Migration of volatile organic compounds from attached garages to residences: A major exposure source

ARTICLE IN PRESS

Environmental Research 104 (2007) 224–240 www.elsevier.com/locate/envres

Migration of volatile organic compounds from attached garages to residences: A major exposure source Stuart Battermana,, Chunrong Jiaa, Gina Hatzivasilisb a

Environmental Health Sciences, 1420 Washington Heights, Room 6075, University of Michigan, Ann Arbor, MI 48019-2029, USA b Ecophysics Inc., Ann Arbor, MI 48108-2200, USA Received 31 May 2006; received in revised form 14 January 2007; accepted 17 January 2007 Available online 12 March 2007

Abstract Vehicle garages often contain high concentrations of volatile organic compounds (VOCs) that may migrate into adjoining residences. This study characterizes VOC concentrations, exposures, airflows, and source apportionments in 15 single-family houses with attached garages in southeast Michigan. Fieldwork included inspections to determine possible VOC sources, deployment of perfluorocarbon tracer (PFT) sources in garages and occupied spaces, and measurements of PFT, VOC, and CO2 concentrations over a 4-day period. Air exchange rates (AERs) averaged 0.4370.37 h1 in the houses and 0.7770.51 h1 in the garages, and air flows from garages to houses averaged 6.575.3% of the houses’ overall air exchange. A total of 39 VOC species were detected indoors, 36 in the garage, and 20 in ambient air. Garages showed high levels of gasoline-related VOCs, e.g., benzene averaged 37739 mg m3. Garage/indoor ratios and multizone IAQ models show that nearly all of the benzene and most of the fuel-related aromatics in the houses resulted from garage sources, confirming earlier reports that suggested the importance of attached garages. Moreover, doses of VOCs such as benzene experienced by non-smoking individuals living in houses with attached garages are dominated by emissions in garages, a result of exposures occurring in both garage and house microenvironments. All of this strongly suggests the need to better control VOC emissions in garages and contaminant migration through the garage–house interface. r 2007 Elsevier Inc. All rights reserved. Keywords: Air exchange; Emissions; Indoor air; Garages; Gasoline; Migration; Volatile organic compounds

1. Introduction High concentrations of volatile organic compounds (VOCs) present in garages, often due to vehicle emissions (Thomas et al., 1993; Lansari et al., 1996; Murphy et al., 1997; Graham et al., 1999, 2004; Noseworthy and Graham, 1999; Tsai and Weisel, 2000), can affect indoor air quality (IAQ) in the occupied space of an attached residence (see especially Fugler et al., 2002; Emmerich et al., 2003; Graham et al., 2004; Batterman et al., 2006; and references therein). In the few studies that have identified and quantified concentrations, VOC compositions in garages reflected the compounds expected for gasoline vapor (e.g., benzene, toluene, ethylbenzene, xylene, and trimethylbenzene), as well as compounds associated with paints, Corresponding author. Fax: +734 763 8095.

E-mail address: [email protected] (S. Batterman). 0013-9351/$ - see front matter r 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.envres.2007.01.008

solvents, cleaners, and other materials used and stored in homes, garages and vehicles (e.g., trichloroethane, trichloroethylene, limonene, a-pinene, and C10–17 n-alkanes). Current knowledge of contaminant migration between residences and garages remains largely qualitative, despite the expressed need for air exchange and migration studies that can be used to better understand and quantify the impacts of attached garages on residential air quality (Emmerich et al., 2003). While concentrations in garages can far exceed risk-based guidelines for certain VOCs (e.g., benzene), few individuals spend large amounts of time in closed garages, and only a fraction of garage emissions should enter occupied spaces. Concentrations or emissions associated with garage sources and garage-to-house migration rates are needed to estimate exposures and risks. Considering a recent average total VOC (TVOC) emission rate estimate of 3.1 g day1 per garage (Batterman et al., 2005) and allowing 10% of these emissions to migrate into

ARTICLE IN PRESS S. Batterman et al. / Environmental Research 104 (2007) 224–240

a medium-sized moderately well-sealed house (volume ¼ 500 m3, air exchange rate (AER) ¼ 0.5 h1) gives an indoor TVOC concentration of 500 mg m3. Assuming that benzene constitutes 2.5% of gasoline vapor yields an indoor benzene concentration of 12.5 mg m3, which falls just below the concentration range (13–45 mg m3) given by US EPA (2003) for a substantial (104) excess lifetime cancer risk. While this estimate does not account for other (outdoor and indoor) emission sources or parameter variability, and the 10% migration rate is unsubstantiated, it demonstrates the potential for adverse exposures. This study investigates the migration of pollutants from residential garages to adjoining houses in 15 homes in southeast Michigan. We previously reported on the VOC sources, concentrations, and emissions in these garages (Batterman et al., 2005). This paper extends the previous study by first describing methods to quantify air flows and contaminant migration (including recirculation) between the garage, living quarters, and outdoor air using a twotracer two-zone system. Next presented are VOC measurements in the occupied portion of the houses and air flow estimates, using a longitudinal study to examine temporal fluctuations and a cross-sectional study to examine variability across 15 houses. We then evaluate effects of AERs and house characteristics on VOC levels and contaminant migration between the garage and the house, apportion VOC concentrations to indoor, garage and outdoor sources, and show the significance of VOC sources in garages to an individual’s total exposure, using benzene as an example. Finally, we discuss the limitations and the implications of this study. 2. Materials and methods We recruited participants from two adjacent cities in southeast Michigan (Ann Arbor and Ypsilanti). Eligible participants were adults living in single-family houses with garages who occupied the house during the study period and promised cooperation with all aspects of the study. Participants were selected as part of a larger study using random digit dialing and snowball recruitment methods. This sample is not random or necessarily representative, although it may capture much of the variation in the sampled communities. Recruitment procedures were approved by the University of Michigan’s Institutional Review Board, and included informed and written consent. An initial longitudinal study was aimed at estimating the spatial and temporal variability within a 2-story house and attached two-car garage. Three people lived in the house, and two cars were parked in the garage. Seven repeated tests were conducted (house/garage #1, samples 1A–1G) over a 5-week period, each using a 4-day sampling period. Typically, five indoor locations (three downstairs, two upstairs) and two garage locations were monitored, and a large number of replicate measurements in each test were taken to estimate precision. The second cross-sectional phase of the study characterized variability across a broad set of houses and garages. Here, 14 additional houses (#2–15) were sampled over 4-day weekday periods. The final measurement at site 1 (1G) was included in these comparisons, since it was conducted just prior to tests on the other houses. From one to three houses were studied simultaneously. AERs and air flows were determined using the constant injection technique, miniature perfluorocarbon tracer (PFT) emitters, and passive samplers placed in houses and garages. PFTs make good tracers as they are chemically stable, nontoxic, normally present at extremely low levels in

225

the environment, and precise measurements can be made over a large concentration range. In garages, hexafluorobenzene (HFB) was emitted (6 mg h1) by two sources held at 40 1C, and in houses, a different PFT, octafluorotoluene (OFT), was emitted (1 mg h1) by two sources in different rooms (Batterman et al., 2006). Concentrations of VOCs and PFTs were measured over a 4-day period in the house, garage and outside air using passive tube-type samplers and Tenax GR adsorbents. Generally, two sites were selected on the house’s first floor, a third site on the second floor, and a fourth site in the garage, each using single or duplicate samplers. If in the same room, the sampler and source were placed on opposite sides of the room. Ambient concentrations were determined at a single location each week, outside one of the study houses (which were usually in the same neighborhood) using duplicate samplers. Previously, we determined that ambient VOC levels in the area studied were low, generally o1 mg m3 for any individual compound. At least one field blank was collected for each house; a total of 25 field blanks were collected and analyzed during the study. Temperature and relative humidity were logged continuously in each house and garage, and for each outdoor sample using an integrated data logger (Hobo HO-8; Onset Computer Corp., Bourne, MA, USA). The carbon dioxide (CO2) concentration was continuously measured at a central location in each house using a sensor (GMW-2; Vaisala Corp., Helsinki, Finland) interfaced to the data logger and calibrated using pure nitrogen and a 1011 ppm CO2 standard. Meteorological data were obtained from the Detroit Metropolitan Airport (Weather Underground, 2005). After sampling, samplers were refrigerated in the laboratory until analysis, which was usually completed within 1–3 days. Sampling tubes and sources were handled carefully to prevent cross-contamination. Tubes were thermally desorbed and analyzed by gas chromatography/mass spectroscopy for 93 target compounds using our standard protocol (Peng and Batterman, 2000; Jia et al., 2006), as well as for HFB and OFT tracers (Batterman et al., 2006). Performance criteria for this method, including method detection limits (MDLs), linearity, dynamic range, and storage stability, have been reported in these papers. Here, MDLs are calculated based on a sampling period of 4 days using three times the standard deviation of low concentration replicates. These and other performance criteria for this study utilize tests for selected compounds, e.g., toluene, benzene, OFT, HFB, as well as both prior and subsequent studies utilizing identical protocols. Non-detects were assigned the compound’s MDL, and replicates were averaged. A walkthrough survey at each house described the configuration of the garage, house, and heating/cooling system, and identified potential VOC sources. Residents filled out a survey to identify activities that might have influenced VOC concentrations, e.g., opening windows. The area of the house was provided by the home owner and confirmed using interior or exterior measurements. Ceiling heights and garage dimensions were measured using an acoustic measuring tape. House volume was estimated by multiplying house area by the average ceiling height; garage volume was calculated using the measured dimensions. The constant injection technique has been described elsewhere (Dietz and Cote, 1982; Sherman and Wilson, 1986; Stymne and Eliasson, 1991; Batterman et al., 2006). The following derivation is based on a mass balance dilution model that assumes fully mixed and steady-state conditions, and a conservative (loss-free) tracer that is not present in outdoor air. Continuity equations represent the flows of each tracer among all zones: Si;k ¼

