This study analyses the effect of the airtightness of the building envelope on the concentration of aerosol particles between 10 and 1000 nm that uncontrollably infiltrate the building from the ambient air. Two buildings were selected on the basis of their physical characteristics, with Building 1 having lower airtightness (S1-NT) and Building 2 having higher airtightness of the building envelope (S2-VT). In each building, particle number concentrations were measured continuously for one week in the selected test room during the non-heating and heating season. In ambient air, traffic is the primary source of particles in the non-heating season, and traffic and combustion are the primary sources in the heating season. As we were mainly interested in the concentration of differently sized particles of outdoor origin in the room and their infiltration rate, we minimised the indoor sources of particles (one person entered the room only when opening and closing the window). For all one-week measurements in the room, we used the same method of opening and closing the window. For the evaluation of the data, we focused on the periods when the window was closed and calculated particle number concentrations (CN is the number of particles in cm–3) of the four fractions (diameter in nm: <100, 100‒200, 201‒500, >500) for these periods. In addition, radon (CRn) and carbon dioxide (CCO2) concentrations, which are indicators of ventilation efficiency, and temperature (Tin) were monitored in the room. In the immediate vicinity of the test room, temperature (Tout), pressure (p), wind speed (v), precipitation (P) and CRn were monitored simultaneously in the ambient air. The airtightness of the two test rooms was calculated using the CO2 tracer gas method. We confirmed that Building 1 has a less airtight building envelope (Ninf = 0.40 h–1) than Building 2 (Ninf = 0.23 h–1). During periods when the window was closed, the average particle number concentrations are as follows: (i) in S1-NT in the non-heating season CN <100 = 1276 cm–3, CN 101–200 = 957 cm–3, CN 201–500 = 408 cm–3, CN >500 = 6 cm–3 and in the heating season CN <100 = 2622 cm–3, CN 101–200 = 1119 cm–3, CN 201–500 = 309 cm–3, CN >500 = 11 cm–3; (ii) in S2-VT in the non-heating season CN <100 = 2060 cm–3, CN 101–200 = 901 cm–3, CN 201–500 = 311 cm–3, CN >500 = 5 cm–3 and in the heating season CN <100 = 4054 cm–3, CN 101–200 = 2558 cm–3, CN 201–500 = 643 cm–3, CN >500 = 13 cm–3. In Building 1, the particle number concentrations in the heating season are higher than in the non-heating season only for specific fractions, while in Building 2 they are significantly higher in the heating season than in the non-heating season for all fractions. The above results also show us that in the heating season, the particle concentrations are higher in Building 2 than in Building 1, indicating that the particles are well infiltrated also through the more airtight envelope of Building 2. We calculated the infiltration rate and found that the concentration of particles <100 nm increases most rapidly from the minimum to the maximum value (S1-NT: 䀆t = 3.8–5.4 h; S2-VT: 䀆t = 7.3–3.4 h), indicating that they pass through the building envelope the fastest among all fractions. If exposed to particles for a prolonged period of time, they have a negative impact on our health. Given that people spend most of their time indoors, knowing the concentrations of particles in ambient air and understanding how they move from ambient air to the indoor environment through the building envelope is crucial for planning improvement measures.
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