#### 2.2. Airtightness Test

To measure the airtightness of the test homes, the fan pressurization method was applied in compliance with ISO 9972 [

25]. The airtightness of the buildings was calculated using the fan pressurization method based on the air flow rate generated by the fan to determine the indoor–outdoor pressure difference for five points between 10 and 60 Pa. The indoor–outdoor pressure difference and the resulting air flow rate can be explained by the power law in Equation (1), and the trend line, which is found by interpolating the measured values with a straight line, can be used to obtain the air leakage coefficient (C) and the pressure exponent. C depends on the leakage characteristics of the building; n is a value between 0.5 and 1: it is close to 0.5 when the inflow air is turbulent and close to 1.0 when it is laminar. The power law is

where Q is the air leakage rate through the building envelope (

${\mathrm{m}}^{3}\xb7{\mathrm{h}}^{-1})$, C is the air leakage coefficient (

${\mathrm{m}}^{3}\xb7{\mathrm{h}}^{-1}\xb7{\mathrm{Pa}}^{-\mathrm{n}}$),

$\Delta \mathrm{P}$ is the induced pressure difference (Pa), and n is the pressure exponent (dimensionless).

${\mathrm{ACH}}_{50}$ (the air change rate at 50 Pa), which is used as a performance indicator of airtightness, can be calculated using the ratio of the air flow rate to the volume of the room, while maintaining the indoor–outdoor pressure difference at 50 Pa through Equation (2). The effective leakage area (ELA) of the units when the pressure difference between the indoors and outdoors is 4 Pa can be calculated using Equation (3). Since the ELA of each unit depends on the size of the unit, the specific ELA, which distributes the ELA over the floor area, was also calculated. The normalized leakage (NL), which allows for comparison of the airtightness between units by accounting for their floor area and height, is calculated by Equation (4) using the ELA, floor area, and floor height:

where

${\mathrm{Q}}_{50}$ is the air flow rate through the building envelope under a pressure difference of 50 Pa(

${\mathrm{m}}^{3}\xb7{\mathrm{h}}^{-1})$, C is the air flow coefficient (

${\mathrm{m}}^{3}\xb7{\mathrm{h}}^{-1}\xb7{\mathrm{Pa}}^{-\mathrm{n}}$),

$\Delta {\mathrm{P}}_{r}$ is the reference pressure difference (Pa), n is the air flow exponent (dimensionless),

$\rho $ is the air density (kg

$\xb7{\mathrm{m}}^{-3}$),

${\mathrm{A}}_{\mathrm{f}}$ is the floor area (

${\mathrm{m}}^{2}$), and H is the floor height (m).

In this study, Retrotec EU6101 with DM32 (USA) was used as the measurement equipment for the fan pressurization method; the measurement error of the wind volume was ±5%. To prevent measurement errors caused by indoor–outdoor pressure differences, the measurement conditions proposed in ISO 9972 were employed, that is, a wind speed of less than 6 m/s and natural conditions with an indoor–outdoor pressure difference of 5 Pa or more. Assuming a single-zone target unit, the interior doors were kept open during the measurement of the blower door, and the air flow rate generated by the fan was measured to create outdoor pressure difference conditions of 10, 20, 30, 40, and 50 Pa. Based on the measurement results of the blower door, the following airtightness indicators were derived: C (leakage coefficient), n (pressure exponent),

${\mathrm{ACH}}_{50}$, ELA (effective leakage area), specific ELA, and NL (normalized leakage). To classify the analysis units by airtightness level, the leakage class was determined according to the airtightness and ventilation requirements presented by ASHRAE 119 [

26], as shown in

Table 2.

#### 2.3. PM 2.5 Infiltration Test

To analyze the effects of building factors on the infiltration of outdoor

${\mathrm{PM}}_{2.5}$, a

${\mathrm{PM}}_{2.5}$ infiltration test was conducted using the blower-door depressurization method [

18], which enables the assessment of outdoor

${\mathrm{PM}}_{2.5}$ infiltration under controlled pressure differences. The main strategy of the blower-door depressurization method is to use a blower door to fix the indoor–outdoor pressure difference at 10 Pa and then to measure the indoor and outdoor

${\mathrm{PM}}_{2.5}$ concentrations. To obtain the indoor

${\mathrm{PM}}_{2.5}$ concentration after the infiltrated outdoor-origin

${\mathrm{PM}}_{2.5}$ had been fully mixed into the indoor air, the indoor and outdoor

${\mathrm{PM}}_{2.5}$ concentration measurements were obtained after operating the blower door for more than one time constant to entirely replace the room air under the controlled indoor–outdoor pressure difference of 10 Pa.

