Indoor Air 2014; 24: 629–638 wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved
© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12117
Effects of types of ventilation system on indoor particle concentrations in residential buildings Abstract The objective of this study was to quantify the influence of ventilation systems on indoor particle concentrations in residential buildings. Fifteen occupied, single-family apartments were selected from three sites. The three sites have three different ventilation systems: unbalanced mechanical ventilation, balanced mechanical ventilation, and natural ventilation. Field measurements were conducted between April and June 2012, when outdoor air temperatures were comfortable. Number concentrations of particles, PM2.5 and CO2, were continuously measured both outdoors and indoors. In the apartments with natural ventilation, I/O ratios of particle number concentrations ranged from 0.56 to 0.72 for submicron particles, and from 0.25 to 0.60 for particles larger than 1.0 lm. The daily average indoor particle concentration decreased to 50% below the outdoor level for submicron particles and 25% below the outdoor level for fine particles, when the apartments were mechanically ventilated. The two mechanical ventilation systems reduced the I/O ratios by 26% for submicron particles and 65% for fine particles compared with the natural ventilation. These results showed that mechanical ventilation can reduce exposure to outdoor particles in residential buildings.
J. S. Park, N.-Y. Jee, J.-W. Jeong Architectural Engineering, Hanyang University, Seoul, Republic of Korea Key words: Particles; Ventilation system; Penetration; Perceived air quality; Residential building; Field measurement.
J. S. Park Architectural Engineering, Hanyang University, 17 Haengdang-dong, Seoul 133-791, Republic of Korea. Tel.: +82-2-2220-1743 Fax: +82-2-2296-5331 e-mail: [email protected] Received for review 15 September 2013. Accepted for publication 3 April 2014.
Practical implications
Results of this study confirm that mechanical ventilation with filtration can significantly reduce indoor particle levels compared with natural ventilation. The I/O ratios of particles substantially varied at the naturally ventilated apartments because of the influence of variable window opening conditions and unsteadiness of wind flow on the penetration of outdoor air particles. For better prediction of the exposure to outdoor particles in naturally ventilated residential buildings, it is important to understand the penetration of outdoor particles with variable window opening conditions.
Introduction
Exposure to airborne particles has a negative effect on the respiratory health of occupants (Schneider et al., 2003). Influence of airborne particles on health effects is strongly related to particle size, and the particle size is also closely related to filtration and penetration in buildings. When there are no indoor particle sources, filtration, deposition, and penetration become important factors to determine indoor airborne particle concentrations (Liu and Nazaroff, 2001). Mechanical ventilation systems are commonly equipped with filters to remove particles from outdoor air and recirculated indoor air. Previous studies showed that buildings with mechanical fans and air filtration have lower indoor levels of airborne particles, compared with outdoor air, in the absence of indoor sources (Stephens and Siegel, 2012; Zhang and Zhu, 2012). Jamriska et al. (2000)
showed that air-handling systems with filters reduced levels of indoor submicron particles by 34% in an office, and field measurements in a children’s day care center showed that the indoor total suspended particulate (TSP) level decreased to 73% below the outdoor level, when it was mechanically ventilated (Partti-Pellinen et al., 2000). Parker et al. (2008) indicated that staying inside a mechanically ventilated building reduces exposure to outdoor submicron particles. Fisk et al. (2002) performed the predictions of energy and total costs for particle air filtration in mechanically ventilated office buildings. Natural ventilation has the potential to reduce operating cost compared with mechanical ventilation. It can provide sufficient air change rate for indoor air quality and thermal environmental conditions (Brager and de Dear, 1998; Heiselberg and Perino, 2010; Wallace et al., 2002). Several previous studies showed that 629
Park et al. the differences in weekly averaged air change rates between natural and mechanical ventilation were small in residential buildings (Oie et al., 1998; Park, 2013; Ruotsalainen et al., 1992). Although natural ventilation has several benefits, the level of indoor airborne particles can be higher in naturally ventilated buildings, because outdoor particles are easily transported indoors through openings and leaks in the building envelope. Levels of indoor particle concentrations within naturally ventilated buildings are significantly influenced by the penetration of outdoor particles through the openings (Tippayawong et al., 2009). For the design of natural ventilation, it is important to improve knowledge of the transport of outdoor air particles into buildings. Comparison with mechanically ventilated buildings can provide better understanding of indoor air quality of naturally ventilated buildings, because different types of ventilation systems might have different effects on indoor particle concentrations. Although previous studies have discussed the impacts of mechanical ventilation systems, they were limited to different operation settings, or simple comparisons of indoor air particle concentration with outdoor air. CFD simulations or theoretical approaches were also developed to demonstrate the effects of penetration and filtration of particles in buildings (Holmberg and Chen, 2003; Nazaroff, 2004; Richmond-Bryant et al., 2006). However, only a few studies have comprised quantitative determination of indoor particle concentrations with different ventilation systems, including fluctuating natural ventilation. A full-scale field measurement can provide reliable data based on real circumstances of indoor environments, including occupant’s behavior and indoor sources. The objective of this study is to quantify indoor particle concentrations in residential buildings with different ventilation systems. The field measurement was conducted in 15 apartments which were ventilated with three different ventilation systems: unbalanced mechanical ventilation, balanced mechanical ventilation, and natural ventilation. Number concentrations of indoor particles, PM2.5, CO2, temperature, and relative humidity were continuously measured both outdoors and indoors.
Methods Sampling sites
Fifteen occupied, single-family apartments were included in this study. The field measurements were undertaken in the moderate climate season, between April and June 2012, when outdoor particle concentrations are generally higher than in the other seasons. The study sites were a convenience sample and were intended to have different ventilation systems that were used extensively in residential buildings. The sample apartments were selected from three sites located in 630
urban and suburban area of Seoul. The sites were apartment complexes which consisted of several apartment buildings. The buildings were typical flat type which contains more than three single-story house units in each floor. The buildings were constructed by reinforced concrete, and the walls between adjacent house units were also filled with concrete. Each house unit was connected to internal stairs and elevators of the building, and more than two external walls of each house unit were facing outdoors. The sample apartments were carefully recruited from the buildings of the three sites. The sample recruitment was based on the number of family members, location, floor number, proximity to main traffic roads, and occupants’ acceptance of 24 h of sampling. Households with smokers, pets, and unusual indoor sources were excluded. Table 1 shows a brief summary of the sample apartments, designated as A–O. Apartments K–O were situated in an urban area, whereas the other apartments were situated in a suburban area. The floor area of the apartments ranged from 99 to 166 m2, and the averaged floor area was slightly higher than that of the overall housing of Seoul. All apartments were singlefamily units, and most of the families consisted of two children and two adults. The floor number of the apartments ranged from the 2nd to 26th floor, and the highest building has 33 floors. Low-, medium-, and high-elevation apartments were included in all sites, as shown in Table 1. The apartments were also selected from both central and border buildings of the sites. Units A–E and K–O were sited within 100 m of a main traffic road that consisted of two roadways with two lanes. Units F–J were more than 200 m away from the main traffic road. The apartments had hydronic radiant floor-heating systems using hot water, which was installed and operated independently of the ventilation system. In each case, the floor was not covered by carpet, which could affect the deposition and resuspension of particles. Sampling was conducted both indoors and outdoors. The instruments for indoor sampling were placed on a small wagon in the center of the master bedroom and living–dining room, sampling from a height of 1.2 m above the floor. Outdoor air was sampled in front of the outdoor intake of the mechanical ventilation system, or on a balcony that was close to operable windows. Ventilation
The three sites were constructed according to Korean ventilation standards for residential buildings. The Korean building code prescribes a ventilation rate of 0.7 air changes per hour (ACH) in occupied spaces of residential buildings. The three sites were ventilated with the three different systems as shown in Table 1. There were two mechanical ventilation systems, and
Indoor particle concentration in residential buildings Table 1 Building characteristics of the sampled apartments
Sample ID
Year built (Year)
Total area (m2)
Residentsa (person)
Floorsb
Locationc
Nearest roadway (m)
Vent.d
A B C D E F G H I J K L M N O
2007 2007 2007 2007 2007 2011 2011 2011 2011 2011 2008 2008 2008 2008 2008
112.2 99.1 111.2 101.9 101.9 166.0 166.0 166.0 166.0 166.0 140.0 140.0 140.0 140.0 140.0
4 (M, F, f, f) 4 (M, F, m, f) 3 (M, F, m) 4 (M, F, m, m) 4 (M, F, m, f) 4 (M, F, m, f) 3 (M, F, m) 5 (M. M, F, F, F) 4 (M, F, f, f) 4 (M, F, m, f) 4 (M, M, F, F) 4 (M, M, M, F) 4 (M, M, F, F) 3 (M, M, F) 2 (F, F)
16/21 2/25 5/21 17/29 13/29 10/12 6/14 10/15 12/13 5/14 15/30 8/33 18/33 26/33 2/33
