# Ventilation Systems and Their Impact on Nanoparticle Concentrations in Office Buildings
The smallest I/O ratio for particle number concentration (PNC) was found with closed windows with the value of 0.33. In Daqing, China, the lower infiltration factors for fine particles (PM\({}_{2.5}\)) in offices and urban residential were mainly caused by closing the windows and using air \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Filter Class According to** & \multicolumn{4}{c}{**Particle Size**} \\
**SS EN 779 1** & **400 nm** & **MPPS 2** & **50 nm** & **20 nm** \\ \hline M5 & 4\% & 2\% & 17\% & 30\% \\ M6 & 20\% & 14\% & 35\% & 63\% \\ F7 & 58\% & 46\% & 62\% & 83\% \\ F8 & 70\% & 55\% & 69\% & 90\% \\ F9 & 81\% & 60\% & 77\% & 94\% \\ \hline \hline \end{tabular}  
* SS EN 779 = Swedish standard for testing and classification of ventilation filters. 2 MPPS = “Most Penetrating Particle Size”. It refers to the particle size that is most difficult to filter. This size varies depending on the structure of the filter and its performance, the air velocity through the material, as well as the properties of the particles.  
\end{table} 
Table 1: The degree of separation in percent for different filters at different particle sizes [16]. conditioning as well as due to a greater air tightness in high buildings [18]. [PERSON] et al. [19] studied the correlations between particle concentrations measured indoors and outdoors in mechanically ventilated buildings in Colorado, USA. The correlations between indoor and outdoor concentrations were higher for UFP (55-100 nm) in comparison with fine particles (0.1-0.7 \(\upmu\)m), especially when the heating, ventilating, and airconditioning (HVAC) system was running. This study also showed that the I/O ratios of particles in the different size fractions were weakly but positively correlated with the amount of ventilation provided to the indoor environment. The I/O relationships for particles in different size modes (nucleation mode, Aitken mode, and accumulation mode), and at different ventilation conditions, have been analyzed in a few studies. Particle size distributions indoors and outdoors were measured in a building in Helsinki downtown during 1999. The I/O ratio for nucleation mode particles (8-25 nm) was mostly around 0.1. For particles in the Aitken mode (25-90 nm), the I/O ratio was mostly between 0.1 and 0.2. For particles in the accumulation mode (90-500 nm), the I/O ratio was mostly around 0.3. When the ventilation flow was switched on, the I/O ratio for particles in accumulation mode increased. However, during nighttime, when the ventilation flow was very low, the I/O ratio for particles in nucleation mode increased [20]. In another study performed in Helsinki, the number concentrations of fine particles were measured indoors and outdoors in an office located in the basement of a building in Helsinki. The measurements took place in the summer of 2000. Particles in the size range of 7-600 nm were measured, and there was a temporal correlation between the particle number concentrations measured indoors and outdoors. The mechanical ventilation system was the main transport route of outdoors particles into the building. The mean penetration factor was 0.41 for nucleation mode particles (7-25 nm), 0.74 for Aitken mode particles (25-100 nm), and 0.87 for particles in accumulation mode (100-600 nm) [21]. In a study performed in California, indoor and outdoor relationships for particles in the size range of 6-220 nm were measured in four apartments within 60 m from the center of the 405 Freeway in Los Angeles. Indoor and outdoor concentrations were measured simultaneously under different ventilation conditions without the presence of any indoor sources that generated particles. Under different air exchange rates, the I/O ratios were highest (0.6-0.9) for particles in the size range of 70-100 nm, while the lowest I/O ratios (0.1-0.4) were found for particles in the size range of 10-20 nm [22].