# Ventilation Systems and Their Impact on Nanoparticle Concentrations in Office Buildings
## 4 Discussion  
### The NP Concentrations and the Impact of the Ventilation Flow  
The NP concentrations measured indoors and the simultaneous ventilation flow presented in Figures 5-10 showed mixed results. Since the NP concentrations measured indoors were a combination of particles generated both indoors and outdoors, the ventilation flow can lead to both increasing and decreasing NP concentrations. The particles generated outdoors that infiltrate the indoor environment through the ventilation system can be assumed to correlate with the amount of ventilation flow. [PERSON] et al. [25] found in energy efficient buildings that the ventilation regime with minimum natural and no mechanical ventilation resulted in the lowest infiltration of outdoor pollutants. However, in this study, this effect appeared only in some of the buildings (e.g., Building 1 and 8) but not in the other buildings. [PERSON] et al. [15] have earlier discussed that particles in different size fractions were filtered to varying degrees by the ventilation filters, and since Figures 5-10 are based on the total NP concentrations, only the outdoor generated particles that were not captured by the filters can be assumed to correlate with the size of the ventilation flow. In Building 8 (Figure 8), a certain degree of correlation could be observed. Building 8 was located relatively close to a busy street, and it was equipped with a sensor-controlled ventilation system switching of during nighttime and on weekends, which may partly explain the results. In Building 7 (Figure 7), the NP concentrations and the ventilation flow exhibited a certain anticorrelation. Building 7 was located furthest away from a busy street and can be assumed to have relatively low concentrations of traffic-generated particles in the outdoor environment, and with a relatively greater impact from indoor sources of NPs. In Buildings 10 and 11 (Figures 9 and 10), the NP concentrations fluctuated sharply despite the fact that the ventilation flows were fairly constant throughout the measurement periods. These buildings were located closest to a busy street where fluctuations in the flow of traffic might have affected the results.  
In the case of indoor sources, the ventilation flow was a crucial factor in removing particles from the buildings, and therefore, anticorrelations between the concentrations of indoor generated particles and the ventilation flow are expected. Since the chemical composition of the NPs measured in this study was not analyzed, it was not possible to distinguish the origin of the particles. However, in addition to NPs with outdoor origin, there are a lot of potential indoor sources of NPs that include electrical appliances [26], bioaerosols from biological materials [27], and laser printers that generate NPs in mainly  
Figure 14: The correlation coefficients (Pearson) between the ventilation flow and the NP number concentration during the one-week measurement period presented for each size fraction in Building 8 during the heating season.  
two size classes: one smaller mode with particles < 10 nm and one larger mode in the size range of 40-100 nm [28]. In the current study, some of the buildings (e.g., Building 8) have sensor-controlled ventilation systems. Most often, they sense CO\({}_{2}\) concentrations indoors, but they are often poorly correlated with the particle concentrations [29]. According to [PERSON] et al. [29], particle sensors could be a better option in terms of level of precision and accuracy. However, they are still expensive, but they are quickly becoming more competitive.