Under natural conditions, the difference between the indoor and outdoor pressures of a building is generally known to be 4 Pa [

27]. In this study, the pressure difference was limited to 10 Pa through the blower door to enable the comparison of the building-specific infiltration factor. This is the minimum recommended pressure difference at which the flow rate is controlled during the blower-door experiment [

27], and it is an indoor–outdoor pressure difference that can be found in mid- and high-rise buildings or that can be caused by external winds in winter [

28,

29]. Based on the living environment in Korea, where the proportion of high-rise multifamily housing units is high, a pressure difference of 10 Pa is therefore judged as suitable for simulating the natural infiltration environment in middle- and high-rise units. Although low indoor–outdoor pressure differences can cause the measured

${\mathrm{PM}}_{2.5}$ infiltration factor to be slightly higher than the actual

${\mathrm{PM}}_{2.5}$ infiltration factor, this study included an infiltration experiment under the same environmental conditions to select the dominant building factors for outdoor

${\mathrm{PM}}_{2.5}$ infiltration through comparison of the units and then evaluated the

${\mathrm{PM}}_{2.5}$ infiltration level.

In this study, the

${\mathrm{PM}}_{2.5}$ infiltration factor as an indicator of outdoor

${\mathrm{PM}}_{2.5}$ infiltration was calculated using the indoor

${\mathrm{PM}}_{2.5}$ mass balance equation. Equation (5) is the indoor

${\mathrm{PM}}_{2.5}$ mass balance equation; it is composed of the outdoor

${\mathrm{PM}}_{2.5}$ infiltration, indoor

${\mathrm{PM}}_{2.5}$ generation, and deposition, resuspension, removal, and exfiltration terms:

where V is the volume of the room (

${\mathrm{m}}^{3}$),

${\mathrm{C}}_{\mathrm{in}}$ is the indoor

${\mathrm{PM}}_{2.5}$ concentration (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{m}}^{-3}$),

${\mathrm{C}}_{\mathrm{out}}$ is the outdoor

${\mathrm{PM}}_{2.5}$ concentration (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{m}}^{-3}$), P is the

${\mathrm{PM}}_{2.5}$ penetration coefficient (dimensionless),

$\mathsf{\lambda}$ is the air change rate (

${\mathrm{h}}^{-1}$), K is the

${\mathrm{PM}}_{2.5}$ deposition rate (

${\mathrm{h}}^{-1}$), E is the indoor

${\mathrm{PM}}_{2.5}$ emission rate (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{h}}^{-1}$),

${\mathrm{R}}_{\mathrm{resus}}$ is the

${\mathrm{PM}}_{2.5}$ resuspension rate (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{h}}^{-1}$), and

${\mathrm{R}}_{\mathrm{rem}}$ is the

${\mathrm{PM}}_{2.5}$ removal rate (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{h}}^{-1}$).

The change in indoor

${\mathrm{PM}}_{2.5}$ concentration is expressed by Equation (6) with the assumption that there is no indoor

${\mathrm{PM}}_{2.5}$ generation source, resuspension, or removal. The indoor

${\mathrm{PM}}_{2.5}$ concentration can be expressed by Equation (7) when the indoor fine dust concentration reaches a steady-state, at which point the

${\mathrm{PM}}_{2.5}$ infiltration factor (

${\mathrm{F}}_{\mathrm{in}}$) can be obtained as the ratio of the indoor and outdoor

${\mathrm{PM}}_{2.5}$ concentrations in the steady-state, as shown in Equation (8):

where V is the volume of the room (

${\mathrm{m}}^{3}$),

${\mathrm{C}}_{\mathrm{in}}$ is the indoor

${\mathrm{PM}}_{2.5}$ concentration (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{m}}^{-3}$),

${\mathrm{C}}_{\mathrm{out}}$ is the outdoor

${\mathrm{PM}}_{2.5}$ concentration (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{m}}^{-3}$), P is the

