The idea of safety within one's home was shattered during the COVID-19 pandemic, as evidenced by an unusual outbreak in Spain. This incident challenged the notion that simply closing doors and avoiding crowds was enough to prevent the spread of the virus. The focus then shifted to the air within the walls, a previously overlooked aspect of indoor safety.
In the city of Santander, a seven-story residential building experienced a sudden surge in COVID-19 cases. What made this outbreak particularly intriguing was the vertical pattern of infections. Residents across four apartments, stacked vertically, tested positive within days, despite not being next to each other.
This pattern led to a groundbreaking discovery. A resident engineer, David Higuera, noticed that the infections followed a precise vertical line, resembling airflow rather than human contact. This revelation prompted a deeper investigation into the building's ventilation system.
The building, constructed in 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. However, this system inadvertently created a pathway for air to travel between homes.
Researchers found that when pressure dropped, airflow could reverse, allowing air from one apartment to enter another through the bathroom vent. This reverse flow, reaching speeds of 42 liters per second, carried virus-carrying aerosols, facilitating the vertical spread of infection.
Computer models confirmed these observations, demonstrating that air from a lower apartment could move up through the shaft and enter an upper apartment. The study also revealed the role of kitchen exhaust fans, which created negative pressure and pulled air from the shared shaft, potentially spreading infection.
Some apartments remained unaffected, thanks to modifications like exhaust fans with one-way flaps or sealed vents. These modifications effectively blocked reverse airflow, highlighting the importance of ventilation design in infection control.
The genetic analysis of the virus further supported the vertical transmission within the building. The nearly identical genetic patterns in infected residents indicated a single transmission chain, reinforcing the idea that the virus spread through the ventilation system.
This case study is not an isolated incident. Similar vertical transmission events have occurred during previous outbreaks, such as SARS in Hong Kong and during the COVID-19 pandemic in Seoul. Many older buildings still use shared ventilation systems, designed for efficiency but lacking infection control measures.
Shelly Miller, the study's first author, emphasizes the broader implications of this finding. She states that even in spaces far from the source of infection, shared air pathways can lead to transmission. This includes multifamily apartment buildings, hotels, office buildings, and cruise ships.
The solution to this issue is surprisingly simple. Installing exhaust fans with one-way flaps can prevent reverse airflow, ensuring that air moves out safely when the fan is on and blocking incoming air when the fan is off. Additionally, allowing fresh air to enter while using kitchen fans can help balance pressure and reduce the risk of infection.
This outbreak has significantly altered our understanding of indoor safety. It demonstrates that walls and doors are not always effective barriers, and hidden pathways within buildings can facilitate the spread of airborne pathogens. As a result, building inspections must now consider airflow systems more carefully, recognizing shared ducts and cavities as potential channels for infection.
In Santander, a simple ventilation shaft became the link between households, revealing the unexpected ways in which infection can move through buildings. This study serves as a reminder that indoor safety is a complex issue, and addressing it requires a comprehensive understanding of ventilation systems and their potential impact on public health.