N X ½Qi;j C j;k  for i ¼ 1 . . . N; k ¼ 1 . . . N,

(1)

j¼1

where Si,k ¼ known emission rate of the kth tracer in the ith zone (mg h1); Qi,i ¼ (unknown) total air flow in or out of the ith zone (m3 h1); Qi,j ¼ (unknown) flow rate from the jth to the ith zone (m3 h1); Cj,k ¼ measured concentration of the kth tracer in the jth zone (mg m3); and N the number of zones. In matrix notation, Eq. (1) becomes S ¼ QC,

(2)

which may be solved for a complete problem, i.e., when a unique tracer is emitted in each zone so that S is diagonal and along with C and Q are

ARTICLE IN PRESS 226

S. Batterman et al. / Environmental Research 104 (2007) 224–240

square matrices of order N; thus, Q ¼ SC1 .

(3)

The effective AERi in the ith zone (h1) is AERi ¼ Qi;i =V i ,

(4)

and account for the asymmetric and nonnormal distributions of AERs and other environmental measurements, Spearman rank correlations were used and reported if po0.05; also, medians and interquartile ranges (IQRs) were computed for G/I ratios and apportionments. Paired and two sample t-tests and other statistics used SYSTAT 10.0; dilution models and AERs were calculated using Microsoft Excel.

3

where Vi is the volume of the ith zone (m ). Note that this estimate includes air drawn from other spaces, i.e., it does not include only outside air. If recirculation between zones is negligible and each tracer is emitted in a single zone, the AER is AERi ¼ S i;i =ðC i;i V i Þ.

(5)

The air flow rate from the jth to the ith zone is Qi;j ¼ Qi;i ðC i;j =C j;j Þ ¼ ðSi;i =C i;i ÞðC i;j =C j;j Þ.

(6)

Lastly, the fraction of the exchange in the ith zone due to migration from the jth zone is simply a concentration ratio: Qi;j =Qi;i ¼ C i;j =C j;j ,

(7)

which is a robust result since it is independent of the PFT emission rate. AERs and interzonal flows were estimated for a 2-zone system using HFB and OFT tracers in garage and houses, respectively, which are considered as separate (but connected) zones, and Eqs. (3) and (4). This approach yields an ‘‘effective’’ AER which accounts for time-varying flows that can result from changes in building, occupant, and meteorological conditions over the testing period (Sherman and Wilson, 1986). A key assumption is the well-mixed condition, which means that the concentration throughout each zone is uniform. Mixing was evaluated using the coefficient of variation (COV) of VOC and PFT concentrations among the multiple locations measured in each house, after averaging replicate (side-by-side) samples. Garage or basement measurements were excluded from the COV calculations. COVs were determined for each house and pollutant separately, and for VOC concentrations X1 mg m3 since replicate variability tends to increase at low concentrations. With simultaneous deployment of PFT emitters and samplers, AERs will be underestimated due to the time required for concentrations to approach steady-state levels. However, biases will be small (o5%) for the AERs likely to be encountered (Batterman et al., 2006), thus no corrections are attempted. After estimating air flows, emission rates of VOC sources in each house and garage were determined using the estimated flows, measured indoor VOC concentrations corrected for outside levels (outdoor levels were subtracted), and Eq. (2). Then, to apportion exposures occurring in houses, concentrations in houses attributable to garage and house emissions were predicted using a dilution model, and indoor concentrations were apportioned to garage, house and outdoor sources by dividing the concentration due to these sources by the measured indoor concentration. AERs were also estimated by fitting CO2 concentration decay curves to a first-order model: C t ¼ C 0 expðktÞ þ C min ,

(8)

where Ct is the concentration (ppm) at time t (h) measured as elapsed time from start of decay, C0 the initial concentration, and k the decay rate (h1). Parameters C0, k, and Cmin were estimated using the nonlinear quasi-Newton solver to minimize squared residuals for selected (2–5 h) periods when a smooth decay in CO2 levels was observed, e.g., in the morning when occupants left the house. Two to four such periods were analyzed at each house and results averaged. In contrast to the effective AERs determined using PFT releases over a 4-day period, these AERs represent short-term measurements. Measurement precision was defined as the percent difference between duplicates. As mentioned, spatial variability was expressed as the COV of concentrations measured at multiple sites within a zone. Temporal variability was expressed as the COV of (average) concentrations within a zone over repeated sampling events. Garage/indoor (G/I) ratios were calculated for each house and pollutant using the average concentration measured in the garage and the occupied space. To obtain robust statistics

3. Results and discussion Tables 1 and 2 show house characteristics, the sampling schedule, and the prevailing meteorological conditions. All of these houses had forced air heating/cooling systems except #8, which had steam radiators. None of the houses had HVAC systems or ducting within the garage, potentially a major entry point for envelope leakage. All houses were located in suburban neighborhoods, most on purely residential streets. One house was on a busy road, but this house (#8) was set back 75 m. None was close to commercial or industrial VOC sources, including gas stations. Of the 15 houses, one (#6) had a detached (free standing) garage (others were attached), and all but one (#13) had non-smoking residents (based on the occupant survey). After describing sampling performance, the longitudinal and cross-sectional studies are discussed in turn. VOC and PFT sampling performance: Table 3 lists the target compounds and the method detection limits (MDLs), which ranged between 0.01 and 1.7 mg m3. Recoveries achieved using the multibed adsorbent and thermal desorption system varied between 75% and 128%. (Note that calibrations used spiked adsorbent tubes and account for deviations from 100%, thus recoveries only indirectly influence results.) Levels in all laboratory and field blanks were below MDLs except for one indoor field blank in which benzene was detected at 0.3 mg m3. Performance for the PFTs HFB and OFT showed excellent linearity (R240.996) over a wide range (up to 1500 mg m3), high recoveries (496%), and low MDLs (o0.03 mg m3). The method performance was consistent with other recent studies examining VOCs (e.g., Peng and Batterman, 2000; Jia et al., 2006) and PFTs (Batterman et al., 2006). Longitudinal study—measurement precision: Replicate precisions averaged 10–11% (COV of replicate samples, seven replicates, 10 VOCs in the house and garage with average concentrations X1 mg m3, Table 4). Performance degraded at low concentrations, but replicate precisions were better than 26% for all compounds except chloroform, which was detected at low concentrations (average of 0.07 mg m3). These results are typical and within acceptance criteria. Spatial variability: The spatial variability of VOC concentrations in the garage, expressed as a COV, was 1173% for VOCs X1 g m3 (six sampling events, 21 VOCs, 2–3 sites), slightly higher than the spatial variation in the house (873%; Table 4). The spatial variability is comparable to replicate precisions, suggesting that the apparent spatial variation is largely attributable to measurement variability. Importantly, results indicate that

3

2

2

3

2

1

3

1A

1B 1C 1D 1E 1F 1G 2

3

4

5

6

7

2

1

2

2

2

2

2

1938

1964

1970

1968

1967

1960

1962

F

F

F

F

F

P

P

544

294

586

430

449

442

521

House/ House and garage characteristics garage ID No. of No. of Year Partial/full House occupants floors in constructed basement volume house (m3)

Two car attached; two adjacent surfaces; entrance to hall Two car attached; two adjacent surfaces; entrance to hall One car attached; one adjacent surface; entrance to family room two car attached; three adjacent surfaces; entrance to kitchen and laundry; living space above Two car detached, freestanding One car attached; three adjacent surfaces; entrance to family room; living space above 1

1

37

2

1

2

2

110

97

126

88

108

2

108 Two car attached; three adjacent surfaces; entrance to hall; living space above

2

2

2

1

2 2 2 2 2 1 1

2

No. cars in Living garage room

1

1

2

2

Family room

2

1 1 1 1 1

Dining room

1 1 1 1 1 1

1

Den

Sampler locations and number of replicates Garage type Garage (no. cars; volume number of (m3) adjacent surfaces; passage door entrance; living space above)

