were also pushed downward into lower floors. Everyday household routines may have been moving viral particles between units in ways no resident could have anticipated.

Apartments with Modified Vents Reported No Infections During COVID Outbreak

Perhaps the most telling evidence was who didn’t get sick. Among the 10 other homes sharing the same vertical duct or patio area, some were unoccupied and others had already modified their bathroom ventilation before the outbreak. Three of those homes had installed individual exhaust fans fitted with a flap that closes automatically when the fan is off, blocking air from flowing backward. A fourth had its bathroom exhaust opening covered entirely. None of those four apartments reported infections. The apartments that lacked such modifications were the ones affected.

Virus sequencing added another layer of evidence. Genetic analysis of samples from three infected residents found their sequences nearly identical, differing from one another by only one or two points in the genome, while differing from the closest sample collected elsewhere in the city by at least eleven points. That similarity points toward a single shared source rather than multiple independent introductions.

Based on their findings, researchers recommended installing forced exhaust fans with non-return flaps in bathroom ducts, noting the fans are generally inexpensive and can be retrofitted into existing buildings. Public health officials, they added, should treat shared ventilation infrastructure as a priority when cases cluster vertically in residential buildings.

While the world debated masks, hand sanitizer, and six-foot distancing, a building’s own hidden ventilation may have been moving the virus floor by floor, bathroom by bathroom, through a duct most residents never thought about.

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Disclaimer: This article is based on a single case study of one residential building and should not be interpreted as proof that shared bathroom ventilation ducts routinely spread COVID-19 or other respiratory illnesses. Researchers identified the duct as the most plausible transmission route, but could not fully rule out other pathways. Readers with concerns about ventilation in their own buildings are encouraged to consult a qualified building engineer or public health professional.

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