Air quality at mass gatherings: assessing ventilation and occupancy in marquees to evaluate airborne infection risk during the COVID-19 pandemic

The COVID-19 pandemic led to the cancellation of mass gathering events to slow the transmission of the SARS-CoV-2 virus, partially due to evidence that the virus can spread via airborne routes, especially in densely occupied, poorly ventilated spaces. Structures such as marquees (large tents), are commonly used at mass gathering events and were a frequently designated “outdoor safe space” during the COVID-19 pandemic. There is, however, scant evidence as to whether semi-outdoor buildings are sufficiently ventilated relative to the occupancy levels to reduce airborne transmission. As part of the largest study of mass gathering events to date, we measured ventilation and occupancy at 80 real events. We compared seven semi-outdoor spaces and one indoor space. Our results showed that most semi-outdoor buildings were sufficiently ventilated relative to the occupancy (mean CO2 <800 ppm). Short peaks in CO2 concentration of up to 1,200 ppm indicated intermittent, but brief, periods of insufficient ventilation relative to the occupancy in some spaces. High occupant density, heterogeneous occupant distribution (crowding), and poor ventilation management strategies negatively influenced the indoor air quality. Event management strategies, such as intervals between events, improved air quality. We conclude that semi-outdoor buildings are not inherently low-risk with respect to long-range airborne pathogen transmission and so require careful consideration for the ventilation provision relative to the occupancy. The evidence presented, using the largest field study of its kind worldwide, provides key evidence to inform revisions to building regulations and pandemic preparedness plans concerning the use of semi-outdoor buildings.