Point source ventilation effectiveness in infection risk-based post-COVID ventilation design

Measurement method for ventilation effectiveness, more specifically, for contaminant removal effectiveness with a point source corresponding to infector is analysed in this study with tracer gas measurements and infection risk calculations. Ventilation effectiveness is needed in infection risk-based ventilation design to take into account air distribution methods deviating from fully mixing. Tracer gas measurements were conducted with two source location in six non-residential spaces. Ventilation effectiveness calculated based on the infection risk probability assessment for every measurement point in the room was compared with calculation from the average concentration and calculation method proposed by REHVA accounting only 50% of measurement points with highest concentration. To conduct infection risk calculation, Wells-Riley model modification providing a relation between infection risk probability and ventilation rate at fully mixing was applied together with infection risk control concept based on the basic reproduction number R0 = 1 during pre-symptomatic infectious period. By applying the required ventilation rate at fully mixing and individual probability of infection in each measurement point, ventilation effectiveness value corresponding to given event reproduction number was solved. With the method developed, the airflow rate at fully mixing and the airflow rate with actual air distribution, calculated with ventilation effectiveness, provide the same event reproduction number. Results show considerable differences compared to calculation based on average measured concentration, which overestimated the ventilation effectiveness and underestimated design ventilation rate. The method proposed by REHVA, taking into account only 50% of measurement points with highest concentration, revealed to be conservative in all studied cases, as ventilation effectiveness values ranged in between 0.34 – 1.29 compared to 0.62 – 1.44 solved from individual risk of all measurement points. Especially in the large open plan office, REHVA method considerably overestimated the design ventilation rate while in smaller spaces all three methods provided similar results. Results indicate that ventilation effectiveness determination from tracer gas measurements with a point source is not a trivial task. Calculation method developed, utilising individual probability of infection in each measurement point can be proposed to improve prediction accuracy.