The concept of home as a sanctuary, a place of refuge from the outside world, was challenged by an unusual COVID-19 outbreak in Spain. This incident, which occurred in a residential building in Santander, forced scientists to delve into the often-overlooked realm of indoor air quality and its potential impact on public health.
The Vertical Mystery
In the summer of 2020, when COVID-19 cases were seemingly under control, an unexpected surge of infections occurred in a seven-story building. What was intriguing was the pattern of these infections: they were not random, but rather, they followed a vertical path. Apartments stacked on top of each other, with no horizontal connection, reported cases.
A resident engineer, David Higuera, noticed this peculiar pattern. He and his wife tested positive, as did their vertical neighbors. The virus seemed to be spreading in a neat column, with cases appearing above and below their apartment. Higuera realized the implications of this:
"If what my wife and I suspected was happening, it could have significant scientific implications for public health."
Airflow: The Unseen Connector
The building, dating back to 1969, had a unique ventilation system. Each apartment had a small bathroom vent connected to a shared vertical shaft, which allowed air to move upward and exit the building due to natural airflow. However, this system also created a shared air pathway between homes, and as we know, viruses can travel through the air.
Researchers studied the airflow and found that under certain conditions, the airflow reversed. When a kitchen exhaust fan was turned on, it could pull air from the shared shaft, bringing with it particles from other homes, including potentially virus-carrying aerosols. This reverse flow was significant, reaching about 42 liters per second.
The Ghost in the Room
In one test, researchers measured carbon dioxide levels in an empty apartment. Carbon dioxide is a marker of human breath, and the levels increased throughout the day, indicating that air was entering the apartment from elsewhere.
"It was like there was a ghost in the room," Higuera said.
This finding confirmed that the air was not just moving out of the building, but also entering apartments, potentially carrying the virus.
The Role of Kitchen Fans
One surprising discovery was the impact of kitchen exhaust fans. These fans, while removing indoor air, also create negative pressure, which can pull air from the shared shaft into the apartment. Bathroom fans had a similar effect, potentially pushing contaminated air to other floors.
Safe Apartments and Genetic Proof
Interestingly, some apartments in the building did not report infections, even though they shared 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 was a key factor in the outbreak.
Scientists also analyzed the virus's genetic patterns, finding nearly identical samples from infected residents. This confirmed that the virus spread within the building and not from external sources.
Broader Implications
This case is not unique. Similar events have occurred during previous 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.
"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," said Shelly Miller, the study's first author.
This can happen in various settings, from multifamily apartment buildings to hotels, office buildings, and even cruise ships.
A Simple Solution
The solution to this problem is surprisingly straightforward. Installing a small exhaust fan with a one-way flap can prevent reverse airflow. Another measure is to allow fresh air to enter while using kitchen fans, such as opening a window to balance pressure.
Rethinking Building Safety
This outbreak highlights the need to reconsider indoor 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, as shared ducts, pipes, and cavities can act as channels for airborne particles.
In conclusion, this study emphasizes the importance of understanding the role of airflow in disease transmission. It shows that infection can move through buildings in ways that are often overlooked, and simple engineering fixes can make a significant difference in preventing the spread of airborne viruses.