Table 1 Description of the houses, sampling locations, and PFT source locations

1

1 1 1 1 1 2

Bedroom

Garage

2

1

1 1 1 1 1

Office

2

Hallway

2

2

2

1

2

2

2

2

2

3 3 3 3 3 3 2

3

Playroom Work area Basement Garage

7

6

6

7

7

6 9 9 9 9 9 7

6

Total

LR

LR

LR

FR

FR

LR LR LR LR LR LR FR

LR

OFT #1

DR

Base

Off

LR

LR

Den Den Den Den Den Den LR

Den

OFT #2

PFT locations

Gar

Gar

Gar

Gar

Gar

Gar Gar Gar Gar Gar Gar Gar

Gar

HFB #1 & #2

1

1

2

3

1

2

5

2.3

9

10

11

12

13

14

15

Mean or total

1.7

2

1

1

2

2

1

1

2

1965

1996

1965

1960

1987

1939

1971

2003

1926

—

F

F

F

F

F

F

P

F

444

493

372

238

465

626

274

385

544

—

118

Two car 99 attached; one adjacent surface; entrance to basement One car 98 attached; one adjacent surface; entrance to kitchen Two car 79 attached; two adjacent surfaces; entrance to hall Two car 241 attached; one adjacent surface; entrance to living room Two car 141 attached; one adjacent surface; entrance to hall One car 58 attached; one adjacent surface; entrance to hall 271 Two car attached; spans two floors; one adjacent surface; entrance to LR; built into hill 111 Two car attached; two adjacent surfaces; entrance to laundry

Garage type Garage (no. cars; volume number of (m3) adjacent surfaces; passage door entrance; living space above)

1.5

29

2

0

2

2

1

2

1

1

2

2

1

1

2

No. cars in Living garage room

16

2

2

2

2

2

Family room

10

1

2

Dining room

8

1

Den

Sampler locations and number of replicates

11

1

2

Bedroom

1

1

Garage

10

2

Office

2

Hallway

3

1

3

2

1

7

2

2

49

2

2

2

2

2

2

2

2

Playroom Work area Basement Garage

149

5

6

7

7

7

7

6

7

Total

—

FR

LR

LR

FR

LR

LR

FR

FR

OFT #1

—

PL

FR

Den

LR

DR

DR

BR

WA

OFT #2

PFT locations

—

Gar

Gar

Gar

Gar

Gar

Gar

Gar

Gar

HFB #1 & #2

Each house had five to nine VOC samplers in the house and garage, two OFT sources in the house, and two HFB sources in the garage. House locations indicated as LR ¼ living room; FR ¼ family room; BR ¼ bedroom; GR ¼ guest room; Off ¼ office; PL ¼ playroom; WA ¼ work area; base ¼ basement. Bold values represent upstairs rooms. ‘‘No. floors in house’’ excludes basement.

4

8

House/ House and garage characteristics garage ID No. of No. of Year Partial/full House occupants floors in constructed basement volume house (m3)

Table 1 (continued )

4/14 4/18 4/21 4/25 4/28 5/2 5/13 5/23 5/23 5/23 5/31 5/31 6/13 6/13 6/13 6/20 6/20 6/21 6/27 6/27 6/28

4/18 4/21 4/25 4/28 5/2 5/6 5/18 5/26 5/26 5/26 6/3 6/3 6/16 6/16 6/16 6/23 6/23 6/24 6/30 6/30 7/1

Stop

18 4

12 15 3 7 7 5 11 13 13 13 17 17 20 20 20 17 17 19 23 23 22 (8–27)

(2–25) (1–27) (1–16) (1–16) (2–14) (6–19) (2–20) (4–22) (4–22) (4–22) (6–25) (6–25) (10–29) (10–29) (10–29) (4–28) (4–28) (4–33) (18–32) (18–32) (16–31) 78 5

45 62 82 72 55 65 74 82 82 82 71 71 81 81 81 74 74 70 81 81 82

Rel. humid (%)

1013 5

1026 1015 1002 1004 1014 1023 1015 1010 1010 1010 1017 1017 1005 1005 1005 1020 1020 1019 1015 1015 1012

Baro. press (kPa)

14 3

13 17 23 18 15 13 15 14 14 14 11 11 18 18 18 11 11 13 12 12 14

Wind speed (km h1)

Temperature range (min–max) shown in parentheses. Ambient data from Weather Underground (2005). NA ¼ not available. a Estimated from previous week with similar weather. b Average and standard deviation includes samples 1G to 15.

Averageb Standard deviationb

1A 1B 1C 1D 1E 1F 1G 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Start

Temperarture (1C)

House/ garage ID

Sampling dates

Ambient

Sites/dates

Table 2 Sampling schedule, weather data, and garage environment during the sampling period

0.02 0.04

0.00 0.00 0.00 0.00 0.00 0.00 0.14 0.05 0.05 0.05 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Precipitation (cm)

25 4

(a) 21 21 19 66 19 20 22 19 27 20 28 26 26 24 25 24 31 20 30 30 (21–30)

(18–24) (19–22) (17–20) (46–68) (17–20) (19–22) (20–25) (17–21) (21–30) (11–33) (25–31) (22–27) (22–29) (22–27) (19–28) (21–27) (24–35) (19–48) (28–32) (24–34)

Temperature (1C)

Indoor

44 10

(a) 39 34 40 39 38 42 41 44 30 61 31 39 54 46 41 54 35 56 50 33

Rel. humid (%)

23 4

(a) 19 13 13 14 14 14a 18 19 18 23 23 23 23 22 NA 24 23 29 28 29

(18–30)

(16–24) (10–18) (11–16) (11–17) (10–18) (10–18)a (15–23) (15–26) (13–25) (19–27) (14–34) (21–25) (20–25) (17–30) NA (16–34) (18–30) (24–44) (22–36) (24–37)

Temperature (1C)

Garage

53 10

(a) 41 50 55 40 38 38a 54 48 52 43 47 58 82 59 NA 44 48 51 48 51

Rel. humid (%)

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S. Batterman et al. / Environmental Research 104 (2007) 224–240

Table 3 List of target VOCs and PFTs Compound

CAS no.

MDL (mg m3)

Compound

CAS no.

MDL (mg m3)

1,2-Dichloroethylene (trans,E) Methyl t-butyl ether 1,1-Dichloroethane Propanenitrile 1,2-Dichloroethylene (Cis, Z) 2-Butanone 2,2-Dichloropropane Bromochloromethane Chloroform Methyl acrylate Ethyl acetate Tetrahydrofuran 1,1,1-Trichloroethane Cyclohexane 1,2-Dichloroethane Butyl chloride 1,1-Dichloropropene Benzene Carbontetrachloride Chloroacetonitrile 1,2-Dichloropropane Trichloroethylene n-Heptane Dibromomethane 2-Nitropropane Bromodichloromethane 2,5-Dimethyl furan Methyl cyclohexane Methyl methacrylate 1,1-Dichloro-2-propanone 1,3-Dichloropropene (Cis, Z) Methyl isobutyl ketone Toluene 1,3-Dichloropropene (Trans, E) 1,1,2-Trichloroethane 1,3-Dichloropropane Dibromochloromethane Ethyl methacrylate 2-Hexanone n-Octane 1,2-Dibromoethane Tetrachloroethene Chlorobenzene 1,1,1,2-Tetrachloroethane Ethylbenzene p,m-Xylene Bromoform

156-60-5 1634-04-4 75-34-3 107-12-0 156-59-2 78-93-3 594-20-7 74-97-5 67-66-3 96-33-3 141-78-6 109-99-9 71-55-6 110-82-7 107-06-2 109-69-3 563-58-6 71-43-2 56-23-5 107-14-2 78-87-5 79-01-6 142-82-5 74-95-3 79-46-9 75-27-4 625-86-5 108-87-2 80-62-6 513-88-2 10061-01-5 108-10-1 108-88-3 10061-02-6 79-00-5 142-28-9 124-48-1 97-63-2 591-78-6 111-65-9 106-93-4 127-18-4 108-90-7 630-20-6 100-41-4 106-42-3 75-25-2

0.252 0.237 0.497 0.190 0.076 0.565 0.081 0.043 0.045 0.516 0.480 0.434 0.055 0.133 0.028 0.082 0.044 0.024 0.146 0.444 0.108 0.059 0.149 0.090 0.241 0.068 0.078 0.043 0.256 0.425 0.104 0.616 0.032 0.079 0.094 0.044 0.054 0.086 0.198 0.067 0.102 0.069 0.038 0.077 0.021 0.038 0.238

