Occupancy-aided ventilation for both airborne infection risk control and work productivity

Abstract

Reducing airborne infectious risk is crucial for controlling infectious respiratory diseases ( e.g. , COVID-19). The airborne transmissibility of COVID-19 is high so that the common ventilation rate may be insufficient to dilute the airborne pathogens, particularly in public buildings with a relatively large occupancy density. Reducing occupancy can reduce the pathogen load thereby reducing airborne infection risk. However, reduced occupancy deteriorates work productivity due to the lost hours of work. This study proposes an occupancy-aided ventilation strategy for constraining the airborne infection risk and minimizing the loss of work productivity. Firstly, two mechanisms of occupancy schedule (alternative changeovers between normal occupancy and reduced occupancy) for reducing the airborne infection risk and loss of work productivity are revealed based on analyzing features of the indoor concentration profile of exhaled aerosols. Secondly, optimization of the occupancy schedule is developed to maximize the total time length of normal occupancy for the minimum loss in work productivity while satisfying the constraint on airborne infection risk ( e.g. , with the reproduction number less than one). The airborne infection risk is evaluated with the rebreathed fraction model. Case studies on COVID-19 in a classroom demonstrate that the proposed occupancy-aided ventilation is effective with an earning ratio of 1.67 (the ratio of the improvement in health outcome to the loss in work productivity) and is robust to the variable occupancy loads and occupancy flexibilities.

Dilution-based evaluation of airborne infection risk - Thorough expansion of Wells-Riley model

“…The well-recognized Wells-Riley model and its modification (the rebreathed-fraction model) estimate the airborne infection risk with the infector's quantum generation rate, considering the biological properties of pathogens implicitly [ 14 ]. However, the Wells-Riley model has no spatial and temporal resolutions and the rebreathed-fraction model has no temporal resolution, resulting in overestimation/underestimation of airborne infection risk [ 14 , 18 , 21 ]. The proposed dilution-based evaluation of airborne infection risk, as a thorough expansion of the Wells-Riley model, has both spatial and temporal resolutions (Sections 2 , 5 ).…”

“…The quantum generation rate of a COVID-19 infector is assigned to be 142 quanta/h and the breathing rate of a standing susceptible is 0.54 m 3 /h [ 26 ]. It should be noted that the quantum generation rate of COVID-19 varies when the activity level and viral load of the infector vary [ 26 ], and the quantum generation rate of 142 quanta/h is used as a specific case [ 21 ] for the application demonstration of the proposed dilution-based evaluation of airborne infection risk. Two scenarios are considered, one with no masks and the other with surgical masks for both the infector and susceptible.…”

“…, the concentrations of exhaled air by the infectors) in the space air and ventilation supply air respectively (ppm); D ms is the dilution ratio under the well-mixed and steady condition; G is the contaminant generation rate in the space ( i.e. , exhaled air by the infectors which can be represented by exhaled CO 2 or other tracer gases) (m 3 /s) [ 18 , 20 , 21 ]; P D,ms is the airborne infection risk estimated by the dilution-based estimation method proposed under the well-mixed and steady condition; V is the volume of the space (m 3 ).…”

Recent research on expiratory particles in respiratory viral infection and control strategies: A review

“…However, this occupancy-aided ventilation may reduce work productivity. Therefore, Zhang et al. (2021) further optimized the occupancy schedule to maximize the total duration of normal occupancy, balancing low airborne infection risks and work productivity.…”

“…(2021) further optimized the occupancy schedule to maximize the total duration of normal occupancy, balancing low airborne infection risks and work productivity. Despite the innovation of occupancy-aided ventilation, there are limitations such as potential corridor blockage, simplification of work productivity evaluation, assumption of uniform virus distribution, and difficulty in obtaining infector numbers ( Zhang et al., 2021 ), and further research is required.…”