A tracer gas study of ventilation and air movement to inform potential SARS-CoV-2 airborne transmission

Tracer gas studies were carried out to investigate the potential for airborne transmission of SARS-CoV-2 variants, and to inform work practises to reduce COVID-19 exposure and transmission. The air exchange of a meeting room was determined in accordance with ASTM E741-11 using sulfur hexafluoride (SF6), a colourless, odorless, non-flammable, and non-toxic tracer gas. Air was generally well mixed in the room, and very little tracer gas escaped through the closed door. The calculated air changes per hour (ACH) of the meeting room (5.8 h-1) was higher than the value recommended for offices (2-3 h-1) and used to assess potential COVID-19 infection risk and calculate maximum occupancy rates using the Airborne Infection Risk Calculator (AIRC) V3 Beta. Results showed that the previous guidance for maximum occupancy rates provided by many Australian States and Territories (the 2 or 4 square meters rule) provides a conservative estimate of the maximum number of occupants within a room for exposure events of up to 2 hours. However, for longer periods, such as a full working day (7.5 hours), and for more infectious strains of SARS-CoV-2, the 2 or 4 square meter rule did not adequately address infection risk. Use of two additional controls, mask wearing and ensuring that all occupants had received a third boosted vaccine dose, was estimated to reduce the probability of infection by 3.75-fold assuming infection with the Omicron variant. Including both controls increased the predicted maximum occupancy to values greater than the 2 and 4 square meter rule for most scenarios. Air movement outside the meeting room was investigated by filling the room with tracer gas and allowing the air to move freely by keeping the door open. Airflow was most influenced by the ventilation system of the laboratories in the north wing of building, rather than laboratories in the south wing. The plug of tracer gas was largely removed from the building within 30 minutes, although detectable concentrations were measured in some areas with lower air flow at the end of the 2-hour experiment. High concentrations were also noted in front of the elevators, however tracer gas concentrations were below detection on other floors. The techniques employed in this study are suitable for measuring air movement and ventilation in other buildings where aerosol transmission of COVID-19 is of concern.