# Ventilation Systems and Their Impact on Nanoparticle Concentrations in Office Buildings
## 4 Discussion
### The Filters Used in the Ventilation Systems and Their Impact on the Particle Concentrations  
As mentioned in Section 2, the filter type F7 was used in all buildings except Building 10 where the filter type M6 was used. Building 11 was also equipped with the filter type F9, but the measurements were carried out in the part of the building where the air was filtered by F7 filters. Building 1 was security classified without information on filter type, but as it was a new building, it had filters \(\geq\)F7. The ability of filter class F7 to absorb particles in different size fractions has also been analyzed in previous studies. The filtration efficiency of particles in F7 filters is mainly driven by diffusion, impaction, and interception processes in the filter media [30]. During test conditions, where an aerosol generator was used in order to generate particles in the nucleation mode (mean particle diameter of 11 nm) and the soot mode (mean particle diameter of 55 nm), the filtration efficiency of F7 filters varied with respect to the size of the particles. The collection efficiency was almost 100% for particles in the size range up to 10 nm, but decreased steeply in the particle size range above 20 nm [15]. Similar results regarding the efficiency of F7 filters have been demonstrated by [PERSON] [30]. The reason for the very high collection efficiency for particles smaller than 20 nm is that the diffusion movement of particles increases in smaller particle sizes, resulting in higher filtration efficiency [15]. Considering the size distributions of the NP concentrations measured in the twelve buildings during the heating and the nonheating season (Figures A1-A24 in Appendix A), the concentrations of NPs in the size range of 10-20 nm were in general relatively low, indicating a high collection efficiency by the F7 filters. However, there were some exceptions with relatively high particle concentrations in the size range of 10-20 nm (Figures A8, A9, A11 and A21). The highest levels of very small NPs appeared in building 11, which was the building closest to a busy street (Figure 1), and where traffic-generated particles can be assumed to be a particularly significant source. However, the peak concentrations measured during the one-week periods in the twelve office buildings were for the most part shown in the size range of 50-60 nm, representing the Aitken mode. It has previously been shown that the average size of NPs measured outdoors are considerably smaller in comparison with the NPs measured indoors [24]. For the NPs measured outdoors during the twelve one-week periods, only the total numbers of particles have been measured based on five-minute intervals, but the numbers of particles in different size fractions are not available. However, it is reasonable to assume that particles in the size range smaller than 50 nm, mainly emitted from traffic, enter the ventilation systems where they are absorbed by the filters. The correlation coefficients calculated between the ventilation flow and the NP concentrations in different size fractions in Figures 11-14 were inconclusive, but the relatively low values in the size range of 10-20 nm indicate an effective filtering by the F7 filters. However, since there were both indoor and outdoor sources contributing to the measured particles, the coefficients could vary depending on whether indoor sources or outdoor sources dominate. The importance of the filtering efficiency and the use of higher efficiency filter to improve indoor air quality have been suggested by several authors [31; 32; 33]. However, the filtering efficiency of traffic exhaust at hot spot locations could be only about one-third [34]. In the case of very high NP concentrations, additional portable air cleaners with HEPA (high efficiency particulate arresting) filters could be used [35]. However, the use of higher class filters results in higher energy demand [36].