Border Border Border Center Center Border Border Border Border Border Border Center Center Center Center
40 45 40 96 96 204 204 200 204 204 76 100 100 100 100
Unbalanced Unbalanced Unbalanced Unbalanced Unbalanced Balanced Balanced Balanced Balanced Balanced Natural Natural Natural Natural Natural
a
M, male adult; F, female adult; m, male below 18 years; f, female below 18 years. Floor number/Total number of floors. c Inner, center area of the complex; outer, border area of the complex. d Unbalanced, unbalanced mechanical ventilation (only mechanical supply fan); balanced, balanced mechanical ventilation (mechanical supply fan and mechanical exhaust fan); natural, natural ventilation. b
the proper installation of filters was checked before sampling. The two mechanical ventilation systems had a dry filter, with minimum efficiency reporting value (MERV) of 11 or equivalent rating, as shown in Table 3. All filters were not replaced after first installation. Apartments A–E were ventilated by an unbalanced mechanical ventilation system. The mechanical supply fan distributed outdoor air to rooms through the supply diffusers mounted at the ceiling, and the supply diffusers were connected to the mechanical supply fan through a single duct located in the ceiling. Indoor air was exhausted through uncontrolled leaks and through exhaust vents located in the kitchen and bathrooms. The fan speed was controlled with a switch with three settings: off, low, and high. The fan was operated at high speed during the sampling period. Apartments F–J each had a balanced mechanical ventilation system with a heat recovery unit. The system had two fans: one was for supplying outdoor air and the other was for exhausting indoor air. The supply diffusers mounted at the ceiling were connected to the mechanical supply fan through the supply duct. Exhaust diffusers were mounted at the ceiling, and those were connected to the mechanical exhaust fan through the return duct. The volume flow rate of supply air was the same as the exhausted airflow rate, and there was no recirculated air. The heat recovery unit was used to transfer sensible and latent heat from exhausted air to the supplied outdoor air. The exhaust air was not mixed with the supply air in the heat recovery unit. There were no mechanical supply and exhaust fan flows supplied to the master bedroom of units F–J. The balance of the mechanical fan flows was checked at the heat recovery unit only by total airflows of the supply and exhaust fans, and the airflow balance in
each room was not confirmed during the field measurements. Apartments K–O were naturally ventilated by operable windows located at different facades, and there were three local exhaust fans in the kitchen and two bathrooms. The operable windows were sliding type, and those were covered with window screen made of metal wire. During the sampling period, the occupants of the naturally ventilated apartments opened or closed the operable windows freely as per their usual daily patterns, whereas those of the mechanically ventilated apartments, A–J, were notified that all windows opening to outdoors should remain closed. The mechanical fans were steadily operated in units A–J during the sampling periods. The occupants of all apartments freely closed or opened interior doors, and the local exhaust fans located at the kitchen and bathrooms were also freely operated during the sampling period. Measurements
Measurements of air tightness and outdoor air supply rates were undertaken before particle sampling. Air tightness was measured using a blower door (Minneapolis Blower Door Model 3; TEC, St. Cloud, MN, USA) according to ASHRAE Standard 119. All exterior windows, diffusers, and vents for local exhaust were closed during the air tightness measurement. The apartments were first depressurized to 50 Pa by a fan of the blower door, and then, the pressure was stepped down in 10-Pa intervals. The air tightness was expressed using a normalized leakage area and effective leakage area at 4 Pa pressure difference. Outdoor supply rates of the two mechanical ventilation systems were measured at the supply diffusers 631
Park et al. using a Balometer capture hood (EBT-721, TSI-ALNOR, Saint Paul, MN, USA). Each measurement was repeated three times, and the ventilation rate of each room was defined as the sum of the supply airflow rates. The ventilation rate of the naturally ventilated apartments, K–O, was estimated using the CO2 decay method. Indoor and outdoor air CO2 concentrations were simultaneously monitored during the sampling period. The indoor air CO2 concentration increased or decreased according to the natural ventilation and activities of occupants. Every natural decay of indoor air CO2 concentration with open windows was used to calculate the natural ventilation rate. There was no deliberate CO2 release and no mixing fan in the rooms, and complete mixing of indoor air was assumed in all calculations. The daily average natural ventilation rate was defined as the average of all decays during the sampling period. The CO2 decay method was also applied to the two mechanical ventilation systems, and the calculated ventilation rates were compared with the measured supply airflows, as shown in Table 2. The sampling was conducted during a 24-h period in each apartment. During sampling, real-time measurements included the following: indoor particle number concentration, PM2.5 mass concentration, CO2, temperature, and relative humidity. The particle number concentrations from a size range of 0.3–0.5 lm to 5.0– 10.0 lm were measured using optical particle counters (Aero Trak 9306; TSI Inc., Shoreview, MN, USA) in outdoor air, in the living–dining room, and in the master bedroom at 10-min intervals. TSI Dust Trak photometers were used to continuously monitor PM2.5
mass concentration in outdoor air and the living–dining rooms. The Dust Trak and Aero Trak were calibrated in the laboratory, before sampling. Indoor air quality monitors (Model 2211; Kanomax, Osaka, Japan) were used to determine the concentration of CO2, as well as the temperature and relative humidity at 1-min intervals. The monitors were calibrated by sampling known concentration of CO2. All measurements were converted into 10-min averages for data reduction and analysis. The occupants also recorded their own activities using time checklist sheets.
Results Leakages and ventilation rates
Table 2 shows the effective leakage area and ventilation rates of the sample apartments. The types of filters that were installed in the two mechanical ventilation systems were noted according to ASHRAE Standard 52.2 (ASHRAE, 2007). The effective leakage area, ELA, quantifies the equivalent amount of opening area in the building envelope and can be used with the LBL model to estimate infiltrating airflows through leakage paths. The normalized leakage area, NL, represents the relative tightness of the sample apartments. The effective leakage area ranged from 31 at O to 191 cm2 at G, with an average across all apartments of 100 cm2. For apartments F–J, with balanced mechanical ventilation systems, the effective leakage areas were 41–91% larger than the average across all apartments. The averaged normalized leakage area was 0.06 0.03 m2/m2, and
Table 2 Leakage and ventilation rate of the sampled apartments Air tightness
Ventilation rate (1/h) Living–dining room
Master bedroom
Sample ID
ELA a (cm2)
NLb (–)
Fan flowsc
CO2d
Fan flows
CO2
Filter efficiencye
Vent.
A B C D E F G H I J K L M N O
44 37 44 149 68 190 191 165 143 142 157 46 47 46 31
0.04 0.03 0.04 0.13 0.06 0.11 0.11 0.10 0.08 0.08 0.11 0.03 0.03 0.03 0.02
0.8 0.9 0.8 1.0 0.9 1.0 0.9 1.0 0.8 0.8 – – – – –
0.9 0.5 0.9 0.9 0.8 0.9 0.9 1.0 0.7 0.7 0.9 1.3 1.2 1.3 0.8
0.9 1.0 1.1 1.1 1.1 – – – – – – – – – –
1.1 0.6 0.8 0.8 0.9 0.3 0.2 0.3 0.5 0.3 0.8 1.0 1.0 0.2 0.1
MERV11 MERV11 MERV11 MERV11 MERV11 MERV < MERV < MERV < MERV < MERV < None None None None None
Unbalanced Unbalanced Unbalanced Unbalanced Unbalanced Balanced Balanced Balanced Balanced Balanced Natural Natural Natural Natural Natural
a
ELA, effective leakage area calculated by LBL Model at 4 Pa. NL, normalized leakage calculated from ASHRAE Standard 119. c Fan flows, ventilation rate measured by supply fan flows; d The average ventilation rate calculated using CO2 concentration decay. e MERV, minimum efficiency reporting value by ASHRAE Standard 52.2 b
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0.4 0.5 0.3 0.5 0.6
0.1 0.1 0.1 0.2 0.1 0.4 0.5 0.4 0.1 0.1
11 11 11 11 11
Indoor particle concentration in residential buildings this corresponds to a leakage of ‘Class A’, for which is recommended full mechanical ventilation in ASHRAE Standard 119. The infiltrating airflows calculated by the LBL model (Sherman and Modera, 1986) were estimated to produce air change rates below 0.1 h 1. Predicted infiltration rates were small relative to measured total ventilation rates in each type of apartment. The mechanical ventilation air change rate measured by the fan airflows ranged from 0.8 to 1.1 h 1 in the living–dining rooms. The differences in the ventilation rates between the CO2 decay method and the fan flows ranged from 0 to 44% with an average of 17%. The differences were considered to be a consequence of inaccuracies in the assumption of complete mixing. The average mechanical ventilation rate of the living– dining rooms was 0.9 h 1, which was slightly higher than the recommendation of the Korean building code. The mechanical ventilation rates of master bedrooms of apartments A–E were close to those of the livingdining rooms. But the ventilation rates of master bedrooms of the apartments F–J were quite low compared with those of the living–dining rooms, likely because there was no mechanical fan airflow in these rooms. The ventilation rates were slightly higher than the infiltration airflows which were calculated by the LBL model. Those results were supposed to be the multizone airflows among the rooms, because the interior door of the master bedrooms was frequently opened during the sampling periods. The opening of the door could influence the airflows of master bedrooms of apartments F–J. The natural ventilation rates of apartments K–O significantly fluctuated according to wind pressure and the opening of windows. The daily average natural ventilation rates of the living–dining room ranged from 0.8 h 1 at apartment O to 1.3 h 1 at apartment L, with an average of 1.1 h 1. The average was slightly higher than the mechanical ventilation rates of apartments A–J. The natural ventilation rates of the master bedrooms were close to those of the living–dining rooms except for N and O. The natural ventilation rates of N and O were below 0.2 h 1.
