The concept of home as a sanctuary, a place of refuge from the outside world, was challenged by an unusual outbreak in Spain. This event, which occurred during the COVID-19 pandemic, forced scientists to delve into a hidden aspect of our living spaces: the air within our walls.
In the summer of 2020, the city of Santander had successfully curbed COVID-19 cases, and life was returning to normal. However, an unexpected surge of infections in a seven-story residential building raised alarms. What was intriguing was the pattern of these infections - they were not horizontally transmitted but vertically stacked, with infected apartments directly above or below each other.
A resident engineer, David Higuera, noticed this peculiar pattern. He and his wife tested positive, as did their neighbors on the same vertical line. It was as if the virus was following a precise, almost architectural, path. This led Higuera to suspect that the building's structure, rather than social interactions, was the key to understanding the outbreak.
The building, dating back to 1969, predated modern ventilation standards in Spain. Each apartment had a small bathroom vent connected to a shared vertical shaft, designed to allow warm air to rise and exit the building. While seemingly efficient, this system created a shared air pathway between homes, a pathway that could potentially carry viral particles.
Researchers studied the airflow in detail, monitoring pressure, airspeed, and carbon dioxide levels. In one intriguing test, an empty apartment showed increasing carbon dioxide levels throughout the day, suggesting that air was entering from other apartments. This 'ghost in the room' phenomenon highlighted the potential for reverse airflow, especially when kitchen or bathroom exhaust fans were in use.
Under certain conditions, airflow reversed, with air rushing into the bathroom vent instead of leaving it. This reverse flow, reaching up to 42 liters per second, could carry virus-carrying aerosols from one apartment to another. The building's design, with its shared ventilation shaft, allowed for vertical transmission of the virus.
Computer models confirmed these observations, showing that air from a lower apartment could move up through the shaft and enter an upper apartment, and vice versa. Infection models also estimated the risk, indicating that in several scenarios, the probability of infection exceeded safe limits.
One surprising finding was the role of kitchen exhaust fans. These devices, while removing indoor air, also create negative pressure, which can pull air from the shared shaft, potentially bringing virus particles into the apartment. Bathroom fans had a similar effect, pushing contaminated air to other floors.
Interestingly, some apartments in the building did not report infections, despite sharing the same ventilation shaft. These homes had small modifications, such as exhaust fans with one-way flaps or sealed vents, which prevented reverse airflow. This contrast provided strong evidence that airflow caused the outbreak.
Genetic analysis of the virus samples from infected residents further supported this theory. The nearly identical genetic patterns confirmed that the virus spread within the building, following a single transmission chain through connected apartments.
This case is not unique. Similar events have occurred during earlier outbreaks, such as the 2003 SARS outbreak in Hong Kong and the COVID-19 outbreaks in Seoul and other cities. Many older buildings still use shared ventilation shafts, designed for efficiency rather than infection control.
As Shelly Miller, the first author of the study from the University of Colorado Boulder, stated, "While this is a special building design more common in Spain, it illustrates a broader concern... Even if you are far from the source, if your air is connected, you can still get sick." This can happen in various settings, from multifamily apartment buildings to hotels, office buildings, and even cruise ships.
The solution, however, is surprisingly simple. Installing a small exhaust fan with a one-way flap can prevent reverse airflow. Additionally, allowing fresh air to enter while using kitchen fans, such as by opening a window, can balance pressure and reduce the risk of pulling air from other apartments.
This outbreak highlights the need to rethink building safety. Walls and doors may not always isolate air, and hidden pathways can connect spaces in unexpected ways. Building inspections should consider airflow systems more carefully, recognizing that shared ducts, pipes, and cavities can act as channels for airborne particles.
In Santander, a simple ventilation shaft became the link between households, demonstrating that infection can move through buildings in ways we rarely consider. This study, published in the journal PLOS One, underscores the importance of understanding the hidden dynamics of our living spaces and the potential impact on public health.
What many people don't realize is that our homes, while providing a sense of security, can also be vulnerable to unseen threats. This outbreak serves as a reminder that we must pay attention to the air we breathe, even within the confines of our own homes. It's a fascinating and somewhat unsettling insight into the hidden world of indoor air transmission.
