**5. Conclusions**

This study performed measurement experiments to evaluate the reduction performance of fine particulate matter concentration in the breathing zone for a method of delivering purified air discharged from an air purifier to the human breathing zone. The results are as follows:

The method of installing a fixed air purifier at a location adjacent to occupants without changing the discharged direction cannot improve the performance in reducing the concentration of fine particulate matter within the breathing zone of occupants; the method of delivering purified air discharged from an air purifier can better reduce the concertation of fine particulate matter in the breathing zone of occupants compared to the concentration of fine particulate matter in the surrounding area, but if the distance from the air purifier to the controlling point is distant and so the velocity of airflow is not su fficient, there is no e ffect in reducing the fine particulate matter concentration.

In the case of a mobile air purifier, the real-time occupant tracking method was e ffective in terms of reducing the concentration of fine particulate matter in the breathing zone of occupants by 51 μg/m<sup>3</sup> compared to the surrounding PM concentration, but there are limits in actual use regarding the moving path of the air purifier or the change in the number of occupants. On the contrary, the operation of the movable air purifier showed that the fine PM concentration of the occupant's respiratory zone to be 68 μg/m<sup>3</sup> lower than other measurement points. Thus, it is more e ffective to divide the target space by zone and move an air purifier around considering the number of occupants and the mobility of an air purifier.

This study compared and evaluated an air purifier's performance in reducing the concentration of fine particulate matter in the breathing zone of occupants against measurement points of other zones by setting measurement points of each zone in one place when delivering purified air discharged from an air purifier. However, the level of reduction performance of fine particulate matter could di ffer according to the range of influencing purified air currents within the same zone. In addition, in the case of the zone controlling method, there are also limitations regarding the moving path and the distance between the occupants. Accordingly, a follow-up study on the range of purified air currents discharged from an air purifier, method of expanding the range, method of installing air purifier on the ceiling, and changing control mode according to the number of occupants will be conducted in future.

**Author Contributions:** Conceptualization, H.P.; methodology, H.P. and S.P.; writing-original draft preparation, H.P.; writing—review and editing, H.P. and S.P.; Supervision, J.S.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Research Foundation of Korea (NRF) gran<sup>t</sup> funded by the Korea governmen<sup>t</sup> (MSIT) (No. 2018R1A2B2007165).

**Conflicts of Interest:** The authors declare no conflict of interest.