Table 3 Outdoor and indoor air temperature, relative humidity, and CO2 concentration in the samplesa Ventilation typesb
Unbalanced (A, B, C, D, E) Outdoor air Temp (°C) 21.5 4.9 RH (%) 50 18 CO2 (ppm) 457 42 Indoor air Living–dining room Temp (°C) 27.4 1.1 RH (%) 43 5 CO2 (ppm) 775 247 |To-Ti|c (°C) 6.2 3.8 Master bedroom Temp (°C) 26.4 0.9 RH (%) 45 5 CO2 (ppm) 1262 316
Balanced (F, G, H, I, J)
Natural (K, L, M, N, O)
20.4 4.0 61 19 452 34
24.2 4.7 45 14 457 25
26.9 48 577 6.8
0.8 6 130 4.1
26.3 0.7 52 5 1256 437
26.9 36 556 4.1
0.9 7 82 2.7
26.7 1.0 40 7 983 272
a
Daily average, 24 h, in each ventilation group. Unbalanced: unbalanced mechanical ventilation (only mechanical supply fan); balanced: balanced mechanical ventilation (mechanical supply and exhaust fans); natural: natural ventilation. c Temperature difference between outdoor and indoor. b
rooms, CO2 concentrations were maintained at below 1,000 ppm with daily average 556–775 ppm. The daily average CO2 concentrations of the naturally ventilated apartments, K–O, were lower than those of the mechanically ventilated apartments, A–J. But the CO2 concentrations of the master bedrooms were significantly higher than those of the living–dining rooms especially in the unventilated apartments, F–J. Figure 1 shows daily averaged outdoor particle number concentrations measured at the three sites. The number concentrations varied over a wide range with time and were lower overnight than during the daytime. The number concentration decreased with particle size fraction, with the order of 103 times difference between the smallest and the largest size intervals. The average number concentrations for 0.3–0.5, 0.5–1.0, 1.0–3.0, 3.0–5.0, and 5.0–10.0 lm size interval were 2.5x108, 4.0x107, 3.2x106, 2.6x105, and 1.2x105 counts/m3, respectively. For each size fraction, daily variation of
Outdoor and indoor environments
Outdoor and indoor environmental conditions measured at the sampling periods are summarized in Table 3. The outdoor climate was mild at the three sites, and the daily averaged outdoor air temperature ranged from 20.4 to 24.2°C. There was no significant difference in indoor air temperature and humidity among the apartments. The indoor air temperature was 2.7–6.5°C higher than the outdoor air temperature. The indoor air relative humidity was below 65% during the sampling periods. The CO2 concentration was significantly influenced by the ventilation rate and the occupants’ activities. In the living–dining
Fig. 1 Daily average outdoor particle number concentrations in three sites (The error bars indicate standard deviations)
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Park et al. the number concentrations corresponded to about 50% of the average values for 0.3–0.5, 3.0–5.0, and 5.0–10.0 lm, and about 80% for 0.5–1.0 and 1.0– 3.0 lm. The differences in daily average number concentrations among the three sites were below 32% for each size fraction. The number concentrations of 0.3– 0.5 and 0.5–1.0 lm size intervals were slightly high at the apartments with balanced mechanical ventilation, F–J. PM2.5 mass concentrations of outdoor air were also at similar levels at the three sites, and the concentration ranged from 0.06 mg/m3 to 0.59 mg/m3 with an average across all sites of 0.20 mg/m3. But an appropriate caution is needed to interpret the measurement results of PM2.5, because Yanosky et al. (2002) showed that the factory calibration of the DustTrak monitor generates substantial bias in reporting mass concentration measurements for typical atmospheric samples.
(a)
(b)
Indoor particle concentrations and sources
An understanding of indoor sources can help to estimate the influence of the three different ventilation systems on indoor particle concentrations. Figure 2 shows hourly variations of outdoor and indoor particle number concentrations for each size fraction. The number concentrations varied over a wide range both outdoors and indoors. But the outdoor number concentrations were consistently higher than the corresponding indoor number concentrations in all size fractions. The indoor number concentrations changed according to the activities of the occupants, and several peaks could be clearly identified during morning and evening periods, at 07:00–09:00 and 18:00–20:00. The peak concentrations decreased with time, because of ventilation, the absence of indoor sources, and deposition. The result showed that indoor sources including occupants’ activities and cooking strongly impacted the indoor particle concentrations. And it was also clearly shown that the patterns of indoor concentrations were significantly different between two units with mechanical ventilation apartments, D and H, and one with natural ventilation, apartment M. Time-averaged indoor-to-outdoor PM2.5 ratios (I/O) in the living–dining rooms are shown in Figure 3. The time-averaged ratios were classified by occupied conditions. The unoccupied period was defined as the absence of the occupants at the living–dining rooms, 00:00–06:00, and the occupied period was subdivided into morning, 07:00–09:00, and evening 18:00–20:00. The I/O ratio of the PM2.5 mass concentration was lower and less variable in the unoccupied period than that of the occupied period, indicating that indoor sources strongly influence PM2.5 concentrations. For the occupied period, obvious indoor sources resulted in an elevated I/O ratio. The I/O ratio increased about 11–38% when the living–dining rooms were occupied. 634
(c)
Fig. 2 Time series of particle number concentrations in outdoor and indoor air in the living–dining room. (a) Unbalanced mechanical ventilation (apartment D). (b) Balanced mechanical ventilation (apartment H). (c) Natural ventilation (apartment M)
The average I/O ratio of the occupied period ranged from 0.5 0.3 for the unbalanced mechanical ventilation units, A–E, to 0.6 0.3 for the natural ventilation units, K–O. Figure 4 shows indoor particle number concentrations for all size fractions in the apartments with
Indoor particle concentration in residential buildings interval, increased considerably compared with that for submicron particles, 0.3–0.5 and 0.5–1.0 lm in the living–dining rooms. For the master bedrooms, there were no significant changes of particle concentrations except for the fine particles. Although the influences of activities of the occupants, such as cooking, dining, and sleeping, were difficult to separate unambiguously from the measured data shown in Figures 3 and 4, it is obvious that the indoor emission rate of particles is significantly changed according to the indoor activities of the occupants. Indoor-to-outdoor particle concentration ratio Fig. 3 Time-average indoor-to-outdoor PM2.5 mass concentration ratios (I/O) in the living–dining rooms (The error bars indicate one standard deviation) (a)
(b)
Fig. 4 Particle number concentrations in the unbalanced mechanical ventilation, in apartments A–E. (The error bars indicate one standard deviation). (a) Living–dining room. (b) Master bedroom
Time-averaged indoor-to-outdoor PM2.5 mass concentration ratios are summarized in Table 4. The statistical significance of differences was tested using the Student’s t test. For all ventilation types, the indoor concentration was lower than the outdoor concentration even in the apartments with natural ventilation. Deposition onto indoor surfaces and filtration in ventilation systems lowered indoor concentrations and dampened the variability seen in outdoor concentrations. With no indoor sources, the ratio is indicative of the contribution of penetration and filtration of the outdoor particles, and it also includes the influence of deposition in the rooms. The mean I/O ratio of PM2.5 ranged from 0.4 for the unbalanced mechanical ventilation to 0.6 for the natural ventilation in the living– dining rooms when they were unoccupied. The occupants of the apartments with natural ventilation commonly opened windows during unoccupied periods as during occupied periods. The I/O ratio with natural ventilation was significantly higher (P < 0.01) than that with unbalanced mechanical ventilation, indicating some benefit from PM2.5 removal by the ventilation system filter. The unbalanced mechanical ventilation showed the lowest I/O ratio among the three ventilation types. The differences of the mean PM2.5 I/O ratio between the balanced mechanical ventilation and the natural ventilation apartments were relatively small compared with the apartments with unbalanced mechanical ventilation. For the occupied periods, the I/O ratios were higher than those of the unoccupied period, probably because of the influence of indoor sources. Daily average I/O ratios for the apartments Table 4 Time-averaged indoor-to-outdoor PM dining rooms (avg s.d.) Unbalanced
unbalanced mechanical ventilation, A–E. The master bedrooms of A–E were mechanically ventilated with an average across 5 apartments of 1.1 h 1 which was the same level as the living–dining rooms. For the occupied periods, the average number concentration for fine particles, such as 1.0–3.0, 3.0–5.0, and 5.0–10.0 lm size
Unoccupied Morningb Eveningc Dailyd
a
0.40 0.52 0.57 0.50
2.5
mass concentration (I/O) in the living–
Balanced *
0.08 0.14 0.19 0.19*
0.55 0.52 0.65 0.58
0.11 0.12 0.11 0.12
Natural 0.60 0.53 0.66 0.61
0.18 0.17 0.20 0.20
a
00:00–06:00, b07:00–09:00, c18:00–20:00, d24-h average.*Statistically significant difference at P < 0.01 (t test).
