Performance of combination of local exhaust system and floor-supply displacement ventilation system as prevention measure of infection in consulting room

Abstract

Droplet nuclei and tiny enough droplets to move as an aerosol are regarded as one of the modes of infection transmission SARS-CoV-2. Various measures have been taken to prevent it worldwide. Nevertheless, many scenarios cannot be avoided close-distance conversations, for example, in a consulting room, restaurant, or crowded train. A consulting room has significant potential for doctors to contact infected patients. Therefore, this study proposes a novel approach combining a local exhaust ventilation (LEV) and floor-supply displacement ventilation system (FSDV) in a consulting room. This study assumes that two persons (doctor and patient) are sitting face to face and talking without a mask in a simple room regarded as a consulting room. The velocity and volume of exhaled air from talking were acquired through field measurements. Then, computational fluid dynamics (CFD) steady analysis was carried out, using the results of exhaled air measurement with various parameters (hood height, hood flow rate, horizontal hood position, and air flow rate). The capture efficiency for tracer gas and contribution distribution for the hood (SVE5: scale for ventilation efficiency 5) have been calculated to reveal the hood's capture performance. In addition, infection risk for the doctor was also calculated using the Wells–Riley model to estimate the infection performance of this ventilation system. By measuring exhaled air from talking, a speed of 0.30 m/s, a volume of 5.21 L/min, and a vertical angle of 11.9° were obtained, and these values were installed into CFD. The CFD results showed that hood flow rate significantly impacts capture efficiency at SA 120 m³/h (6 ACH), and horizontal hood position significantly impacts at SA 1000 m³/h (50 ACH). SVE5 also showed hood's effective area is greatly influenced by the flow rate balance between the hood and the other exhaust routes. Under high air supply conditions: SA 1000 m³/h (50 ACH), there was almost no airborne transmission risk for a doctor with or without a hood. However, under 120 m³/h (6 ACH) conditions, the combination of the hood and FSDV system could reduce an infection risk sufficiently. The hood should be located above the infected person's head to keep the counter person's infectious risk low, indicating that the introduction of the hood is reasonable in the consulting room, where it is easy to find where the infected person is.