Styrene o-Xylene n-Nonane 1,1,2,2-Tetrachloroethane 1,2,3-Trichloropropane Isopropylbenzene Bromobenzene 1,4-Dichlor-2-butene (trans, E) a-Pinene 2-Chlorotoluene n-Propylbenzene 4-Chlorotoluene 4-Ethyl toluene 1,3,5-Trimethylbenzene Pentachloroethane 2-Ethyl toluene Phenol tert-Butylbenzene 1,2,4-Trimethylbenzene n-Decane 1,3-Dichlorobenzene 1,4-Dichlorobenzene sec-Butylbenzene 1,2,3-trimethyl benzene p-Isopropyltoluene d-Limonene 1,2-Dichlorobenzene n-Butylbenzene o-Cresol Hexachloroethane p,m-Cresol 1,2-Dibromo-3-chloropropane Nitrobenzene n-Undecane 1,2,4-Trichlorobenzene Naphthalene n-Dodecane 1,2,3-Trichlorobenzene Hexachlorobutadiene n-Tridecane n-Tetradecane n-Pentadecane n-Hexadecane n-Heptadecane

100-42-5 95-47-6 111-84-2 79-34-5 96-18-4 98-82-8 108-86-1 110-57-6 7785-70-8 95-49-8 103-65-1 106-43-4 622-96-8 108-67-8 76-01-7 611-14-3 108-95-2 98-06-6 95-63-6 124-18-5 541-73-1 106-46-7 135-98-8 526-73-8 99-87-6 5989-27-5 95-50-1 104-51-8 95-48-7 67-72-1 106-44-5 96-12-8 98-95-3 1120-21-4 120-82-1 91-20-3 112-40-3 87-61-6 87-68-3 629-50-5 629-59-4 629-62-9 544-76-3 629-78-7

0.022 0.026 0.137 0.057 0.074 0.020 0.039 0.572 0.032 0.022 0.040 0.058 0.052 0.034 0.135 0.056 1.488 0.039 0.031 0.043 0.020 0.015 0.038 0.038 0.042 0.045 0.061 0.061 1.596 0.114 1.596 0.128 1.702 0.056 0.036 0.081 0.047 0.038 0.037 0.043 0.037 0.032 0.046 0.045

Hexafluorobenzene Octafluorotoluene

392-56-3 434-64-0

0.033 0.008

Also shown are the Chemical Abstract System (CAS) number and the method detection limit (MDL) based on 4-day sampling period.

both garage and occupied (house) zones were well-mixed, a condition in the AER determinations. (Still, to confirm the mixing assumption in the second phase, we continued to monitor indoor locations at multiple sites.) This finding does not apply to unoccupied basements and other poorly coupled spaces, e.g., VOC levels in the basement were less than half the levels found on other floors, and the indoor tracer OFT was only one-fourth as high (trial 1G). Temporal variability: Concentrations of most VOCs showed only modest temporal variability. In the garage,

concentrations varied by 28710% (Table 4). Compounds showing greater variability were mostly measured at low concentrations, e.g., styrene and tetrachloroethene. Limonene also showed more variation (54%), likely reflecting intermittent use of cleaning compounds. We previously apportioned the total (apparent) temporal variation to changes in source emissions (accounting for 50% of the variation); changes in AERs (28%), and measurement errors (23%; Batterman et al., 2005). Temporal variation in VOC emissions was the major factor accounting for

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Table 4 Replicate precision, temporal variability, and spatial variability of concentration measurements in the pilot study (at house/garage #1) measured over seven trials for selected VOCs VOCs/AERs/flows

Concentration variability (%) Benzene Toluene p-Xylene, m-Xylene 1,2,4-Trimethylbenzene Styrene Naphthalene Chloroform Carbontetrachloride Tetrachloroethene 1,1,1-Trichloroethane a-Pinene d-Limonene n-Nonane n-Hexadecane Average Std. dev. AER/flows: average (standard dev.) AER (h1) House-to-garage flow (%) Garage-to-house (%)

House

Garage

Replicate precision

Temporal variability

Spatial variability

Replicate precision

Temporal variability

7 6 9 12 19a 17a 88a 20 23a 12a 9 10 26a 14a

18 23 22 22 23a 13a 91a 39 102a 20a 14 45 29a 30a

6 7 6 7 16a 9a 45a 15 21a 14a 6 7 31a 18a

11 14 10 11 21a 11 nd 19 13a 7 8 9 10 nd

26 23 26 25 58a 21 nd 32 51a 18 27 54 27 nd

10 4

26 11

8 3

11 3

28 10

0.18 — 13.16

(0.06) — (5.03)

— — —

0.65 2.97 —

(0.12) (1.55) —

Average replicate precision (%) shown for replicate samples. Temporal and spatial variability measured as COVs (%). Temporal variability shown for AERs and exchange fractions as determined using seven trials, with mean and standard deviation (latter in parentheses). a Average concentrations o1 mg m3, excluded from the average and standard deviation

fluctuations in VOC concentrations. In the house, the temporal variability was comparable (26711%, Table 4) to that in the garage. Again, higher variability (45%) was observed for limonene. In comparison to the 4-day samples used, short-term measurements will demonstrate much more variability (e.g., Thomas et al., 1993). AERs over the 5-week study period averaged 0.657 0.12 h1 in the garage and 0.1870.06 h1 in the house (Table 4, n ¼ 7). The temporal variability was relatively small despite considerable meteorological variation over the study period, e.g., 4-day temperatures ranged from 3 to 15 1C (Table 2). Garage-to-house and reverse flows averaged 1375% and 371%, respectively, of the total exchange. Only weak correlation was seen between garage AERs and meteorological variables. Overall, the temporal variation in air flows during the measurement period was not large, and a single 4-day measurement provided representative results. Cross-sectional study—VOC concentrations in the 15 houses and garages: A total of 39 target VOCs were detected in houses, 36 in the garages, and 20 in ambient air (Tables 5 and 6; VOCs found in only one home, e.g., tetrahydrofuran and 1,4-dichlorobenzene are excluded). Detection frequencies approached or reached 100% for most VOCs in both houses and garages. In houses, the most prevalent VOCs included aromatics, terpenes and

n-alkanes, and concentrations of individual compounds ranged from roughly 1 to 10 mg m3 (toluene and limonene generally had higher concentrations). In garages, the same VOCs were detected, but concentrations of aromatics were considerably higher. Outdoors, concentrations were almost alwayso1 mg m3 with the exception of toluene. The low outdoor levels reflect the suburban neighborhoods studied and the lack of strong sources. Precisions determined using replicate VOC samples at each house averaged 10%, comparable to that obtained in the longitudinal study. The apparent spatial variability at the 15 houses averaged 16%, just slightly exceeding precisions, once again indicating that the houses were well mixed. Excluding the three houses with the highest spatial COVs (#8, 12 and 14), the spatial COV dropped to 10%. Considering only the indoor tracer (OFT), the spatial COV was 26% (precision was 10%). Excluding the same three houses, the average spatial COV for OFT dropped to 19%. These three houses had several characteristics that increased the spatial differences: #8 was the only house with steam radiators; #12 had the heating/cooling vents and door closed in an upstairs office where a VOC sample was taken; and #14 was built on a hill with the family room on a lower floor than the living room, an unusual arrangement. For other houses, the mixing assumption necessary to derive AERs was satisfied. Because our intent was to

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Table 5 VOC levels in the house, outdoors, and in the garage VOC

Chloroform 1,1,1-Trichloroethane Benzene Carbontetrachloride n-Heptane Methyl cyclohexane Toluene n-Octane Tetrachloroethene Ethylbenzene m, p-Xylene Styrene o-Xylene n-Nonane Isopropylbenzene a-Pinene n-Propylbenzene 4-Ethyl toluene 1,3,5-Trimethylbenzene 2-Ethyl toluene 1,2,4-Trimethylbenzene n-Decane 1,2,3-Trimethyl benzene p-Isopropyltoluene d-Limonene n-Butylbenzene n-Undecane Naphthalene n-Dodecane n-Tridecane n-Tetradecane n-Pentadecane n-Hexadecane n-Heptadecane

Indoor concentration (mg m3)

Outdoor concentration (mg m3)

Garage concentration (mg m3)

DF (%)

Mean

S. Dev.

Max

DF (%)

Mean

S. Dev.

Max

DF (%)

Mean

S. Dev.

Max

80 53 100 100 93 100 100 93 73 100 100 100 100 93 100 100 100 100 100 100 100 100 100 100 100 87 93 100 93 87 87 87 87 87

0.3 5.4 2.0 1.4 2.8 0.7 26.5 0.9 0.6 2.3 8.3 1.0 2.9 1.3 0.2 14.1 0.7 4.6 1.1 0.9 3.7 2.5 1.0 1.8 25.6 0.1 1.9 8.3 0.7 0.8 1.9 0.9 0.6 0.4

0.3 19.4 1.9 1.2 2.2 0.5 22.4 0.5 1.2 1.7 6.9 1.9 2.4 1.7 0.2 24.2 0.8 4.8 1.2 0.9 4.1 3.1 0.8 3.0 25.5 0.1 1.9 23.7 0.7 1.1 3.0 0.5 0.3 0.3

0.9 75.4 7.6 5.6 8.2 1.9 87.0 1.7 4.4 5.2 21.3 7.3 7.9 6.4 0.5 93.7 2.9 18.9 4.3 3.4 15.9 11.4 2.7 12.3 93.0 0.3 6.3 91.7 2.7 4.5 11.7 1.8 1.1 1.3

