The idea of safety inside one’s home shaped much of how people responded to COVID-19. Close the door, avoid crowds, and the risk drops. But one unusual outbreak in Spain challenged that belief and forced scientists to look at something most people ignore: the air inside walls. In the summer of 2020, the city of Santander had nearly eliminated COVID-19 cases. Life had started to feel normal again. Then, in a seven-story residential building, infections appeared suddenly. Fifteen residents across four apartments tested positive within days. The pattern made little sense at first. These apartments were not next to each other. They were stacked vertically. Each infected home sat directly above or below another. This unusual layout hinted at something hidden. The connection was not social – it was structural. A resident engineer, David Higuera, noticed the pattern early. He and his wife tested positive. So did neighbors on the same vertical line of apartments. Cases appeared above and below them in a neat column. “I knew that if what my wife and I suspected was happening, it could have significant scientific implications for public health,” said Higuera. This was not random spread. It followed a path that looked more like airflow than human contact. Old design meets a new virus The building dated back to 1969. It came before modern ventilation standards in Spain. Each apartment had a small bathroom vent connected to a shared vertical shaft. This shaft ran from the bottom floor to the roof. It worked using natural airflow. Warm air rises, so the system allowed air to move upward and exit the building. It seemed simple and effective. But it also created a shared air pathway between homes. Airflow inside buildings is not constant. It shifts with temperature, weather, and human activity. Opening a window or turning on a fan can change pressure inside a room. When pressure drops, airflow can reverse. Instead of air moving out, it moves in. In this building, that meant air from one apartment could enter another through the bathroom vent. Clues in an empty apartment Researchers studied the airflow in detail. They monitored pressure, airspeed, and carbon dioxide levels. Carbon dioxide is a marker of human breath. In one test, they measured an empty apartment. It should have had almost no carbon dioxide. Instead, levels increased throughout the day. “It was like there was a ghost in the room,” Higuera said. The air was not coming from outside. It was coming from other apartments. The virus spread through vents Under certain conditions, airflow reversed strongly. When a kitchen exhaust fan ran at full power, air rushed into the bathroom vent instead of leaving it. This reverse flow reached about 42 liters per second. Along with the air came particles from other homes, including virus-carrying aerosols. The building’s design allowed air to move both ways. That made it possible for infection to travel vertically. Researchers used computer models to test their observations. One model recreated airflow between two stacked bathrooms. Another simulated the entire building. The results matched real measurements. Air from a lower apartment could move up through the shaft and enter an upper apartment. The same process could happen in reverse. They also used infection models to estimate risk. In several scenarios, the probability of infection increased beyond safe limits. The role of kitchen fans One surprising finding involved kitchen exhaust fans. These devices remove indoor air, but they also create negative pressure. When someone turned on a kitchen hood, it pulled air from the shared shaft. If that air carried virus particles, it entered the apartment. Bathroom fans had a similar effect. They could push contaminated air to other floors. Everyday actions like cooking or ventilating a bathroom could unintentionally spread infection. Apartments that were safe Some apartments in the building did not report infections. These homes shared the same ventilation shaft but had small modifications. Three apartments had exhaust fans with one-way flaps. These flaps allowed air to leave but blocked it from entering. Another apartment had sealed its vent completely. None of these homes had COVID-19 cases. This contrast gave strong support to the idea that airflow caused the outbreak. Genetic proof of spread Scientists also analyzed the virus itself. Samples from infected residents showed nearly identical genetic patterns. They were different from other cases in the city at the time. This confirmed that the virus spread within the building rather than from outside sources. The outbreak followed a single transmission chain through connected apartments. Broader implications of the study This case is not isolated. Similar events occurred during earlier outbreaks. In 2003, SARS spread through a housing complex in Hong Kong. In Seoul and other cities, vertical transmission appeared again during COVID-19. Many older buildings still use shared ventilation shafts. These systems were designed for efficiency, not infection control. “While this is a special building design more common in Spain, it illustrates a broader concern, that even if you are far from the source, if your air is connected, you can still get sick,” said Shelly Miller, first author of the study from the University of Colorado Boulder. “This can happen in a multifamily apartment building through the ducts, in a hotel between the hallway and rooms off the hallway, in office buildings between offices or on a cruise ship.” A simple engineering fix The solution is surprisingly simple. Installing a small exhaust fan with a one-way flap can stop reverse airflow. The flap blocks incoming air when the fan is off. When the fan runs, it pushes air out safely. Another measure is to allow fresh air to enter while using kitchen fans. Opening a window can balance pressure and reduce the chance of pulling air from other apartments. Rethinking building safety This outbreak changes how we think about indoor safety. Walls and doors do not always isolate air. Hidden pathways can connect spaces in unexpected ways. Building inspections need to consider airflow systems more carefully. Shared ducts, pipes, and cavities can act as channels for airborne particles. In Santander, a simple ventilation shaft became the link between households. It showed that infection can move through buildings in ways people rarely consider. The study is published in the journal PLOS One. Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.
