On-site quantification of re-entry ratio of ventilation exhausts in multi-family residential buildings and implications

In the worldwide spread of Severe Acute Respiratory Syndrome (SARS) in 2003, cluster of cases occurred in a number of large high-rise residential building blocks, especially in Hong Kong. In this study, we examined one of the most likely virus-spread mechanisms, which is related to the inter-flat or inter-zonal airflow through open-windows caused by buoyancy effects. Dual tracer gases of CO2 and SF6 are employed simultaneously to quantify the amount of the exhaust air coming out of the upper part of the window of a floor that re-enters the lower part of the open-window at the immediate upper floor. It was found that the room air could contain up to 7% of the exhaust air from the lower floor, and this occurs at low wind conditions with a combination of indoor-outdoor temperature difference. The results can well explain the earlier governmental finding that DNA strings of SARS Corono-Virus were detected within the sampled deposits on the window sills of the upper floors of the two index patients' flats. The preliminary and yet alarming conclusion may be that, in high-rise buildings, windows flush with a flat facade can be a major route for the vertical spread of pathogen-containing aerosols.