0 0 100 100 0 13 100 0 0 100 100 0 100 13 25 88 75 88 88 88 88 13 75 25 25 0 13 0 0 0 0 0 0 0

0.0 0.0 0.40 0.9 0.0 0.0 1.2 0.0 0.0 0.2 0.7 0.0 0.2 0.1 0.0 0.3 0.0 0.3 0.1 0.1 0.3 0.1 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.1 0.2 0.0 0.0 0.5 0.0 0.0 0.1 0.3 0.0 0.1 0.2 0.0 0.2 0.0 0.1 0.1 0.1 0.1 0.3 0.1 0.0 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.6 1.4 0.0 0.1 1.8 0.0 0.0 0.4 1.0 0.0 0.3 0.6 0.0 0.5 0.1 0.4 0.2 0.2 0.4 0.7 0.2 0.0 0.3 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0 40 100 100 100 100 100 100 33 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 80 53 53 47 40 40

0.0 1.3 36.6 1.3 18.8 6.7 214.3 6.3 0.3 28.0 114.0 0.6 38.0 3.6 1.6 6.8 8.1 43.2 12.4 9.6 44.0 3.9 10.3 0.8 6.5 1.6 2.7 8.9 1.2 1.5 1.0 0.3 0.2 0.1

0.0 2.0 38.5 0.5 15.3 4.9 180.3 6.4 0.5 23.7 97.0 0.8 32.7 2.5 1.3 10.7 6.7 35.9 10.3 7.8 37.6 4.1 8.8 0.5 5.2 1.7 3.6 8.7 1.7 4.4 2.0 0.4 0.2 0.1

0.0 5.0 159.3 2.6 57.2 18.0 729.1 20.8 1.6 91.1 371.1 2.8 123.9 8.9 4.5 41.0 24.3 131.3 38.3 28.6 138.9 15.0 32.5 1.6 16.9 6.6 13.6 34.4 6.8 17.2 7.6 1.3 0.6 0.3

DF ¼ detection frequency.

study occupied houses, we removed the anomalous measurement at house #12 (taken in the closed-off office) from the house average used in subsequent calculations. Garage/indoor concentration ratios: Fig. 1 shows the median and IQR of garage/indoor (G/I) concentration ratios calculated across the 14 houses with attached garages. G/I ratios spanned three orders of magnitudes, e.g., the median ratio ranged from 0.027 for OFT (the indoor tracer) to 23 for HFB (the garage tracer). G/I ratios provide information related to the location of emission sources:

 

G/I ratios o1 suggest sources in the house, e.g., limonene used in many cleaning products, soaps, and shampoos. G/I ratios near 1 might arise if sources in the house and garage generated similar concentrations (possible but unlikely) or if the VOC is due exclusively to outdoor sources and negligible losses occur from reaction, sorption, etc. The latter applies to carbon tetrachloride, a stable and globally distributed VOC, for which the median ratio was almost exactly 1.



G/I ratios 41 signify garage sources. Ratios will increase with strong sources in the garage and few, if any, sources in the house. G/I ratios for aromatics, e.g., benzene and xylene, exceeded 10, showing the dominance of garage sources.

The G/I ratios for benzene (median ¼ 16.6) and HFB (median ¼ 22.6) were expected to be equal since the only known sources were in garages. With one exception, G/I ratios of benzene and HFB were closely tracked, unlike limonene, for example, which originated primarily from sources elsewhere (Fig. 2). The exception, house #13, contained smokers and G/I ratios for benzene (1.8) and HFB (19.2) diverged. HFB and benzene G/I ratios should have been identical if outdoor levels were small, garage-tohouse transfers for both compounds were identical (likely if both zones are well-mixed), sinks that remove VOCs were negligible, and experimental errors were small. The discrepancy in G/I ratios is explained by the outdoor concentrations of benzene. After subtracting the outdoor

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Table 6 Concentrations of selected VOCs and CO2 in houses and garages (4-day average), air exchange rates, and interzonal flows (as percent of total exchange) for each house/garage Site measurement

House/Garage Number 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

House concentrations: VOCs (mg m3) and Chloroform 0.1 0.7 1,1,1-Trichloroethane 0.5 0.0 Benzene 2.5 1.0 Carbontetrachloride 0.6 0.9 Toluene 34.3 10.4 n-Octane 1.1 0.6 Tetrachloroethene 0.1 0.0 Ethylbenzene 2.4 0.8 m, p-Xylene 9.1 3.2 Styrene 0.7 0.1 o-Xylene 3.1 1.0 n-Nonane 0.5 0.5 a-Pinene 4.6 1.3 1,3,5-Trimethylbenzene 0.8 0.3 2-Ethyl toluene 0.6 0.3 1,2,4-Trimethylbenzene 2.6 1.2 n-Decane 0.9 0.6 1,2,3-Trimethyl benzene 0.7 0.3 d-Limonene 51.2 15.1 n-Undecane 1.1 0.5 Naphthalene 0.6 0.5 n-Dodecane 0.7 0.3 n-Tridecane 0.6 0.3 n-Tetradecane 1.2 0.3 n-Pentadecane 1.1 0.3 CO2 (ppm) 752 530

CO2 (ppm) 0.1 0.2 1.1 0.0 1.0 3.7 0.7 0.9 5.9 28.4 0.5 0.9 0.0 0.7 0.5 3.6 1.5 14.2 0.2 0.2 0.5 4.5 0.5 0.5 1.1 2.2 0.2 0.9 0.2 0.7 0.9 2.9 0.9 0.5 0.2 0.7 2.2 10.6 1.2 0.4 0.4 0.6 0.4 0.3 0.3 0.3 0.6 1.0 0.6 0.8 NA 630

0.8 0.9 4.2 1.8 56.0 1.3 2.3 5.2 20.3 0.2 7.3 1.4 5.5 2.9 2.3 9.3 2.0 2.5 31.6 1.5 1.4 0.5 0.5 1.1 1.0 NA

0.2 1.4 0.6 1.7 14.7 0.6 0.1 0.5 1.7 0.2 0.6 0.9 3.2 0.4 0.2 0.9 1.6 0.3 64.0 1.5 91.7 0.5 0.5 1.2 1.1 597

0.0 0.0 1.2 0.9 20.5 1.7 4.4 4.1 15.5 1.0 4.9 1.1 21.2 2.0 1.4 5.7 2.0 1.5 14.9 1.9 1.2 1.2 1.0 1.0 1.0 787

0.2 0.1 0.6 0.9 3.0 0.0 0.0 0.2 0.9 0.1 0.3 0.4 1.2 0.3 0.3 0.9 1.7 0.4 2.2 1.5 0.1 0.7 0.0 0.0 0.0 479

0.0 0.0 1.3 1.0 29.4 0.8 0.3 2.1 6.8 7.3 2.3 2.6 93.7 0.9 0.6 2.5 4.9 0.8 18.8 6.3 1.1 2.7 4.5 5.1 1.6 556

0.3 0.0 1.2 1.3 87.0 0.8 0.2 1.1 3.8 0.3 1.1 6.4 3.3 0.4 0.4 1.7 11.4 0.6 12.9 4.5 21.5 0.6 1.6 11.7 1.8 537

0.6 0.0 7.6 1.3 41.2 1.0 0.3 4.8 21.3 0.4 7.9 0.0 2.5 1.5 1.2 5.5 0.7 1.2 19.9 0.6 1.6 0.5 0.6 0.7 0.7 625

0.1 0.0 1.1 1.2 5.7 0.3 0.0 0.9 2.9 0.0 1.0 0.3 1.2 0.3 0.2 1.0 0.3 0.2 4.5 0.0 0.3 0.0 0.0 0.0 0.0 508

0.7 75.4 0.7 5.6 26.4 1.6 0.9 2.6 9.1 2.9 3.0 3.1 19.4 1.1 0.9 3.0 7.5 1.4 93.0 5.3 0.6 1.6 0.8 1.3 1.0 NA

0.0 0.5 0.8 1.3 7.8 0.5 0.0 1.7 3.3 0.9 1.3 0.3 16.7 0.3 0.3 1.5 0.9 0.7 12.6 0.7 1.4 0.3 0.7 1.2 0.7 692

0.9 1.0 2.0 1.2 26.1 1.4 0.3 3.7 11.8 0.9 4.2 0.8 34.7 4.3 3.4 15.9 1.1 2.7 30.9 1.0 1.3 0.8 0.8 1.6 1.6 694

