The relationship between ventilation rates in schools and the indoor airborne transmission potential of COVID-19

The requirement for natural ventilation strategies in classrooms to perform in respect to both their inherent and adaptive capacity is necessary to mitigate the risk of transmission associated with infectious airborne pathogens such as COVID-19. This paper assesses the performance of ten classrooms considering (a) the designed versus actual ventilation capacity; (b) the probability of airborne disease transmission by unit time resulting from calculated ventilation rates and (c) the reduction of transmission risk associated with mask wearing by all occupants. Monitored air changes per hour (ACH) range from 0.26–1.7, demonstrating that none of the classrooms meet designed ventilation rate whilst in operation. If the designed ventilation rate of 5 ACH were to be achieved, it would represent an infection risk of less than 1% during standard class durations and 5% over an 8-hour school day. The minimum achieved ventilation rate represents an infection risk of 7–12% (per class) and 75–100% (per school day) dependent on classroom specific parameters. Protection is afforded by the wearing of masks within the indoor school setting. During specified class durations in settings with sub-optimal ventilation rates, probability of infection can be reduced from 7–12% to 1–5%. Over an 8-hour school day, probability of infection can be reduced from 75–100% to 18–43%. These reductions are possible dependent on mask choice.