Table 1.

Typical building ventilation system guidelines in different countries to reduce the risk of COVID-19 transmission.

Region & institution	Typical building ventilation system guidelines
Canada Canadian Committee on Indoor Air Quality (Aug. 2020 version) (Canadian Committee on Indoor Air Quality, 2020)	•Increase the amount of air exchanged per hour •Ventilation systems should operate beyond the hours of occupancy (two hours before and after) or around the clock. •Use high performance filters •Ensure proper air distribution and circulation.
USA ASHRAE/CDC (May. 2020 version) (Schoen, 2020)	•Increase outdoor air ventilation (use caution in highly polluted areas) •Disable demand-controlled ventilation •Further open minimum outdoor air dampers, which can eliminate recirculation) •Improve central air filtration. •Keep systems running longer hours, if possible 24/7.
European REHVA (Nov. 2020 version) (REHVA, 2020)	•Increase air supply and exhaust ventilation; in demand-controlled ventilation system, change the CO2 setpoint to 400 ppm. •Ventilation system should start at the nominal speed at least 2 hours before the building opening time and switch off or to a lower speed 2 hours after the building usage time. •No use of central recirculation.
China National Health Commission of the People's Republic of China (Jul. 2020 version) (National Health Commission of the People's Republic of China, 2020)	For all-air air conditioning system: •Ventilation system should operate at the maximum fresh air flow rates and reduce or even eliminate return air based on the performance of the filtration or disinfection. •For buildings with a large number of occupants, e.g., mall or office, start ventilation at least 1 hour before the building opening time and switch off 1 hour after the building usage time.
WHO (Mar. 2021 version) (WHO, 2020)	For non-residential spaces: •The minimum mechanical ventilation rate should be higher than 10 L/s/person. •The indoor air should be changed as uniformly as possible. •For facilities with non-occupancy periods, the HVAC system should be operated with maximum outside airflow for 2 hours before and after occupied times. •Increase the percentage of outdoor air in recirculation; guarantee air separation in heat recovery unit.