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Park et al. with unbalanced mechanical ventilation, balanced mechanical ventilation, and natural ventilation were 0.50, 0.58, and 0.61, respectively. The difference between unbalanced mechanical ventilation and natural ventilation was statistically significant (P < 0.01). Figure 5 shows the trend of the time-averaged I/O ratios in the living–dining rooms, and the I/O ratios for each size interval are summarized in Table 5. During unoccupied periods, significant differences were clearly found in all size fractions between apartments (a)
(b)
(c)
Fig. 5 Time-averaged I/O ratios of particle number concentrations at living–dining room (The error bars indicate one standard deviation). (a) Unoccupied. (b) Occupied. (c) Daily
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with unbalanced mechanical ventilation and those with natural ventilation. The I/O ratios of the apartments with natural ventilation were 2–3 times higher for 1.0– 3.0, 3.0–5.0, and 5.0–10.0 lm size intervals than those with unbalanced mechanical ventilation. And there were also significant differences in the I/O ratios for 0.3–0.5, 0.5–1.0, and 1.0–3.0 lm size intervals between the unbalanced and balanced ventilation systems. Those differences were considered to result from high leakage area and low filtration efficiency of the filter. The average effective leakage area of the apartments with balanced mechanical ventilation was about 60% larger than in case with unbalanced mechanical ventilation. The outdoor particle concentrations for 0.3–0.5 and 0.5–1.0 lm size intervals were also 10–38% higher with balanced mechanical ventilation than with unbalanced mechanical ventilation, as shown in Figure 1. During occupied periods, however, significant differences were not found between the apartments with unbalanced mechanical ventilation and those with natural ventilation. The differences in the I/O ratios among the three ventilation types were considerably decreased during periods of occupancy compared with those of the unoccupied period, because of the influence of indoor sources. The daily average I/O ratios of the apartments with natural ventilation were significantly higher (P < 0.01) in all size intervals than those of the apartments with either type of mechanical ventilation system. The I/O ratios for 0.3–0.5, 0.5–1.0, 1.0–3.0, 3.0–5.0, and 5.0– 10.0 lm size intervals were 0.6, 0.7, 0.6, 0.3, and 0.5, respectively, in the natural ventilation case. Those corresponded to 123%, 141%, 162%, 216%, and 161% of the average I/O ratios in the two sets of apartments with mechanical ventilation. For the master bedroom, the daily average I/O ratios for all size intervals were also significantly higher (P > 0.01) with natural ventilation than with mechanical ventilation. Discussions
The results of this study substantiate an expectation of significant improvement in indoor particle concentrations of outdoor origin when apartments are mechanically ventilated with filtration. The daily average indoor particle concentration decreased to 50% below the outdoor level for submicron particles and 25% below the outdoor level for fine particles, when the apartments were mechanically ventilated. The two types of mechanical ventilation reduced the I/O ratios by 26% for submicron particles and 65% for fine particles compared with natural ventilation. These results show that indoor airborne particles are sometimes dominated by outdoor air (Franck et al., 2003) and also that mechanical ventilation can be effective for removing incoming outdoor particles (Bolster and Linden, 2009; Nazaroff, 2004).
Indoor particle concentration in residential buildings Table 5 Time-averaged I/O ratios of particle number concentrations in the sampled apartments Living–dining room Unbalanced Unoccupied/Asleepa 0.3–0.5 lm 0.5–1.0 lm 1.0–3.0 lm 3.0–5.0 lm 5.0–10.0 lm Occupied 0.3–0.5 lm 0.5–1.0 lm 1.0–3.0 lm 3.0–5.0 lm 5.0–10.0 lm Dailyb 0.3–0.5 lm 0.5–1.0 lm 1.0–3.0 lm 3.0–5.0 lm 5.0–10.0 lm
Master bedroom Balanced
Natural
Unbalanced
Balanced
Natural
0.39 0.32 0.19 0.06 0.13
0.11* 0.09* 0.08* 0.04 0.08
0.46 0.53 0.30 0.09 0.18
0.07* 0.16* 0.13* 0.04 0.11
0.51 0.64 0.58 0.24 0.51
0.09* 0.20* 0.16* 0.09* 0.21*
0.50 0.35 0.15 0.19 0.13
0.11 0.14 0.10 0.15* 0.10*
0.59 0.36 0.11 0.08 0.05
0.10 0.07 0.04 0.07* 0.08*
0.66 0.46 0.29 0.40 0.25
0.12* 0.18* 0.09* 0.17* 0.12*
0.52 0.50 0.47 0.22 0.43
0.14 0.18 0.27 0.16 0.38
0.51 0.58 0.37 0.13 0.28
0.11 0.20 0.15 0.06* 0.14*
0.56 0.72 0.60 0.25 0.51
0.11 0.24 0.20 0.11 0.24
0.52 0.38 0.23 0.33 0.23
0.16* 0.12 0.17 0.29 0.20
0.72 0.46 0.16 0.19 0.13
0.14 0.13 0.06* 0.06 0.08*
0.80 0.64 0.38 0.48 0.30
0.07* 0.07* 0.04* 0.09* 0.08*
0.51 0.44 0.33 0.15 0.30
0.19 0.20 0.24 0.15 0.28
0.49 0.55 0.35 0.12 0.26
0.10 0.17 0.14 0.06 0.16
0.62 0.69 0.55 0.29 0.45
0.18* 0.23* 0.21* 0.15* 0.24*
0.61 0.39 0.18 0.24 0.16
0.25 0.19 0.14 0.24* 0.17*
0.67 0.40 0.15 0.16 0.11
0.16 0.12 0.06 0.12* 0.11*
0.77 0.56 0.33 0.42 0.26
0.20* 0.20* 0.14* 0.20* 0.14*
a
Unoccupied is 00:00–06:00 for living–dining room, and sleep is for master bedroom. 24-h data in each ventilation type. *Statistically significant difference from other ventilation type at P < 0.01 (t test). b
A previous study showed that the HVAC system of an office building reduced the submicron particle concentration by 34% (Jamriska et al., 2000). Filtration of recirculated airflow can reduce indoor particle concentrations (Fisk et al., 2002). The I/O ratios summarized in Tables 4 and 5 can be compared with other studies. For the naturally ventilated schools with operable windows and doors, measured I/O ratios ranged from 0.74 to 0.88 for submicron particles (Tippayawong et al., 2009), whereas the I/O ratios were quite low in a mechanically ventilated school building (Parker et al., 2008). The results of this study indicate that mechanical ventilation can reduce exposure to outdoor particles in residential buildings. With natural ventilation, the daily average I/O ratios ranged from 0.56 to 0.72 for submicron particles and from 0.25 to 0.60 for fine particles. Without indoor sources, the I/O ratios varied from 0.51 to 0.64 for submicron particles and from 0.24 to 0.58 for fine particles. Previous studies showed that an average penetration ratio through infiltration was 0.47 0.15 in 19 residential buildings (Stephens and Siegel, 2012). However, in the naturally ventilated apartments studied here, the average effective leakage area (ELA) was 66 cm2, only 9% of the average ELA in the previous studies. Therefore, the penetration through infiltration in the naturally ventilated apartments was unimportant relative to the penetration through operable windows. Penetration through operable windows depends on several factors, including the geometry of openings, wind direction and speed, pressure difference between outdoors and indoors, and dynamic properties of particles (Chen and Zhao, 2011; Liu and Nazaroff, 2001;
Rim et al., 2010). The geometrical size of openings and unsteadiness of wind flow substantially influence not only the natural ventilation rate, but also the penetration of incoming outdoor particles (Park, 2013). The I/O ratios in the apartments with the natural ventilation shown in Figure 5 included variable window opening conditions, and the results are only applicable to times when outdoor air temperatures are comfortable. From the results of this study, it was difficult to quantify the influences of variable opening size and unsteadiness of wind flows on the penetration of outdoor particles. Better characterization of the variable opening windows and wind flows is needed to evaluate the outdoor particle penetration in naturally ventilated residential buildings. Previous studies showed that understanding the influence of occupant window opening behavior on ventilation rate is important when estimating human exposure to indoor air pollutants (Howard-Reed et al., 2002). Outdoor air temperature substantially influences on the occupant window opening behavior. For the case of the two mechanical ventilation options studied here, the I/O ratios shown in Tables 4 and 5 might be different from those in real conditions, because occupants frequently use operable windows instead of mechanical fans when the outdoor air temperature is comfortable (Park and Kim, 2012). Conclusion
This study evaluated the influences of ventilation system type on indoor particle concentrations in residential buildings. The daily average indoor particle 637
Park et al. concentration decreased to 50% below the outdoor level for submicron particles and 25% below the outdoor level for fine particles, when the apartments were mechanically ventilated. The two mechanical ventilation systems reduced the I/O ratios by 26% for submicron particles and 65% for fine particles compared with natural ventilation, and this finding is consistent with previous studies. This study contributes to understanding the exposure to outdoor particles in naturally ventilated residential buildings under moderate climate conditions.
Acknowledgements
This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (NRF-2013R1A2A2A05005131). The author thanks C.S. Roh, J. E. Lee, and B. D. Seo, graduate students, who helped with the field measurements.
