Ventilation and air quality measurements were concurrently obtained in the living area and the child’s bedroom of each house during baseline and each seasonal assessments (summer was defined as June, July and August; fall as September, October and November; winter as December, January, February, and spring as March, April and May.) ACRs were estimated using the multizone constant injection method [16 ,17 ], which allows determination of “local” ACRs. The method used two different perfluorocarbon tracers (PFTs) and concentration measurements of the tracers in both zones. Two passive emitters of hexafluorobenzene (HFB) were placed in the living area, and two emitters of octafluorotoluene (OFT) in the sleeping area. Emitters were typically placed in opposite corners of rooms, releasing the PFT at a constant rate over the weeklong sampling period. PFT concentrations were measured using passive samplers at central locations in the two rooms, placed away from the emitters [18 (link)]. The passive samplers were analyzed using thermal desorption, cryofocusing and GC-MS analysis [19 (link)]. Figure 1 depicts flows Q1 to Q4 determined by this method; flows Q5 and Q6 are obtained by flow balance. Note that zone 1, called the living area in this paper, refers to all rooms other than the (study) bedroom.
ACRs for the house and bedroom, and flows between these zones, were determined using the measured concentrations and the volumes of the house and bedroom as follows [20 (link)]:
where Q1 and Q2 = air flows into zone 1 and 2, respectively, from outdoors (m3·h−1); Q3 = air flow rate from zone 1 to 2 (m3·h−1); Q4 = air flow rate from zone 2 to 1 (m3·h−1); CHFB,1 and COFT,1 = concentrations of PFT tracers HFB and OFT in zone 1 (mg·m−3); CHFB,2 and COFT,2 = concentrations of HFB and OFT in zone 2 (mg·m−3); and EHFB,1 and EOFT,2 = emission rates of HFB and OFT in zones 1 and 2 (mg·h−1), respectively. This result assumes that outdoor PFT concentrations are zero, and that the PFTs are inert (removed only by airflows and not by settling, deposition, filtration or reaction). The ACRi (h−1) in zone i was calculated as Qi/Vi (i = 1,2), where Vi = volume of zone i (m3).
Interzonal flows. Interzonal flows transport pollutants between zones, e.g., cigarette smoke emitted in the living area that is brought to the bedroom. Interzonal flows Q3 and Q4 from the two zone model are expressed as interzonal flow proportions 𝛼HB and 𝛼BH (dimensionless, ratios between 0 and 1):
where 𝛼HB = fraction of the air coming into the bedroom that arises from the (remainder of the) house, and similarly, 𝛼BH = fraction of house air coming from the bedroom. These proportions provide a simple way to compare the magnitude of interzonal flows among buildings of different sizes.
Indoor and outdoor measurements. PM concentrations in bedrooms were measured as sequential 24-h filter samples during the sampling week collocated with the PFT samplers in each season. As detailed elsewhere [15 (link)], PM samples were collected at 15 L/min on 1 µm-rated PTFE filters installed in static-free polypropylene cassettes (Omega Specialty Instruments Co., Houston, TX, USA) for gravimetric analysis. The inlets on these cassettes are not designed to be size selective, and they essentially capture the total suspended particulate (TSP) fraction. In addition, particle number counts (PNCs) in 0.3–1.0 µm and 1–5 µm dia size ranges were measured continuously using an optical particle counter (GT-521, MetOne, Grants Pass, OR, USA). PM concentrations and 0.3–1.0 µm PNCs were significantly correlated [15 (link)]. The PM data were reduced to weeklong averages.
Outdoor PM2.5 measurements were obtained from air quality monitoring sites in Detroit selected to be representative of population exposure. These included daily data from four sites (Allen Park, Ambassador Bridge, Dearborn, Newberry School), and every third day data from five additional sites (Southwest High School, Linwood, East 7 Mile, Livonia, Wyandotte). These sites were operated by the Michigan Department of Natural Resources and the Environment using protocols that followed standard federal reference methods. Meteorological data, including wind speed, direction, temperature, humidity, and barometric pressure, were obtained from the Detroit City Airport site located near the middle of the study area.
In addition to the PFT tracers, the same passive samplers measured two tracers of environmental tobacco smoke (ETS), i.e., 2,5-dimethylfuran and 3-ethenylpyridine used to confirm the presence of ETS, along with about 100 other volatile organic compounds (VOCs), e.g., naphthalene, BTEX (sum of benzene, toluene, ethylbenzene and xylenes), and total volatile organic compounds (TVOC, sum of target compounds) [15 (link),18 (link),20 (link),21 (link)]. These samplers were placed in bedrooms and living areas for a 1-week period, and analyzed using thermal desorption, gas chromatography and mass spectrometry. Temperatures and relative humidity also were monitored in both bedrooms and living rooms, and CO2 was monitored in the bedroom using an infrared sensor. These variables were monitored continuously and reduced to 1-week averages.