Garage concentrations: VOCs (mg m3) Chloroform 0.0 0.0 1,1,1-Trichloroethane 4.3 0.5 Benzene 19.0 22.4 Carbontetrachloride 1.5 1.4 Toluene 114.8 121.2 n-Octane 3.0 3.7 Tetrachloroethene 0.3 0.0 Ethylbenzene 15.3 18.5 M, p-Xylene 65.8 77.7 Styrene 0.1 2.8 o-Xylene 22.0 24.9 n-Nonane 1.6 6.7 a-Pinene 0.9 0.9 1,3,5-Trimethylbenzene 5.5 7.2 2-Ethyl toluene 4.5 5.5 1,2,4-Trimethylbenzene 19.7 27.0 n-Decane 1.3 11.3 1,2,3-Trimethylbenzene 4.4 5.9 d-Limonene 7.9 14.4 n-Undecane 0.7 8.2 Naphthalene 3.4 9.5 n-Dodecane 0.3 0.8 n-Tridecane 0.0 0.6 n-Tetradecane 0.0 0.4 n-Pentadecane 0.0 0.0

0.0 0.0 20.5 1.0 70.4 0.8 0.0 5.3 19.2 0.0 6.6 0.8 0.6 2.7 2.3 9.7 0.8 2.2 0.2 0.3 2.0 0.0 0.0 0.0 0.0

0.0 0.0 27.2 1.1 351.9 5.1 0.1 40.2 159.5 0.3 57.5 4.5 0.8 21.8 18.4 75.1 4.7 19.7 16.9 1.9 12.2 0.3 0.0 0.0 0.0

0.0 0.0 159.3 1.3 729.1 20.8 0.0 91.1 371.1 0.4 123.9 8.9 8.2 38.3 28.6 138.9 5.6 32.5 1.8 3.7 34.4 2.2 1.8 1.2 0.4

0.0 1.0 24.0 2.1 170.0 19.9 1.6 46.1 169.6 2.0 64.0 6.2 41.0 20.5 16.2 72.9 4.0 17.6 12.6 2.3 10.9 6.8 17.2 7.6 1.3

0.0 5.0 29.4 2.6 120.6 2.6 0.0 11.4 43.7 0.6 13.4 1.5 2.1 8.4 6.8 28.1 2.1 6.5 11.0 1.1 2.7 0.0 0.0 0.0 0.0

0.0 0.0 8.1 1.1 121.6 1.2 0.0 8.8 37.3 0.2 12.3 0.6 17.0 3.6 2.8 12.6 1.1 2.9 4.9 0.8 3.2 0.5 0.6 1.3 0.4

0.0 0.0 28.4 1.1 115.7 2.9 0.0 6.0 23.1 0.1 7.7 1.5 1.7 4.5 3.4 14.7 1.9 3.6 5.0 1.0 6.9 0.6 0.3 3.0 0.6

0.0 0.0 82.5 0.9 409.5 12.9 0.0 51.5 222.6 0.4 71.4 4.5 8.4 25.1 19.0 93.3 2.8 19.9 7.2 1.7 15.8 0.9 0.6 0.4 0.3

0.0 0.0 39.3 1.0 165.1 6.0 0.0 24.5 106.0 0.1 33.5 2.7 0.7 10.3 7.3 35.8 1.5 7.9 1.2 0.7 6.3 0.4 0.0 0.0 0.0

0.0 4.8 1.4 1.3 16.8 0.9 0.8 4.0 15.0 0.2 4.3 6.3 1.8 1.9 1.6 5.2 15.0 2.4 2.8 13.6 0.5 2.3 0.0 0.0 0.0

0.0 4.0 41.0 1.2 275.0 6.0 0.0 40.7 169.1 0.6 57.0 2.7 5.9 17.3 13.1 64.0 1.9 14.5 2.7 1.0 17.7 1.1 0.8 1.0 0.4

0.0 0.0 26.4 0.9 315.3 5.1 0.0 39.1 159.2 0.4 47.0 4.4 10.9 12.1 8.6 40.3 3.7 8.8 7.1 2.6 4.8 1.6 0.7 0.6 0.4

Air exchange rates and flows House AER (h1) 0.21 Garage AER (h1) 0.80 Garage-house flow (%) 12.6 House-garage flow (%) 6.0

0.54 0.30 4.1 15.1

0.42 1.11 3.9 1.2

0.0 0.0 20.0 0.6 117.5 4.1 1.0 17.9 70.9 0.6 24.0 1.7 0.8 6.4 5.4 23.0 0.9 5.1 1.5 0.4 3.5 0.0 0.0 0.0 0.0 0.37 0.58 11.0 0.7

House #6 had detached garage and no interzonal flows.

0.15 1.80 18.5 0.0

0.34 0.49 — —

0.41 0.73 2.6 22.3

1.65 0.38 1.6 0.0

0.29 0.33 12.4 2.9

0.50 1.54 3.1 4.0

0.26 0.83 8.9 2.4

0.67 0.42 1.8 4.2

0.11 1.56 5.2 0.2

0.35 0.16 0.8 7.5

0.19 0.58 4.8 2.5

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Fig. 1. Garage/indoor concentration ratios for VOCs. Median and interquartile range for 14 homes. TMB is trimethylbenzene.

1000 Benzene

G/I Ratio (Benzene or Limonene) .

Limonene (R)-(+) 100

10

 1

 0.1 1

10

100

1000

G/I Ratio of HFB Fig. 2. Garage to indoor (G/I) ratios of benzene and limonene versus HFB at all houses except #6.

benzene levels (0.470.1 mg m3) from both house and garage measurements, the benzene (median ¼ 23.0) and HFB ratios were essentially identical. This shows that PFT tracers released in the garage and measured in both garages and attached houses accurately reflect garage-to-house flows and, for contaminants released exclusively in the garage, are good predictors of concentrations. Also, after adjustment for ambient concentrations, benzene can serve as a tracer of garage-to-house migration in houses without benzene sources, e.g., those houses without smokers. VOC sources: Based on VOC composition, identified sources, correlations and G/I ratios, the following VOCs and sources groups were identified:



Gasoline-related aromatic compounds (e.g., benzene, toluene, xylene, 1,2,4-trimethylbenzene, naphthalene, but not styrene) had much higher concentrations in garages than houses. These compounds are major gasoline constituents and have been found in garages

 

 

and associated with vehicle emissions (Graham et al., 2004). Similarly, n-alkanes up to C9 (heptane, octane, nonane) were elevated in all but one garage (#13), again most likely due to gasoline sources. VOC compositions in garages more closely matched profiles of evaporative gasoline than VOC exhaust (Batterman et al., 2006), suggesting the importance of emissions from gasolinepowered equipment (vehicles, lawnmowers, chainsaws, etc.) and gasoline storage containers in garages. Chloroform was detected in houses, largely due to water use activities such as showering and clothes washing (Nuckols et al., 2005). This compound is unlikely to be found in garages or outdoors (Agency for Toxic Substances and Disease Registry (ATSDR), 2004). Carbon tetrachloride, now banned from all but limited industrial applications, was detected at similar concentrations in houses in garages. This VOC, which was previously was used in aerosol cans, pesticides, fire extinguishers, and as a degreasing agent (ATSDR, 2004), is found at background levels of 0.8 mg m3. Tetrachloroethene, a dry cleaning residue, averaged 0.671.2 mg m3 in houses and about half that in garages. Terpenes (e.g., pinene and limonene), present in many cleaning products as well as tobacco smoke and flavor applications, had relatively high concentrations in houses, and usually lower (in 11 of 15 cases) but still elevated levels in garages. n-alkanes from C10 to C13, were elevated in most garages and possibly represent evaporated fuel and oils. n-alkanes from C14 to C17, were elevated in most houses and possibly reflect off-gassing from paints and varnishes.

AERs and interzonal flows: The effective AERs based on PFT measurements averaged 0.4370.37 h1 with a wide distribution across the 15 houses (Fig. 3). AERs incorporating recirculation were only slightly (o2%) larger than AERs without recirculation (results not shown) since interzonal flows were small. Where windows were reported to be opened at least some of the time, AERs increased slightly (average of 0.3870.11 versus 0.3070.20 h1 in

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houses with closed windows), but this difference was not statistically significant. AERs were negatively correlated with the 4-day minimum temperatures (0.28). Earlier studies have shown larger effects of window openings (Wallace et al., 2002), lower AERs for houses built in the 1970s, and negative correlation between AER and house volume (Oie et al., 1998). AERs derived from CO2 concentration decay curves averaged 0.6570.37 h1 (n ¼ 12, CO2 measurements unavailable at houses 3, 5, and 13). These values were reasonably well correlated with the effective AERs (r ¼ 0.74) though 76% higher. AERs derived from CO2 and PFT measurements differ for several reasons. First, CO2-derived AERs utilized only a few hours of data, typically when people leave the house, that may be affected by door openings and other factors associated with departures, while the effective AERs are 4-day measurements that include periods when houses may be unoccupied, closed, and HVAC systems inoperative. Second, when AERs are time-varying, a time-weighted AER (e.g., derived using a sequence of short-term measurements) will exceed the effective AER (Batterman et al., 2006). Additional reasons might include time varying CO2 emission rates (specifically, elevated emissions associated with occupant departures), estimation uncertainty (e.g., multiple solutions can give similar fits), incomplete mixing and/or differential distribution of CO2 and PFTs, and measurement errors. Finally, CO2 concentrations in the houses were not particularly high, i.e., averages ranged from 480 to 790 ppm and peaks from 580 to 900 ppm, and it was sometimes difficult to find periods with smooth concentration decays.