${\mathrm{PM}}_{2.5}$ penetration coefficient (dimensionless),

${\mathrm{ACH}}_{10}$ is the air change rate at 10 Pa (

${\mathrm{h}}^{-1}$), K is the

${\mathrm{PM}}_{2.5}$ deposition rate (

${\mathrm{h}}^{-1}$),

${\mathrm{C}}_{\mathrm{in},\mathrm{ss}}$ is the indoor

${\mathrm{PM}}_{2.5}$ concentration at steady-state (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{m}}^{-3}$), and

${\mathrm{C}}_{\mathrm{out},\mathrm{ss}}$ is the outdoor

${\mathrm{PM}}_{2.5}$ concentration at steady-state (

$\mathsf{\mu}\mathrm{g}\xb7{\mathrm{m}}^{-3}$).

To conduct the

${\mathrm{PM}}_{2.5}$ infiltration test using the fan pressurization method, Retrotec EU6101 with DM32 (USA) was used for the blower door, and a light-scattering-type AM510 (TSI, Shoreview, MN, USA), which has been used for continuous measurement of

${\mathrm{PM}}_{2.5}$ concentration in previous studies [

30,

31], was used for the measurements. The measurement error of the PM

_{2.5} concentration was 1

$\mathsf{\mu}\mathrm{g}/{\mathrm{m}}^{3}$ over 24 h. At a measurement interval of 3 min, the indoor and outdoor

${\mathrm{PM}}_{2.5}$ concentrations were measured at one point in the center of the unit and at one point in the outdoor area close to the unit. To prevent the resuspension of indoor

${\mathrm{PM}}_{2.5}$ caused by air flow through the blower door, cleaning was carried out to remove indoor

${\mathrm{PM}}_{2.5}$ sources before the measurements, and the measurements were conducted in the absence of indoor

${\mathrm{PM}}_{2.5}$ sources or resuspension activities in the room. The

${\mathrm{PM}}_{2.5}$ concentration was obtained after one time constant at a 10 Pa pressure difference at the steady-state of the indoor

${\mathrm{PM}}_{2.5}$ concentration, and the infiltration factor of

${\mathrm{PM}}_{2.5}$ was calculated using Equation (8).

The

${\mathrm{PM}}_{2.5}$ infiltration test with the blower-door depressurization procedure was conducted to minimize the impact of environmental factors on the outdoor

${\mathrm{PM}}_{2.5}$ infiltration when comparing the

${\mathrm{PM}}_{2.5}$ infiltration factors of multifamily homes according to their building characteristics. Nevertheless, as the factors affecting the outdoor

${\mathrm{PM}}_{2.5}$ infiltration, the outdoor

${\mathrm{PM}}_{2.5}$ concentration conditions varied at the time of the measurements. When the outdoor

${\mathrm{PM}}_{2.5}$ concentration is low, the margin of error in the calculation of the

${\mathrm{PM}}_{2.5}$ infiltration factor may even increase to the level of the device measurement error (1

${\mathsf{\mu}\mathrm{g}/\mathrm{m}}^{3}$) due to the small difference between the indoor and outdoor

${\mathrm{PM}}_{2.5}$ concentrations. When the outdoor

${\mathrm{PM}}_{2.5}$ concentration changes drastically, the infiltration factor may be overestimated or underestimated depending on the pattern of change. The analysis was thus performed by classifying the outdoor

${\mathrm{PM}}_{2.5}$ concentration and its fluctuations as they are expected to affect the outdoor

${\mathrm{PM}}_{2.5}$ infiltration (

Table 3). OPC-1 denotes the combination of concentrations that exceed the “bad” level of a daily average of 35

${\mathsf{\mu}\mathrm{g}/\mathrm{m}}^{3}$ presented by the MOE in Korea and the U.S. EPA with low fluctuation, i.e., measurements with a deviation of less than 10% of the average outdoor

${\mathrm{PM}}_{2.5}$ concentration, and this case was adopted for statistical analysis. Moreover, based on the measurement results for OPC-2, which includes average outdoor

${\mathrm{PM}}_{2.5}$ concentrations below 35

${\mathsf{\mu}\mathrm{g}/\mathrm{m}}^{3}$, and OPC-3, which includes outdoor

${\mathrm{PM}}_{2.5}$ concentration deviations of 10% or more than the average, trends in the measurement results were investigated according to the outdoor

${\mathrm{PM}}_{2.5}$ concentration conditions.