In this case study, we investigate the building infiltration rate and indoor aerosol concentration levels in two buildings equipped with passive house technology and one “conventional” house in Ócsa, Hungary. We have aimed to determine the indoor aerosol pollution level and its elemental composition, establish the relationship between the indoor and outdoor concentration levels, and study how the different ventilation rates and modes affect the indoor particulate matter (PM) contamination. Our results indicate that the measured PM concentration levels were well below the recommended limits overall. In particular, the mean PMfine (aerodynamic diameter < 2.5 µm) concentration was around 5 µg m–3 while the outdoor PMfine level was 20 µg m–3. The mean indoor concentration of the coarse fraction aerosols (aerodynamic diameter > 2.5 µm) varied between 2.5 and 7 µg m–3, with higher values corresponding to better airtightness of the house. As assessed by the indoor/outdoor elemental ratios and mass size distribution data, the filtration of the coarse mode particles was adequate in the passive houses. However, the PMfine fraction could get through the filters unhindered, as indicated by PMfine levels independent of the ventilation modes. The coarse mode particles inside the passive houses mainly originated from indoor sources.