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© 2014 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12117
Effects of types of ventilation system on indoor particle concentrations in residential buildings Abstract The objective of this study was to quantify the influence of ventilation systems on indoor particle concentrations in residential buildings. Fifteen occupied, single-family apartments were selected from three sites. The three sites have three different ventilation systems: unbalanced mechanical ventilation, balanced mechanical ventilation, and natural ventilation. Field measurements were conducted between April and June 2012, when outdoor air temperatures were comfortable. Number concentrations of particles, PM2.5 and CO2, were continuously measured both outdoors and indoors. In the apartments with natural ventilation, I/O ratios of particle number concentrations ranged from 0.56 to 0.72 for submicron particles, and from 0.25 to 0.60 for particles larger than 1.0 lm. The daily average indoor particle concentration decreased to 50% below the outdoor level for submicron particles and 25% below the outdoor level for fine particles, when the apartments were mechanically ventilated. The two mechanical ventilation systems reduced the I/O ratios by 26% for submicron particles and 65% for fine particles compared with the natural ventilation. These results showed that mechanical ventilation can reduce exposure to outdoor particles in residential buildings.
J. S. Park, N.-Y. Jee, J.-W. Jeong Architectural Engineering, Hanyang University, Seoul, Republic of Korea Key words: Particles; Ventilation system; Penetration; Perceived air quality; Residential building; Field measurement.
J. S. Park Architectural Engineering, Hanyang University, 17 Haengdang-dong, Seoul 133-791, Republic of Korea. Tel.: +82-2-2220-1743 Fax: +82-2-2296-5331 e-mail: [email protected] Received for review 15 September 2013. Accepted for publication 3 April 2014.
Practical implications
Results of this study confirm that mechanical ventilation with filtration can significantly reduce indoor particle levels compared with natural ventilation. The I/O ratios of particles substantially varied at the naturally ventilated apartments because of the influence of variable window opening conditions and unsteadiness of wind flow on the penetration of outdoor air particles. For better prediction of the exposure to outdoor particles in naturally ventilated residential buildings, it is important to understand the penetration of outdoor particles with variable window opening conditions.
Introduction
Exposure to airborne particles has a negative effect on the respiratory health of occupants (Schneider et al., 2003). Influence of airborne particles on health effects is strongly related to particle size, and the particle size is also closely related to filtration and penetration in buildings. When there are no indoor particle sources, filtration, deposition, and penetration become important factors to determine indoor airborne particle concentrations (Liu and Nazaroff, 2001). Mechanical ventilation systems are commonly equipped with filters to remove particles from outdoor air and recirculated indoor air. Previous studies showed that buildings with mechanical fans and air filtration have lower indoor levels of airborne particles, compared with outdoor air, in the absence of indoor sources (Stephens and Siegel, 2012; Zhang and Zhu, 2012). Jamriska et al. (2000)
showed that air-handling systems with filters reduced levels of indoor submicron particles by 34% in an office, and field measurements in a children’s day care center showed that the indoor total suspended particulate (TSP) level decreased to 73% below the outdoor level, when it was mechanically ventilated (Partti-Pellinen et al., 2000). Parker et al. (2008) indicated that staying inside a mechanically ventilated building reduces exposure to outdoor submicron particles. Fisk et al. (2002) performed the predictions of energy and total costs for particle air filtration in mechanically ventilated office buildings. Natural ventilation has the potential to reduce operating cost compared with mechanical ventilation. It can provide sufficient air change rate for indoor air quality and thermal environmental conditions (Brager and de Dear, 1998; Heiselberg and Perino, 2010; Wallace et al., 2002). Several previous studies showed that 629
Park et al. the differences in weekly averaged air change rates between natural and mechanical ventilation were small in residential buildings (Oie et al., 1998; Park, 2013; Ruotsalainen et al., 1992). Although natural ventilation has several benefits, the level of indoor airborne particles can be higher in naturally ventilated buildings, because outdoor particles are easily transported indoors through openings and leaks in the building envelope. Levels of indoor particle concentrations within naturally ventilated buildings are significantly influenced by the penetration of outdoor particles through the openings (Tippayawong et al., 2009). For the design of natural ventilation, it is important to improve knowledge of the transport of outdoor air particles into buildings. Comparison with mechanically ventilated buildings can provide better understanding of indoor air quality of naturally ventilated buildings, because different types of ventilation systems might have different effects on indoor particle concentrations. Although previous studies have discussed the impacts of mechanical ventilation systems, they were limited to different operation settings, or simple comparisons of indoor air particle concentration with outdoor air. CFD simulations or theoretical approaches were also developed to demonstrate the effects of penetration and filtration of particles in buildings (Holmberg and Chen, 2003; Nazaroff, 2004; Richmond-Bryant et al., 2006). However, only a few studies have comprised quantitative determination of indoor particle concentrations with different ventilation systems, including fluctuating natural ventilation. A full-scale field measurement can provide reliable data based on real circumstances of indoor environments, including occupant’s behavior and indoor sources. The objective of this study is to quantify indoor particle concentrations in residential buildings with different ventilation systems. The field measurement was conducted in 15 apartments which were ventilated with three different ventilation systems: unbalanced mechanical ventilation, balanced mechanical ventilation, and natural ventilation. Number concentrations of indoor particles, PM2.5, CO2, temperature, and relative humidity were continuously measured both outdoors and indoors.
Methods Sampling sites
Fifteen occupied, single-family apartments were included in this study. The field measurements were undertaken in the moderate climate season, between April and June 2012, when outdoor particle concentrations are generally higher than in the other seasons. The study sites were a convenience sample and were intended to have different ventilation systems that were used extensively in residential buildings. The sample apartments were selected from three sites located in 630
urban and suburban area of Seoul. The sites were apartment complexes which consisted of several apartment buildings. The buildings were typical flat type which contains more than three single-story house units in each floor. The buildings were constructed by reinforced concrete, and the walls between adjacent house units were also filled with concrete. Each house unit was connected to internal stairs and elevators of the building, and more than two external walls of each house unit were facing outdoors. The sample apartments were carefully recruited from the buildings of the three sites. The sample recruitment was based on the number of family members, location, floor number, proximity to main traffic roads, and occupants’ acceptance of 24 h of sampling. Households with smokers, pets, and unusual indoor sources were excluded. Table 1 shows a brief summary of the sample apartments, designated as A–O. Apartments K–O were situated in an urban area, whereas the other apartments were situated in a suburban area. The floor area of the apartments ranged from 99 to 166 m2, and the averaged floor area was slightly higher than that of the overall housing of Seoul. All apartments were singlefamily units, and most of the families consisted of two children and two adults. The floor number of the apartments ranged from the 2nd to 26th floor, and the highest building has 33 floors. Low-, medium-, and high-elevation apartments were included in all sites, as shown in Table 1. The apartments were also selected from both central and border buildings of the sites. Units A–E and K–O were sited within 100 m of a main traffic road that consisted of two roadways with two lanes. Units F–J were more than 200 m away from the main traffic road. The apartments had hydronic radiant floor-heating systems using hot water, which was installed and operated independently of the ventilation system. In each case, the floor was not covered by carpet, which could affect the deposition and resuspension of particles. Sampling was conducted both indoors and outdoors. The instruments for indoor sampling were placed on a small wagon in the center of the master bedroom and living–dining room, sampling from a height of 1.2 m above the floor. Outdoor air was sampled in front of the outdoor intake of the mechanical ventilation system, or on a balcony that was close to operable windows. Ventilation
The three sites were constructed according to Korean ventilation standards for residential buildings. The Korean building code prescribes a ventilation rate of 0.7 air changes per hour (ACH) in occupied spaces of residential buildings. The three sites were ventilated with the three different systems as shown in Table 1. There were two mechanical ventilation systems, and
Indoor particle concentration in residential buildings Table 1 Building characteristics of the sampled apartments
Sample ID
Year built (Year)
Total area (m2)
Residentsa (person)
Floorsb
Locationc
Nearest roadway (m)
Vent.d
A B C D E F G H I J K L M N O
2007 2007 2007 2007 2007 2011 2011 2011 2011 2011 2008 2008 2008 2008 2008
112.2 99.1 111.2 101.9 101.9 166.0 166.0 166.0 166.0 166.0 140.0 140.0 140.0 140.0 140.0
4 (M, F, f, f) 4 (M, F, m, f) 3 (M, F, m) 4 (M, F, m, m) 4 (M, F, m, f) 4 (M, F, m, f) 3 (M, F, m) 5 (M. M, F, F, F) 4 (M, F, f, f) 4 (M, F, m, f) 4 (M, M, F, F) 4 (M, M, M, F) 4 (M, M, F, F) 3 (M, M, F) 2 (F, F)
16/21 2/25 5/21 17/29 13/29 10/12 6/14 10/15 12/13 5/14 15/30 8/33 18/33 26/33 2/33
