Supply air filtration and fine particle levels in indoor air of occupied dwellings

Indoor Air quality (IAQ) of dwellings is the result of several sources and processes, and the impact of ventilation system is the one amongst many others. Definition and metrics of IAQ are several and we choose in this study to focus on airborne particle levels. Our question was: How the filtration of supply air impacts particle levels in indoor air?  
On-site measurement campaigns were carried out in occupied dwellings between 2016 and 2019. Three houses and one flat located in the city and in the countryside were instrumented during three weeks. Two of the dwellings were equipped with balanced mechanical ventilation systems, one with mechanical exhaust air system and the last one with mechanical supply air system. Optical particle counters, measuring in number particle concentrations between 0.3 and 3 µm, were used to monitor particle levels of outdoor air, supply air and extract air, every 30 seconds. Supply air filtration was changed every week in order to follow the impact of the level of filtration on indoor air particle concentrations: fine filter, coarse filter or no filter on supply air. To be able to understand fluctuations over time, occupants were asked to fulfil questionnaires informing about activities (cooking, cleaning, airing, etc.) and room occupancies.  
The first finding is that on-site measurement in occupied dwellings can imply strong variations on external parameters, but also on occupant-related sources. It makes it difficult the comparison between measurement periods. Therefore, both time curves and statistical indicators, based on box plot or weekly curves between indoor and outdoor concentrations for each size of particles, have been used to analyze the results. Moreover measurements during occupancy and during night time have been analyzed separately.  
Regarding the filter efficiency, on-site measurements of filter penetration index (ratio between supply air and outdoor air concentrations) are consistent with efficiencies measured in the laboratory, for each particle size category. In particular, medium filters have a low impact on particles smaller than 1 µm.  
The main conclusion of this study is that indoor air quality does not only depend on the quality of the air entering the dwelling. The behaviors of the occupants, the opening of doors and windows, the cooking activities with, for example, the use of recirculating kitchen hoods are important elements. However, the contribution of the supply air filtration on indoor air particles concentrations is real and positive, linked with the efficiency for each size of particles. We haven’t seen much impact, using medium filters, while the impact was positive with fine filters. To keep this impact positive, attention should be drawn to the need for good maintenance of these filters to avoid a decrease in airflow due to clogging, and to air renewal.