Quality assurance (QA). PFT, VOC and ETS tracer measurements used duplicate samplers and showed good agreement, i.e., replicate precision averaged 11 ± 12% for the PFTs, 15 ± 16% for VOCs, and 14 ± 13% for the ETS tracers. Field blanks for passive sampling tubes were deployed at each household each week, and showed negligible contamination. Emitters were weighed periodically to determine emission rates, and samplers were temperature corrected. ACR measurements that were excessively large (≥10 h−1) or unrealistically small (≤0.1 h−1) probably resulted from incomplete mixing or other reasons, and thus were omitted from analyses. (As shown later, such values constituted a very small fraction of measurements.) Further description of QA is provided elsewhere [15 (link)].
ACRs for the house and bedroom, and flows between these zones, were determined using the measured concentrations and the volumes of the house and bedroom as follows [20 (link)]:
where Q1 and Q2 = air flows into zone 1 and 2, respectively, from outdoors (m3·h−1); Q3 = air flow rate from zone 1 to 2 (m3·h−1); Q4 = air flow rate from zone 2 to 1 (m3·h−1); CHFB,1 and COFT,1 = concentrations of PFT tracers HFB and OFT in zone 1 (mg·m−3); CHFB,2 and COFT,2 = concentrations of HFB and OFT in zone 2 (mg·m−3); and EHFB,1 and EOFT,2 = emission rates of HFB and OFT in zones 1 and 2 (mg·h−1), respectively. This result assumes that outdoor PFT concentrations are zero, and that the PFTs are inert (removed only by airflows and not by settling, deposition, filtration or reaction). The ACRi (h−1) in zone i was calculated as Qi/Vi (i = 1,2), where Vi = volume of zone i (m3).
Interzonal flows. Interzonal flows transport pollutants between zones, e.g., cigarette smoke emitted in the living area that is brought to the bedroom. Interzonal flows Q3 and Q4 from the two zone model are expressed as interzonal flow proportions 𝛼HB and 𝛼BH (dimensionless, ratios between 0 and 1):
where 𝛼HB = fraction of the air coming into the bedroom that arises from the (remainder of the) house, and similarly, 𝛼BH = fraction of house air coming from the bedroom. These proportions provide a simple way to compare the magnitude of interzonal flows among buildings of different sizes.
Indoor and outdoor measurements. PM concentrations in bedrooms were measured as sequential 24-h filter samples during the sampling week collocated with the PFT samplers in each season. As detailed elsewhere [15 (link)], PM samples were collected at 15 L/min on 1 µm-rated PTFE filters installed in static-free polypropylene cassettes (Omega Specialty Instruments Co., Houston, TX, USA) for gravimetric analysis. The inlets on these cassettes are not designed to be size selective, and they essentially capture the total suspended particulate (TSP) fraction. In addition, particle number counts (PNCs) in 0.3–1.0 µm and 1–5 µm dia size ranges were measured continuously using an optical particle counter (GT-521, MetOne, Grants Pass, OR, USA). PM concentrations and 0.3–1.0 µm PNCs were significantly correlated [15 (link)]. The PM data were reduced to weeklong averages.
Outdoor PM2.5 measurements were obtained from air quality monitoring sites in Detroit selected to be representative of population exposure. These included daily data from four sites (Allen Park, Ambassador Bridge, Dearborn, Newberry School), and every third day data from five additional sites (Southwest High School, Linwood, East 7 Mile, Livonia, Wyandotte). These sites were operated by the Michigan Department of Natural Resources and the Environment using protocols that followed standard federal reference methods. Meteorological data, including wind speed, direction, temperature, humidity, and barometric pressure, were obtained from the Detroit City Airport site located near the middle of the study area.
In addition to the PFT tracers, the same passive samplers measured two tracers of environmental tobacco smoke (ETS), i.e., 2,5-dimethylfuran and 3-ethenylpyridine used to confirm the presence of ETS, along with about 100 other volatile organic compounds (VOCs), e.g., naphthalene, BTEX (sum of benzene, toluene, ethylbenzene and xylenes), and total volatile organic compounds (TVOC, sum of target compounds) [15 (link),18 (link),20 (link),21 (link)]. These samplers were placed in bedrooms and living areas for a 1-week period, and analyzed using thermal desorption, gas chromatography and mass spectrometry. Temperatures and relative humidity also were monitored in both bedrooms and living rooms, and CO2 was monitored in the bedroom using an infrared sensor. These variables were monitored continuously and reduced to 1-week averages.
Quality assurance (QA). PFT, VOC and ETS tracer measurements used duplicate samplers and showed good agreement, i.e., replicate precision averaged 11 ± 12% for the PFTs, 15 ± 16% for VOCs, and 14 ± 13% for the ETS tracers. Field blanks for passive sampling tubes were deployed at each household each week, and showed negligible contamination. Emitters were weighed periodically to determine emission rates, and samplers were temperature corrected. ACR measurements that were excessively large (≥10 h−1) or unrealistically small (≤0.1 h−1) probably resulted from incomplete mixing or other reasons, and thus were omitted from analyses. (As shown later, such values constituted a very small fraction of measurements.) Further description of QA is provided elsewhere [15 (link)].
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