As a common feature of most global cities, tall buildings offer a number of advantages, whether it be to provide offices or homes when urban space is not available or to make an iconic statement on the world stage. However, these “cities in the sky” can also facilitate the rapid spread of infection from one person to another due the high density of people in a single building.
Using advanced numerical modelling techniques, researchers at PRoBE were able to confirm the World Health Organisation conjecture that a high cluster of cases of SARS, reported at the Amoy Gardens residences in Hong Kong in 2003, was caused by the vertical transmission of the virus between apartments via both the sanitary plumbing system and the service risers. The 321 confirmed cases of SARS and 42 fatalities suffered by the residents highlight the risk of infection spread within buildings, especially the significance of a vertical transmission route which is particularly unique to tall buildings.
By modelling air flow movement and air pressure wave propagation within the sanitary plumbing system, the PRoBE research was able to demonstrate the likely circumstances within the building that resulted in the rapid spread of the virus. The index patient (the first resident to become infected with the virus) lived on the 16th floor of the 36 storey building. Infected faecal particles were discharged to the sanitary plumbing system during the diarrhoeal phase of the infection. The flushing of the WC caused the generation of airborne bioaerosols within the system vertical stack. A number of dry floor drains, together with some appliances with no fitted U-bends, provided a route for the ingress of infected bioaerosols from the sanitary plumbing system and into the bathroom, driven by the transient pressures prevailing within the system, natural buoyancy, and the negative pressure created by the bathroom extract fan. After passing through the extract fan, the virus-laden bioaerosols were then exhausted into the external service riser which acted as a channel to spread the virus to upper and lower apartments via open windows.
This mechanism of viral spread was attributed to the infection of residents in some 11 apartments below the index patient and 27 apartments above the index patient – firmly highlighting the significant risk of vertical transmission of infection in tall buildings.