Figure 3 shows CFD modeling for the vertical movement of contaminated air in the air shaft between unit 7 (Ad) and unit 8 (Ab) in building E and the three-dimensional spread of the viral plume to buildings B, C, and D. Moist, warm air flowed from the bathroom of the index unit (unit 7 in building E, with windows facing direction Ad) and established a plume in the air shaft that spread the airborne virus upward. The predicted decay in the concentration of the contaminated aerosols in the plume was 25 percent at the top of the air shaft where a northeasterly wind was blowing at the velocity of 2 m per second. On reaching the top of the building, the contaminated air was carried by this northeasterly wind toward other buildings. A substantial amount of the contaminated air passed between buildings C and D at the height of the middle-level floors.
In summary, our epidemiologic analysis, experimental studies, and airflow simulations support the probability of an airborne spread of the SARS virus in the outbreak in Amoy Gardens.
Virus-laden aerosols generated in the vertical soil stack of unit 7 in building E returned to the bathroom through the dried-up seals of the floor-drain traps and then
entered the air shaft, probably by means of suction created by an exhaust fan. The aerosols
moved upward owing to the buoyancy of the warm, humid air within the air shaft and could
enter apartment units that bordered the air shaft on the upper floors because of the negative pressure created by the exhaust fans or the action of wind flows around the building.
The horizontal spread of infection to other units in building E was by movement of the air between apartment units. After the plume reached the top of the air shaft in building E, the virus was spread to some units at certain heights in buildings B, C, and D by the action of a predominant northeasterly wind.