Infection-prevention performance of local exhaust ventilation under three different underfloor air distribution systems during a face-to-face conversation

This study proposes using a local exhaust ventilation system (LEV) to prevent airborne infections, especially for short-range conversations. We compared the performance of a hood in three different underfloor air distribution systems (UFAD): floor-supply displacement ventilation (FSDV), horizontal flow-type floor diffuser (HFD), and swirling flow-type floor diffuser (SFD). Two situations were considered: Case A, a consulting room, and Case B, a restaurant or meeting room. The difference in infection risk assessment between using CO2 and artificial saliva particles as tracers of exhaled breath was also discussed. Results indicate that the distribution of exhaled air and infection risk for doctors decreased in the order FSDV < HFD < SFD. Although the effect of introducing hoods was confirmed to a certain degree for the three ventilation methods in Case A, the effect of the hoods on the quanta concentration of the facing person was small in Case B. Comparing airborne infection risks between gas and particles, particle-based airborne infection was smaller in the FSDV due to the more significant impact of particle adhesion and falling. As a limitation, the ventilation rate in the experiment was 1000 m3/h (50 ACH). Therefore, the air supply method had a more significant impact on the results than the hood method. A practical implication of this experiment is that even under high ventilation volumes (50 ACH), the FSDV can reduce the horizontal distribution of the patient's exhaled air and prevent airborne infection. These results should be adapted to smaller spaces such as examination rooms and meeting rooms.