# Ventilation Systems and Their Impact on Nanoparticle Concentrations in Office Buildings  
[PERSON]  
1  
[PERSON]  
2 Section of Sustainable Health, Department of Public Health and Clinical Medicine, Faculty of Medicine, Umea University, 901 87 Umea, Sweden; [EMAIL_ADDRESS]  
[PERSON]  
1  
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pollutants generated in each part of the building should also be removed in an efficient manner [10]. There are three methods that are used in order to ventiate a building: natural ventilation (NV), mechanical ventilation (MV), and hybrid ventilation (HV). NV is driven by thermal forces that arise as a result of differences in the density of the indoor and outdoor air. The outdoor air flows in through purpose-built openings that include windows, doors, solar chimneys, wind towers, and trickle ventilators. The efficiency of NV largely depends on the temperature difference between the indoor and outdoor air. MV uses fans in order to create pressure, which in turn creates air exchange within a building. MV can be based on either an overpressure system, a negative pressure system, or a combination of these. An overpressure system uses fans at the air intake where the air is supplied to the building. A negative pressure system uses fans that suck the air out of the building. HV is a combination of natural and mechanical ventilation. HV relies on natural driving forces in order to maintain an air exchange within a building, but it is equipped with fans that can be activated when the natural driving forces become insufficient [10]. The effects of ventilation methods on particle concentrations indoors have been addressed in a number of studies. In a study from Sydney, Australia, air pollutants were measured indoors during one year in eleven occupational buildings with ventilation systems representing NV, MV and HV. In general, the buildings equipped with an MV system showed the lowest average concentrations of both TSP (total suspended particles), particulate matter (PM\({}_{10}\)), and fine particles (PM\({}_{2.5}\)) [11]. In another study in Seoul, South Korea, where the influence of different ventilation systems on indoor particle concentrations was analyzed in 15 single-family apartments, it appeared that MV reduced the exposure to outdoor particles more than NV. When the apartments were mechanically ventilated, the daily average indoor particle concentration decreased to 50% below the outdoor level for submicrometer particles (<1 \(\upmu\)m) and 25% below the outdoor level for fine particles [12]. In general, mechanical ventilation equipped with filters reduces the particle concentrations indoors, but the magnitude of this effect depends on the efficiency, location, and the number of filters applied [13; 14]. [PERSON] et al. [15] found that the concentration of indoor nanoparticles originating outdoors was largely determined by the filtration efficiency of the filters used in the ventilation system. The filters are classified according to their ability to capture particles at different sizes, defined as filtration efficiency. The abbreviations M and F stand for medium and fine where the filters classified as fine have a higher degree of separation in comparison with the filters classified as medium. The filtration efficiency for different filters at different particles sizes is presented in Table 1.  
Moreover, different ventilation conditions (closed windows, windows opened for short periods, and windows tilted open all day long) might influence indoor air quality. [PERSON] et al. [17] analyzed a hospital building in Erfurt, Germany, and found that the lowest indoor/outdoor (I/O) ratios for all pollutants were found with closed windows.