100.0

AER (hr-1) or Fraction (%)

Garage-to-House fraction (%) House-to-Garage Fraction (%) Garage AER (hr-1) House AER (hr-1)

10.0

1.0

0.1 0

10

20

30

40

50 60 Percentile

70

80

90

100

Fig. 3. Distributions of garage-to-house and house-to-garage fractions, and garage and house AERs (n ¼ 15).

235

AERs depend on temperature, wind speed, building and ventilation system design and operation, degree of sheltering, window and door openings, and other factors (Oie et al., 1998; Johnson et al., 2004). Previously, Murray and Burmaster (1995) reported annual average AERs of 0.55746 and 0.6570.57 h1 in regions just north and south of the study areas, with AERs lower by 20% in the spring season; Pandian et al. (1993) estimated an annual average AER of 0.60 h1 and seasonal AERs of 5.4, 0.4, and 0.5 h1 in summer, fall, and winter, respectively, in northeast US homes. The houses sampled here appear slightly ‘‘tighter,’’ possibly because the literature values are 410 years old, most Michigan houses are now well insulated, including the installation of replacement windows, and because the generally moderate temperatures and wind speeds during the study period would have minimize stack effects and infiltration. The present study was not designed to investigate factors affecting airtightness, rather to investigate VOC levels and sources. AERs in garages: The effective AERs in garages averaged 0.7770.51 h1, significantly higher than in houses (2 sample t-test p ¼ 0.02; paired t-test p ¼ 0.09), as expected given the rapid ventilation expected when garage doors are opened and the generally loose seals on garage doors. However, AERs in four houses (#2, 8, 12, and 14) were lower than those in the corresponding attached garage. In these cases, garage AERs were very low (0.16–0.42 h1), and in three of the cases (not #12) windows in the house were regularly opened. As in houses, AERs in garages were negatively correlated with the minimum 4-day temperatures (r ¼ 0.22), as well as positively correlated with garage–outdoor temperature differences (r ¼ 0.60). The literature regarding AERs in garages is scarce. Four earlier studies suggest that reasonable estimates of AERs in garages are roughly 2–10 times higher than AERs in the house (Batterman et al., 2006). Like houses, AERs in garages will be affected by meteorology, garage design, operation, maintenance, and measurement method. Based on our limited sample, AERs in garages show a large range and are likely affected by many factors that defy simple generalizations. Interzonal flows: Air flows from the garage to the house averaged 9.375.7 m3 h1 (equivalent to 6.575.3% of the house’s total air exchange). The reverse (house-togarage) flows were smaller, 2.672.2 m3 h1 (4.976.4% of the garage’s total air exchange). In two houses (#5, 8), no house-to-garage flows could be detected, and these flows were very small (o1 m3 h1) in three other houses (#4, 9, 13; Fig. 3). Garage-to-house air flows, expressed as a percentage of the houses air exchange, tended to increase in houses that had lower AERs (r ¼ 0.47) and higher volumes (r ¼ 0.34), thus, both larger and ‘tighter’ homes draw a larger fraction of air from the attached garage. The garageto-house percentage was also correlated with the garage’s AER (r ¼ 0.37). Multiple regression models using house volume, house AER and garage AER as explanatory

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236

variables maintained these trends and explained 67% of the variation in the garage-to-house air flow fraction; however, estimated coefficients were not statistically significant, probably due to the small sample size and many potential and unmeasured cofactors. Possibly, ‘leakier’ garages might either more easily permit winds to enter and/or be closer to atmospheric pressure, thus increasing the garageto-house pressure differential that drives contaminant migration (assuming the house is below atmospheric pressure), and ‘tighter’ and larger houses might obtain a larger proportion of ventilation air from attached garages than leakier and smaller houses. We raise but cannot confirm these possibilities because our sample size was small, airtightness was not measured, and meteorological and/or other factors may have confounded results. Further investigation is suggested, especially since concentrations from garage and internal sources could be higher in ‘tighter’ houses, as discussed below. Very few measurements of garage-to-house flows were identified in the literature. Fugler et al. (2002) studied 25 houses with attached garages and found that housegarage interface was equal in leakiness to the rest of the house envelope. Sherman and Chan (2004) review two Belgian studies that also show significant leakage through the garage–house interface and discuss factors affecting airtightness. Relationship between CO2, VOCs and ventilation: VOC concentrations attributable to indoor sources should be inversely proportional to the AER. Fig. 4 shows this relationship for the study houses and for four pollutants with best-fit regression lines (shown as curves on the loglinear plot). Only approximate agreement is expected since houses varied in size and source strength. Still, the inverse relationship is approximately satisfied for CO2 and

limonene, which largely arise from sources within the house. However, this relationship does not apply to benzene, which is associated primarily with garage sources, or to toluene, which arises from both garage and houses sources. Additional information is needed to explain concentrations of these VOCs. While source control may be the most important strategy to limit concentrations of VOCs and other pollutants, the role of (whole house) ventilation, which ‘‘dilutes the unavoidable contaminant emissions from people, from materials, and from background processes’’ (American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc (ASHRAE), 2004) should not be overlooked, and minimum standards apply to low-rise residential (and other) buildings that depend on floor area and the number of bedrooms. For a 200 m2 four bedroom house, the ASHRAE minimum is 35 L s1, equivalent to an AER of 0.2 h1. This AER was achieved 13 of the 15 houses tested. Apportionment of VOC sources: Concentrations in the houses were apportioned to emission sources in the garage, house and outdoors, as displayed in Fig. 5 using the VOC sequence shown earlier for G/I ratios (Fig. 1). VOCs can be classified by source as follows:





1000



Concentration

100



10

1

CO2 (ppm) Limonene (ug/m3)

TVOC (ug/m3) Benzene (ug/m3)

0.1 0

1

2

3

4

5 6 1/AER (hr)

7

8

9

10

Fig. 4. CO2, TVOC, limonene, and benzene concentrations versus inverse AERs (n ¼ 15 houses).

Outdoor sources: Carbon tetrachloride was the only VOC due primarily to outdoor sources, which contributed 81725% of indoor concentrations. Contributions from sources in houses (17722%) and garages (275%) were small, and possibly negligible. House sources: VOCs attributed nearly entirely to house sources included limonene, C12–C17 alkanes, and isopropyltoluene. Pinene also was primarily due to indoor sources (85715%); outdoor sources (11712%) and garage sources (477%) made small (possibly negligible) contributions. These VOCs had small G/I ratios (median p1). Garage sources: This group included trimethylbenzene, xylene, ethyl toluene, cyclohexane, isopropyltoluene, and benzene. For most of these VOCs, garage sources contributed above 50%, house sources below 30%, and outdoor sources below 10%, though benzene showed larger outdoor contributions (25712%). These VOCs had high G/I ratios (median X10). Mixtures of primarily garage and house sources: This included C8–C10 alkanes, naphthalene, toluene, and surprisingly, 1,2,3-trimethylbenzene. The variability for this last VOC was very high, thus it is likely associated with gasoline sources in the garage.

The analysis shows that garage-related emissions are the dominant source of benzene and several other VOCs in the study homes. Estimated benzene emissions in the garages averaged 3.174.2 mg h1 (maximum of 16 mg h1 in garage 11). For contrast, toluene emissions averaged 19724 mg h1 in garages (maximum of 81 mg h1 also in garage 11) and 1.772.8 mg h1 in houses (maximum of

ARTICLE IN PRESS 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0 100 90 80 70 60 50 40 30 20 10 0

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Outside

Garage

House

n-Heptadecane n-Pentadecane Limonene n-Hexadecane n-Tetradecane a-Pinene n-Tridecane Isopropyltoluene Styrene Carbontet n-Dodecane n-Undecane n-Decane n-Nonane n-Octane Methyl_cyclohexane Isopropylbenzene Naphthalene 1,2,3 TMB Toluene 2-Ethyl toluene 4-Ethyl toluene 1,2,4 TMB o-Xylene 1,3,5 TMB n-Propylbenzene Ethylbenzene m, p-Xylene Benzene

Apportionment (%)

Apportionment (%)

Apportionment (%)

S. Batterman et al. / Environmental Research 104 (2007) 224–240

Fig. 5. Apportionments of VOC concentrations in houses to outside, garage, and house emission sources. Order of VOCs ranked by G/I ratio as shown in Fig. 1. Values represent median, 25th, and 75th percentile. Derived using 2-zone model (n ¼ 15 houses).

