During the COVID-19 pandemic, airborne transmission of SARS-CoV-2 via respiratory aerosols was a critical concern in indoor environments. In the city of Santander, Spain, an outbreak in a multi-family residential building during a period of low community transmission revealed vertical clustering of 15 cases in four homes. The building' s design included single interior bathrooms without windows in each home, ventilated by a shared vertical bathroom duct system. Field measurements, computational...