Measurement of ventilation effectiveness and indoor air quality in toilets at mass gathering events

Mass gathering events were closed in 2020 to reduce the spread of SARS-CoV-2. These events included music concerts, theatre shows, and sports matches. It is known, however, that the long-range aerosol transmission of pathogens, such as SARS-CoV-2, can be reduced with sufficient ventilation indoors. This paper examines the risk of reopening these mass gathering events by measuring the CO2 concentration, as a proxy for ventilation effectiveness, at 58 events, with a specific focus on small enclosed spaces with short occupancy. Toilets (sanitary accommodation) are spaces that are densely and continuously occupied for short durations throughout the events, such as during theatre intervals or half-time at sports events. The results showed that the average air quality in toilets was good at most events. There were, however, considerable peaks in CO2 concentration of up to 3431 ppm in toilets at times when occupancy was presumed high, indicating that the risk of exposure to exhaled breath, which may contain virus-laden aerosols, is higher in toilets than elsewhere in the venue (although occupancy duration will be much lower). Recommendations are provided to encourage building designers and operators to be mindful of the ventilation strategies used in toilets given their occupancy and size.