Border Border Border Center Center Border Border Border Border Border Border Center Center Center Center
40 45 40 96 96 204 204 200 204 204 76 100 100 100 100
Unbalanced Unbalanced Unbalanced Unbalanced Unbalanced Balanced Balanced Balanced Balanced Balanced Natural Natural Natural Natural Natural
a
M, male adult; F, female adult; m, male below 18 years; f, female below 18 years. Floor number/Total number of floors. c Inner, center area of the complex; outer, border area of the complex. d Unbalanced, unbalanced mechanical ventilation (only mechanical supply fan); balanced, balanced mechanical ventilation (mechanical supply fan and mechanical exhaust fan); natural, natural ventilation. b
the proper installation of filters was checked before sampling. The two mechanical ventilation systems had a dry filter, with minimum efficiency reporting value (MERV) of 11 or equivalent rating, as shown in Table 3. All filters were not replaced after first installation. Apartments A–E were ventilated by an unbalanced mechanical ventilation system. The mechanical supply fan distributed outdoor air to rooms through the supply diffusers mounted at the ceiling, and the supply diffusers were connected to the mechanical supply fan through a single duct located in the ceiling. Indoor air was exhausted through uncontrolled leaks and through exhaust vents located in the kitchen and bathrooms. The fan speed was controlled with a switch with three settings: off, low, and high. The fan was operated at high speed during the sampling period. Apartments F–J each had a balanced mechanical ventilation system with a heat recovery unit. The system had two fans: one was for supplying outdoor air and the other was for exhausting indoor air. The supply diffusers mounted at the ceiling were connected to the mechanical supply fan through the supply duct. Exhaust diffusers were mounted at the ceiling, and those were connected to the mechanical exhaust fan through the return duct. The volume flow rate of supply air was the same as the exhausted airflow rate, and there was no recirculated air. The heat recovery unit was used to transfer sensible and latent heat from exhausted air to the supplied outdoor air. The exhaust air was not mixed with the supply air in the heat recovery unit. There were no mechanical supply and exhaust fan flows supplied to the master bedroom of units F–J. The balance of the mechanical fan flows was checked at the heat recovery unit only by total airflows of the supply and exhaust fans, and the airflow balance in
each room was not confirmed during the field measurements. Apartments K–O were naturally ventilated by operable windows located at different facades, and there were three local exhaust fans in the kitchen and two bathrooms. The operable windows were sliding type, and those were covered with window screen made of metal wire. During the sampling period, the occupants of the naturally ventilated apartments opened or closed the operable windows freely as per their usual daily patterns, whereas those of the mechanically ventilated apartments, A–J, were notified that all windows opening to outdoors should remain closed. The mechanical fans were steadily operated in units A–J during the sampling periods. The occupants of all apartments freely closed or opened interior doors, and the local exhaust fans located at the kitchen and bathrooms were also freely operated during the sampling period. Measurements
Measurements of air tightness and outdoor air supply rates were undertaken before particle sampling. Air tightness was measured using a blower door (Minneapolis Blower Door Model 3; TEC, St. Cloud, MN, USA) according to ASHRAE Standard 119. All exterior windows, diffusers, and vents for local exhaust were closed during the air tightness measurement. The apartments were first depressurized to 50 Pa by a fan of the blower door, and then, the pressure was stepped down in 10-Pa intervals. The air tightness was expressed using a normalized leakage area and effective leakage area at 4 Pa pressure difference. Outdoor supply rates of the two mechanical ventilation systems were measured at the supply diffusers 631
Park et al. using a Balometer capture hood (EBT-721, TSI-ALNOR, Saint Paul, MN, USA). Each measurement was repeated three times, and the ventilation rate of each room was defined as the sum of the supply airflow rates. The ventilation rate of the naturally ventilated apartments, K–O, was estimated using the CO2 decay method. Indoor and outdoor air CO2 concentrations were simultaneously monitored during the sampling period. The indoor air CO2 concentration increased or decreased according to the natural ventilation and activities of occupants. Every natural decay of indoor air CO2 concentration with open windows was used to calculate the natural ventilation rate. There was no deliberate CO2 release and no mixing fan in the rooms, and complete mixing of indoor air was assumed in all calculations. The daily average natural ventilation rate was defined as the average of all decays during the sampling period. The CO2 decay method was also applied to the two mechanical ventilation systems, and the calculated ventilation rates were compared with the measured supply airflows, as shown in Table 2. The sampling was conducted during a 24-h period in each apartment. During sampling, real-time measurements included the following: indoor particle number concentration, PM2.5 mass concentration, CO2, temperature, and relative humidity. The particle number concentrations from a size range of 0.3–0.5 lm to 5.0– 10.0 lm were measured using optical particle counters (Aero Trak 9306; TSI Inc., Shoreview, MN, USA) in outdoor air, in the living–dining room, and in the master bedroom at 10-min intervals. TSI Dust Trak photometers were used to continuously monitor PM2.5
mass concentration in outdoor air and the living–dining rooms. The Dust Trak and Aero Trak were calibrated in the laboratory, before sampling. Indoor air quality monitors (Model 2211; Kanomax, Osaka, Japan) were used to determine the concentration of CO2, as well as the temperature and relative humidity at 1-min intervals. The monitors were calibrated by sampling known concentration of CO2. All measurements were converted into 10-min averages for data reduction and analysis. The occupants also recorded their own activities using time checklist sheets.
Results Leakages and ventilation rates
Table 2 shows the effective leakage area and ventilation rates of the sample apartments. The types of filters that were installed in the two mechanical ventilation systems were noted according to ASHRAE Standard 52.2 (ASHRAE, 2007). The effective leakage area, ELA, quantifies the equivalent amount of opening area in the building envelope and can be used with the LBL model to estimate infiltrating airflows through leakage paths. The normalized leakage area, NL, represents the relative tightness of the sample apartments. The effective leakage area ranged from 31 at O to 191 cm2 at G, with an average across all apartments of 100 cm2. For apartments F–J, with balanced mechanical ventilation systems, the effective leakage areas were 41–91% larger than the average across all apartments. The averaged normalized leakage area was 0.06 0.03 m2/m2, and
Table 2 Leakage and ventilation rate of the sampled apartments Air tightness
Ventilation rate (1/h) Living–dining room
Master bedroom
Sample ID
ELA a (cm2)
NLb (–)
Fan flowsc
CO2d
Fan flows
CO2
Filter efficiencye
Vent.
A B C D E F G H I J K L M N O
44 37 44 149 68 190 191 165 143 142 157 46 47 46 31
0.04 0.03 0.04 0.13 0.06 0.11 0.11 0.10 0.08 0.08 0.11 0.03 0.03 0.03 0.02
0.8 0.9 0.8 1.0 0.9 1.0 0.9 1.0 0.8 0.8 – – – – –
0.9 0.5 0.9 0.9 0.8 0.9 0.9 1.0 0.7 0.7 0.9 1.3 1.2 1.3 0.8
0.9 1.0 1.1 1.1 1.1 – – – – – – – – – –
1.1 0.6 0.8 0.8 0.9 0.3 0.2 0.3 0.5 0.3 0.8 1.0 1.0 0.2 0.1
MERV11 MERV11 MERV11 MERV11 MERV11 MERV < MERV < MERV < MERV < MERV < None None None None None
Unbalanced Unbalanced Unbalanced Unbalanced Unbalanced Balanced Balanced Balanced Balanced Balanced Natural Natural Natural Natural Natural
a
ELA, effective leakage area calculated by LBL Model at 4 Pa. NL, normalized leakage calculated from ASHRAE Standard 119. c Fan flows, ventilation rate measured by supply fan flows; d The average ventilation rate calculated using CO2 concentration decay. e MERV, minimum efficiency reporting value by ASHRAE Standard 52.2 b
632
0.4 0.5 0.3 0.5 0.6
0.1 0.1 0.1 0.2 0.1 0.4 0.5 0.4 0.1 0.1
11 11 11 11 11
Indoor particle concentration in residential buildings this corresponds to a leakage of ‘Class A’, for which is recommended full mechanical ventilation in ASHRAE Standard 119. The infiltrating airflows calculated by the LBL model (Sherman and Modera, 1986) were estimated to produce air change rates below 0.1 h 1. Predicted infiltration rates were small relative to measured total ventilation rates in each type of apartment. The mechanical ventilation air change rate measured by the fan airflows ranged from 0.8 to 1.1 h 1 in the living–dining rooms. The differences in the ventilation rates between the CO2 decay method and the fan flows ranged from 0 to 44% with an average of 17%. The differences were considered to be a consequence of inaccuracies in the assumption of complete mixing. The average mechanical ventilation rate of the living– dining rooms was 0.9 h 1, which was slightly higher than the recommendation of the Korean building code. The mechanical ventilation rates of master bedrooms of apartments A–E were close to those of the livingdining rooms. But the ventilation rates of master bedrooms of the apartments F–J were quite low compared with those of the living–dining rooms, likely because there was no mechanical fan airflow in these rooms. The ventilation rates were slightly higher than the infiltration airflows which were calculated by the LBL model. Those results were supposed to be the multizone airflows among the rooms, because the interior door of the master bedrooms was frequently opened during the sampling periods. The opening of the door could influence the airflows of master bedrooms of apartments F–J. The natural ventilation rates of apartments K–O significantly fluctuated according to wind pressure and the opening of windows. The daily average natural ventilation rates of the living–dining room ranged from 0.8 h 1 at apartment O to 1.3 h 1 at apartment L, with an average of 1.1 h 1. The average was slightly higher than the mechanical ventilation rates of apartments A–J. The natural ventilation rates of the master bedrooms were close to those of the living–dining rooms except for N and O. The natural ventilation rates of N and O were below 0.2 h 1.