11 mg h1 in house 10), while limonene emissions averaged 0.4970.75 mg h1 in garages and 2.771.6 mg h1 in the houses. Based on the sum of detected VOCs, emission rates from garage sources (62777 mg h1) greatly exceeded rates from house sources (12717 mg h1). In all of these estimates, the large variability reflects the range across the 15 houses and garages (though estimation uncertainty can also be significant, as discussed later). The apportionments demonstrate the significance of contaminant migration from attached garages. Migration occurs intermittently when passage doors are opened, and continuously due to gaps in the garage–house interface. While our results cannot separate these pathways, we speculate that penetrations in the garage–house interface may be significant, especially since passage doors are opened only momentarily. While interzonal flows are not large, garages often contain high emitting sources, e.g., fuel evaporating from lawn mowers and cars. While ventilation standards recognize the importance of attached garages, applicable standards in building codes are fairly minimal, e.g., ‘‘migration of contaminants should be prevented by gasketed or substantially airtight’’ (using weather stripping) doors between garages and occupied

spaces (ASHRAE, 2004). Of course, houses contain many other VOC sources (Brown et al., 1994; Hoddinott and Lee, 2000; Van Winkle and Scheff, 2001), but this study shows that the presence of an attached garage is one of the more important factors affecting residential air quality for VOCs. Apportionment of exposures: Since most individuals spend more time at home than in any other ‘‘microenvironment’’ (US EPA, 1997), exposure in homes may contribute the largest portion of the cumulative VOC dose. Table 7 estimates benzene doses using typical concentrations, activity durations, and breathing rates that apply to non-occupationally exposed and non-smoking adults. Urban monitoring networks show benzene concentrations of 1–2 mg m3 (MDEQ, 2005); For commuting in light and heavy traffic, we use 2 and 4.5 mg m3, respectively, typical values in the Detroit area (Batterman et al., 2002a, b). The outdoor (non-commuting) exposures uses the nationwide ‘‘background’’ estimate of benzene concentration, 0.48 mg m3 (US EPA (US Environmental Protection Agency), 1996), though our study community had slightly lower levels, 0.40 mg m3. Concentrations in homes use the average from this study. Concentrations in garages use

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Table 7 Estimated benzene dose for non-smoking adults due to exposures in six microenvironments Microenvironment

Activity duration Median (min)

Benzene concen (mg m3)

Dose (mg day1)

0.5 0.5 1.6 1.0 1.0 1.0

2.0 36.6 0.5 2.0 4.5 0.5

13.5 16.0 1.2 1.5 1.3 3.4

36.6 43.5 3.2 4.0 3.6 9.2

17.7

2.7

36.9

100.0

Inhalation rates (m3 h1) Adj. Median (min)

Home Garage Outdoors Vehicle Heavy traffic Office/factory

985 60 105 70 20 485

810 53 92 44 18 425

Sum per day

1725

1440

Dose fraction (%)

Reference

15–315 15–116 15–32 15–33 15–44 15–36

Adjusted median duration is normalized to 1440 min per day. Inhalation rates from Table 5–23 in US EPA 1997. Reference gives table in US EPA, 1997 for median duration.

one-half of the average in this study, reduced since garage doors are likely to be open at least some of the time when individuals are in garages. Office/workplace exposures use 1 mg m3, between background and urban levels. Breathing rates and activity levels for each microenvironment use recommended values (US EPA, 1997). Time budgets use median values from the same source. All of these parameters are approximate. With the selected parameters and activities, benzene sources in garages (including migration to the house) constitute the major share (70%) of the total daily dose. Because outdoor and commuting times are short, the corresponding exposures are small (o10% of the total). We were surprised that the median time spent in garages was so high (60 min d1); this is based on a small sample (n ¼ 193) and may not be representative. Even if an individual spends little time in a garage, houses will remain a significant and generally the dominant microenvironment for benzene exposure. While the apportionments have some uncertainties (see below) and a high degree of variability across the housing stock is expected, these results confirm the importance of garage sources. Such exposure is widespread given that many North American houses have attached garages. We could not identify the number or fraction of homes with attached garages, but we note that 59% of US households live in single-family houses, and that 55% of single-family houses and mobile homes have garages or carports (US DOE (US Department of Energy), 2001). Highly exposed individuals are of greatest concern, i.e., those living in houses that have high emissions in an attached garage, a high migration rate to the house, and a low house AER. House #13 in our sample may fit into this category: it had the highest benzene concentration in the house (8 mg m3), which resulted from the second highest garage concentration (83 mg m3), a low house AER (0.26 h1), and a high house-garage flow fraction (9%). Nearly all (95%) of the benzene in this house was apportioned to garage sources. Study strengths and limitations: This is one of the first studies to use multigas tracer techniques to estimate air flows in houses and attached garages, along with simultaneous

VOC measurements. The derived ‘‘effective’’ AERs, interzonal flows, apportionments and emission rates account for time-varying flows, and measurements appeared robust and reliable. A wide set of VOCs were detected, mixing assumptions were confirmed using multiple sites in each house, and measurement precisions were excellent. Limitations include a small sample in a single season collected in one geographic area, which limits the ability to generalize results. In particular, we expect that importance of air and contaminant flows from garages is likely to depend on climate, construction methods, and other factors. VOC concentrations in garages spanned a large range, and additional variability is expected for AERs and interzonal flows. No pollutants were monitored other than VOCs and CO2. Errors may have been introduced by the use of estimated house volumes, and by the assumptions of steady-state conditions and complete mixing. Additionally, most of the houses tested had basements, and preliminary observations suggest that basement air was only partially mixed with the remainder of air in the house, e.g., tracer gas concentrations were generally lower in the basement. Because we did not include the basement area in the calculations, AERs for the houses may be biased; however, the calculated air migration rates and the exchange fractions are unaffected. Because basements appear at least somewhat decoupled from the remainder of the house, their AERs and migration rates would be best estimated as a separate zone in the house. This was not attempted in the present study which prioritized the living area of the houses. The apportionments depend on VOC measurements in house, garage and outside air, as well as AERs and interzonal flows and, in cases, derived apportionments were sensitive to these variables and small negative estimates were obtained (set to zero). However, median and interquartile range are robust statistics. The use of house-specific outdoor measurements might reduce apportionment uncertainties. Since assumed or averaged parameter values were used, the dose apportionment is approximate. The benzene apportionment also excludes events that can produce high concentrations, e.g., cigarette

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smoke exposure and vehicle refueling, which might be included in a more refined analysis. 4. Conclusions Measured using a dual tracer system, effective AERs across 15 houses averaged 0.4370.37 and 0.77751 h1 in homes and garages, respectively. These AERs represent 4day integrated averages. Short-term AER estimates derived using CO2 measurements yielded values 76% higher. Garage-to-house flows averaged 9.375.7 m3 h1, equal to 6.575.3% of the houses’ AERs. In 4-day samples, a total of 39 VOCs were detected in houses, 36 in garages, and 20 in ambient air. Garage/indoor ratios and a two-zone dilution/mixing model were used to apportion VOC emission sources. For benzene, concentrations in houses were nearly entirely due to the migration of contaminants from the garage, and exposures in houses and garages accounted for the major share of an individual’s cumulative dose. This study confirms several previous reports (e.g., Fugler et al., 2002; Emmerich et al., 2003; Batterman et al., 2005) suggesting that houses with attached garages have higher levels of VOCs, and the quantitative analysis shows that attached garages are the primary source of many compounds found in the occupied portion of residences. While house-to-house variability in AERs, interzonal flows, and VOC concentrations can be significant, an important finding is that tighter houses tend to have both higher garage-to-house flows and higher VOC levels due to emissions in the house and garage. Identifying pollutant sources is an initial step in controlling IAQ management, and this study shows the need to further investigate a broader set of houses and to identify households most at risk. Actions that can be taken for houses with attached garages to minimize VOC exposures include: eliminating or reducing VOC sources in garages (e.g., removing or sealing VOC sources, not idling or warming vehicles in the garage); sealing the garage–house interface (e.g., establishing quantitative targets for migration rates in building/ventilation standards); providing dilution ventilation in the garage (e.g., using natural or mechanical means); using exhaust ventilation for chemicals stored in the garage; and maintaining a positive pressure differential between the house and garage. Proposed recommendations to reduce benzene levels in conventional gasoline and emissions and spillage from gasoline containers will also reduce exposure (US EPA, 2006). Other actions have been suggested, e.g., ventilating garages at 250 CFM (118 l s1) for 15 min after a vehicle in the garage has been started or turned off, and the use of a continuously depressurized cavity between house and garage (ALA, 2004). However, strategies must consider that evaporative emissions require continuous controls, and that migration occurs through passage doors as well as hidden penetrations in the garage–house interface. Finally, the effectiveness of these and other mitigation measures should be evaluated.

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Acknowledgments The authors thank Sergei Chernyak, Chris Godwin, and Scott Roberts for their laboratory assistance and data management, and Simone Charles with her review. Financial support was provided by the American Chemistry Council (Grant 2401). Funding Sources. Financial support was provided by the American Chemistry Council (Grant 2401). Ethics. Recruitment procedures were approved by the University of Michigan’s Institutional Review Board, and included informed and written consent.

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