Table 3 Outdoor and indoor air temperature, relative humidity, and CO2 concentration in the samplesa Ventilation typesb
Unbalanced (A, B, C, D, E) Outdoor air Temp (°C) 21.5 4.9 RH (%) 50 18 CO2 (ppm) 457 42 Indoor air Living–dining room Temp (°C) 27.4 1.1 RH (%) 43 5 CO2 (ppm) 775 247 |To-Ti|c (°C) 6.2 3.8 Master bedroom Temp (°C) 26.4 0.9 RH (%) 45 5 CO2 (ppm) 1262 316
Balanced (F, G, H, I, J)
Natural (K, L, M, N, O)
20.4 4.0 61 19 452 34
24.2 4.7 45 14 457 25
26.9 48 577 6.8
0.8 6 130 4.1
26.3 0.7 52 5 1256 437
26.9 36 556 4.1
0.9 7 82 2.7
26.7 1.0 40 7 983 272
a
Daily average, 24 h, in each ventilation group. Unbalanced: unbalanced mechanical ventilation (only mechanical supply fan); balanced: balanced mechanical ventilation (mechanical supply and exhaust fans); natural: natural ventilation. c Temperature difference between outdoor and indoor. b
rooms, CO2 concentrations were maintained at below 1,000 ppm with daily average 556–775 ppm. The daily average CO2 concentrations of the naturally ventilated apartments, K–O, were lower than those of the mechanically ventilated apartments, A–J. But the CO2 concentrations of the master bedrooms were significantly higher than those of the living–dining rooms especially in the unventilated apartments, F–J. Figure 1 shows daily averaged outdoor particle number concentrations measured at the three sites. The number concentrations varied over a wide range with time and were lower overnight than during the daytime. The number concentration decreased with particle size fraction, with the order of 103 times difference between the smallest and the largest size intervals. The average number concentrations for 0.3–0.5, 0.5–1.0, 1.0–3.0, 3.0–5.0, and 5.0–10.0 lm size interval were 2.5x108, 4.0x107, 3.2x106, 2.6x105, and 1.2x105 counts/m3, respectively. For each size fraction, daily variation of
Outdoor and indoor environments
Outdoor and indoor environmental conditions measured at the sampling periods are summarized in Table 3. The outdoor climate was mild at the three sites, and the daily averaged outdoor air temperature ranged from 20.4 to 24.2°C. There was no significant difference in indoor air temperature and humidity among the apartments. The indoor air temperature was 2.7–6.5°C higher than the outdoor air temperature. The indoor air relative humidity was below 65% during the sampling periods. The CO2 concentration was significantly influenced by the ventilation rate and the occupants’ activities. In the living–dining
Fig. 1 Daily average outdoor particle number concentrations in three sites (The error bars indicate standard deviations)
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Park et al. the number concentrations corresponded to about 50% of the average values for 0.3–0.5, 3.0–5.0, and 5.0–10.0 lm, and about 80% for 0.5–1.0 and 1.0– 3.0 lm. The differences in daily average number concentrations among the three sites were below 32% for each size fraction. The number concentrations of 0.3– 0.5 and 0.5–1.0 lm size intervals were slightly high at the apartments with balanced mechanical ventilation, F–J. PM2.5 mass concentrations of outdoor air were also at similar levels at the three sites, and the concentration ranged from 0.06 mg/m3 to 0.59 mg/m3 with an average across all sites of 0.20 mg/m3. But an appropriate caution is needed to interpret the measurement results of PM2.5, because Yanosky et al. (2002) showed that the factory calibration of the DustTrak monitor generates substantial bias in reporting mass concentration measurements for typical atmospheric samples.
(a)
(b)
Indoor particle concentrations and sources
An understanding of indoor sources can help to estimate the influence of the three different ventilation systems on indoor particle concentrations. Figure 2 shows hourly variations of outdoor and indoor particle number concentrations for each size fraction. The number concentrations varied over a wide range both outdoors and indoors. But the outdoor number concentrations were consistently higher than the corresponding indoor number concentrations in all size fractions. The indoor number concentrations changed according to the activities of the occupants, and several peaks could be clearly identified during morning and evening periods, at 07:00–09:00 and 18:00–20:00. The peak concentrations decreased with time, because of ventilation, the absence of indoor sources, and deposition. The result showed that indoor sources including occupants’ activities and cooking strongly impacted the indoor particle concentrations. And it was also clearly shown that the patterns of indoor concentrations were significantly different between two units with mechanical ventilation apartments, D and H, and one with natural ventilation, apartment M. Time-averaged indoor-to-outdoor PM2.5 ratios (I/O) in the living–dining rooms are shown in Figure 3. The time-averaged ratios were classified by occupied conditions. The unoccupied period was defined as the absence of the occupants at the living–dining rooms, 00:00–06:00, and the occupied period was subdivided into morning, 07:00–09:00, and evening 18:00–20:00. The I/O ratio of the PM2.5 mass concentration was lower and less variable in the unoccupied period than that of the occupied period, indicating that indoor sources strongly influence PM2.5 concentrations. For the occupied period, obvious indoor sources resulted in an elevated I/O ratio. The I/O ratio increased about 11–38% when the living–dining rooms were occupied. 634
(c)
Fig. 2 Time series of particle number concentrations in outdoor and indoor air in the living–dining room. (a) Unbalanced mechanical ventilation (apartment D). (b) Balanced mechanical ventilation (apartment H). (c) Natural ventilation (apartment M)
The average I/O ratio of the occupied period ranged from 0.5 0.3 for the unbalanced mechanical ventilation units, A–E, to 0.6 0.3 for the natural ventilation units, K–O. Figure 4 shows indoor particle number concentrations for all size fractions in the apartments with
Indoor particle concentration in residential buildings interval, increased considerably compared with that for submicron particles, 0.3–0.5 and 0.5–1.0 lm in the living–dining rooms. For the master bedrooms, there were no significant changes of particle concentrations except for the fine particles. Although the influences of activities of the occupants, such as cooking, dining, and sleeping, were difficult to separate unambiguously from the measured data shown in Figures 3 and 4, it is obvious that the indoor emission rate of particles is significantly changed according to the indoor activities of the occupants. Indoor-to-outdoor particle concentration ratio Fig. 3 Time-average indoor-to-outdoor PM2.5 mass concentration ratios (I/O) in the living–dining rooms (The error bars indicate one standard deviation) (a)
(b)
Fig. 4 Particle number concentrations in the unbalanced mechanical ventilation, in apartments A–E. (The error bars indicate one standard deviation). (a) Living–dining room. (b) Master bedroom
Time-averaged indoor-to-outdoor PM2.5 mass concentration ratios are summarized in Table 4. The statistical significance of differences was tested using the Student’s t test. For all ventilation types, the indoor concentration was lower than the outdoor concentration even in the apartments with natural ventilation. Deposition onto indoor surfaces and filtration in ventilation systems lowered indoor concentrations and dampened the variability seen in outdoor concentrations. With no indoor sources, the ratio is indicative of the contribution of penetration and filtration of the outdoor particles, and it also includes the influence of deposition in the rooms. The mean I/O ratio of PM2.5 ranged from 0.4 for the unbalanced mechanical ventilation to 0.6 for the natural ventilation in the living– dining rooms when they were unoccupied. The occupants of the apartments with natural ventilation commonly opened windows during unoccupied periods as during occupied periods. The I/O ratio with natural ventilation was significantly higher (P < 0.01) than that with unbalanced mechanical ventilation, indicating some benefit from PM2.5 removal by the ventilation system filter. The unbalanced mechanical ventilation showed the lowest I/O ratio among the three ventilation types. The differences of the mean PM2.5 I/O ratio between the balanced mechanical ventilation and the natural ventilation apartments were relatively small compared with the apartments with unbalanced mechanical ventilation. For the occupied periods, the I/O ratios were higher than those of the unoccupied period, probably because of the influence of indoor sources. Daily average I/O ratios for the apartments Table 4 Time-averaged indoor-to-outdoor PM dining rooms (avg s.d.) Unbalanced
unbalanced mechanical ventilation, A–E. The master bedrooms of A–E were mechanically ventilated with an average across 5 apartments of 1.1 h 1 which was the same level as the living–dining rooms. For the occupied periods, the average number concentration for fine particles, such as 1.0–3.0, 3.0–5.0, and 5.0–10.0 lm size
Unoccupied Morningb Eveningc Dailyd
a
0.40 0.52 0.57 0.50
2.5
mass concentration (I/O) in the living–
Balanced *
0.08 0.14 0.19 0.19*
0.55 0.52 0.65 0.58
0.11 0.12 0.11 0.12
Natural 0.60 0.53 0.66 0.61
0.18 0.17 0.20 0.20
a
00:00–06:00, b07:00–09:00, c18:00–20:00, d24-h average.*Statistically significant difference at P < 0.01 (t test).
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Park et al. with unbalanced mechanical ventilation, balanced mechanical ventilation, and natural ventilation were 0.50, 0.58, and 0.61, respectively. The difference between unbalanced mechanical ventilation and natural ventilation was statistically significant (P < 0.01). Figure 5 shows the trend of the time-averaged I/O ratios in the living–dining rooms, and the I/O ratios for each size interval are summarized in Table 5. During unoccupied periods, significant differences were clearly found in all size fractions between apartments (a)
(b)
(c)
Fig. 5 Time-averaged I/O ratios of particle number concentrations at living–dining room (The error bars indicate one standard deviation). (a) Unoccupied. (b) Occupied. (c) Daily
636
with unbalanced mechanical ventilation and those with natural ventilation. The I/O ratios of the apartments with natural ventilation were 2–3 times higher for 1.0– 3.0, 3.0–5.0, and 5.0–10.0 lm size intervals than those with unbalanced mechanical ventilation. And there were also significant differences in the I/O ratios for 0.3–0.5, 0.5–1.0, and 1.0–3.0 lm size intervals between the unbalanced and balanced ventilation systems. Those differences were considered to result from high leakage area and low filtration efficiency of the filter. The average effective leakage area of the apartments with balanced mechanical ventilation was about 60% larger than in case with unbalanced mechanical ventilation. The outdoor particle concentrations for 0.3–0.5 and 0.5–1.0 lm size intervals were also 10–38% higher with balanced mechanical ventilation than with unbalanced mechanical ventilation, as shown in Figure 1. During occupied periods, however, significant differences were not found between the apartments with unbalanced mechanical ventilation and those with natural ventilation. The differences in the I/O ratios among the three ventilation types were considerably decreased during periods of occupancy compared with those of the unoccupied period, because of the influence of indoor sources. The daily average I/O ratios of the apartments with natural ventilation were significantly higher (P < 0.01) in all size intervals than those of the apartments with either type of mechanical ventilation system. The I/O ratios for 0.3–0.5, 0.5–1.0, 1.0–3.0, 3.0–5.0, and 5.0– 10.0 lm size intervals were 0.6, 0.7, 0.6, 0.3, and 0.5, respectively, in the natural ventilation case. Those corresponded to 123%, 141%, 162%, 216%, and 161% of the average I/O ratios in the two sets of apartments with mechanical ventilation. For the master bedroom, the daily average I/O ratios for all size intervals were also significantly higher (P > 0.01) with natural ventilation than with mechanical ventilation. Discussions
The results of this study substantiate an expectation of significant improvement in indoor particle concentrations of outdoor origin when apartments are mechanically ventilated with filtration. The daily average indoor particle concentration decreased to 50% below the outdoor level for submicron particles and 25% below the outdoor level for fine particles, when the apartments were mechanically ventilated. The two types of mechanical ventilation reduced the I/O ratios by 26% for submicron particles and 65% for fine particles compared with natural ventilation. These results show that indoor airborne particles are sometimes dominated by outdoor air (Franck et al., 2003) and also that mechanical ventilation can be effective for removing incoming outdoor particles (Bolster and Linden, 2009; Nazaroff, 2004).
Indoor particle concentration in residential buildings Table 5 Time-averaged I/O ratios of particle number concentrations in the sampled apartments Living–dining room Unbalanced Unoccupied/Asleepa 0.3–0.5 lm 0.5–1.0 lm 1.0–3.0 lm 3.0–5.0 lm 5.0–10.0 lm Occupied 0.3–0.5 lm 0.5–1.0 lm 1.0–3.0 lm 3.0–5.0 lm 5.0–10.0 lm Dailyb 0.3–0.5 lm 0.5–1.0 lm 1.0–3.0 lm 3.0–5.0 lm 5.0–10.0 lm
Master bedroom Balanced
Natural
Unbalanced
Balanced
Natural
0.39 0.32 0.19 0.06 0.13
0.11* 0.09* 0.08* 0.04 0.08
0.46 0.53 0.30 0.09 0.18
0.07* 0.16* 0.13* 0.04 0.11
0.51 0.64 0.58 0.24 0.51
0.09* 0.20* 0.16* 0.09* 0.21*
0.50 0.35 0.15 0.19 0.13
0.11 0.14 0.10 0.15* 0.10*
0.59 0.36 0.11 0.08 0.05
0.10 0.07 0.04 0.07* 0.08*
0.66 0.46 0.29 0.40 0.25
0.12* 0.18* 0.09* 0.17* 0.12*
0.52 0.50 0.47 0.22 0.43
0.14 0.18 0.27 0.16 0.38
0.51 0.58 0.37 0.13 0.28
0.11 0.20 0.15 0.06* 0.14*
0.56 0.72 0.60 0.25 0.51
0.11 0.24 0.20 0.11 0.24
0.52 0.38 0.23 0.33 0.23
0.16* 0.12 0.17 0.29 0.20
0.72 0.46 0.16 0.19 0.13
0.14 0.13 0.06* 0.06 0.08*
0.80 0.64 0.38 0.48 0.30
0.07* 0.07* 0.04* 0.09* 0.08*
0.51 0.44 0.33 0.15 0.30
0.19 0.20 0.24 0.15 0.28
0.49 0.55 0.35 0.12 0.26
0.10 0.17 0.14 0.06 0.16
0.62 0.69 0.55 0.29 0.45
0.18* 0.23* 0.21* 0.15* 0.24*
0.61 0.39 0.18 0.24 0.16
0.25 0.19 0.14 0.24* 0.17*
0.67 0.40 0.15 0.16 0.11
0.16 0.12 0.06 0.12* 0.11*
0.77 0.56 0.33 0.42 0.26
0.20* 0.20* 0.14* 0.20* 0.14*
a
Unoccupied is 00:00–06:00 for living–dining room, and sleep is for master bedroom. 24-h data in each ventilation type. *Statistically significant difference from other ventilation type at P < 0.01 (t test). b
A previous study showed that the HVAC system of an office building reduced the submicron particle concentration by 34% (Jamriska et al., 2000). Filtration of recirculated airflow can reduce indoor particle concentrations (Fisk et al., 2002). The I/O ratios summarized in Tables 4 and 5 can be compared with other studies. For the naturally ventilated schools with operable windows and doors, measured I/O ratios ranged from 0.74 to 0.88 for submicron particles (Tippayawong et al., 2009), whereas the I/O ratios were quite low in a mechanically ventilated school building (Parker et al., 2008). The results of this study indicate that mechanical ventilation can reduce exposure to outdoor particles in residential buildings. With natural ventilation, the daily average I/O ratios ranged from 0.56 to 0.72 for submicron particles and from 0.25 to 0.60 for fine particles. Without indoor sources, the I/O ratios varied from 0.51 to 0.64 for submicron particles and from 0.24 to 0.58 for fine particles. Previous studies showed that an average penetration ratio through infiltration was 0.47 0.15 in 19 residential buildings (Stephens and Siegel, 2012). However, in the naturally ventilated apartments studied here, the average effective leakage area (ELA) was 66 cm2, only 9% of the average ELA in the previous studies. Therefore, the penetration through infiltration in the naturally ventilated apartments was unimportant relative to the penetration through operable windows. Penetration through operable windows depends on several factors, including the geometry of openings, wind direction and speed, pressure difference between outdoors and indoors, and dynamic properties of particles (Chen and Zhao, 2011; Liu and Nazaroff, 2001;
Rim et al., 2010). The geometrical size of openings and unsteadiness of wind flow substantially influence not only the natural ventilation rate, but also the penetration of incoming outdoor particles (Park, 2013). The I/O ratios in the apartments with the natural ventilation shown in Figure 5 included variable window opening conditions, and the results are only applicable to times when outdoor air temperatures are comfortable. From the results of this study, it was difficult to quantify the influences of variable opening size and unsteadiness of wind flows on the penetration of outdoor particles. Better characterization of the variable opening windows and wind flows is needed to evaluate the outdoor particle penetration in naturally ventilated residential buildings. Previous studies showed that understanding the influence of occupant window opening behavior on ventilation rate is important when estimating human exposure to indoor air pollutants (Howard-Reed et al., 2002). Outdoor air temperature substantially influences on the occupant window opening behavior. For the case of the two mechanical ventilation options studied here, the I/O ratios shown in Tables 4 and 5 might be different from those in real conditions, because occupants frequently use operable windows instead of mechanical fans when the outdoor air temperature is comfortable (Park and Kim, 2012). Conclusion
This study evaluated the influences of ventilation system type on indoor particle concentrations in residential buildings. The daily average indoor particle 637
Park et al. concentration decreased to 50% below the outdoor level for submicron particles and 25% below the outdoor level for fine particles, when the apartments were mechanically ventilated. The two mechanical ventilation systems reduced the I/O ratios by 26% for submicron particles and 65% for fine particles compared with natural ventilation, and this finding is consistent with previous studies. This study contributes to understanding the exposure to outdoor particles in naturally ventilated residential buildings under moderate climate conditions.
Acknowledgements
This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (NRF-2013R1A2A2A05005131). The author thanks C.S. Roh, J. E. Lee, and B. D. Seo, graduate students, who helped with the field measurements.
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