Attached Garages as a Source of Volatile Organic Compounds in New
Homes
Francis J. Offermann1, Alfred T. Hodgson2, Peggy L. Jenkins3, Ryan D. Johnson3 and
Thomas J. Phillips3
1
Indoor Environmental Engineering, San Francisco, CA, USA
Berkeley Analytical, Richmond, CA, USA
3
California Air Resources Board, Sacramento, CA, USA
2
*
Corresponding email: [email protected]
KEYWORDS
Air leakage, benzene, motor vehicle, residence, xylene
SUMMARY
A majority of new single-family residences in the USA are constructed with attached garages.
Many are two story structures with garages situated under living spaces. The air in garages
communicates with air in living spaces through passage doorways and unsealed structural
gaps and cracks. Consequently, benzene and other air pollutants originating from vehicles
parked in garages contaminate indoor air. This study examines home-to-garage leakage areas
and coupling and indoor concentrations of garage related VOCs for a sample of 105 new
California, USA homes. Homes with garages under living spaces had higher home-to-garage
coupling and higher emission rates of benzene and xylenes compared to homes with garages
to the side. A sub-study found that the typical impact ratio (i.e., I-O / G-O concentrations) of
garage pollutants on indoor air quality was 0.05 – 0.08. A comparison with previous studies
suggests that typical recent construction practices have not substantially reduced garagerelated impacts.
1 INTRODUCTION
A majority of single-family homes and a large fraction of multi-family homes in the USA
have attached garages. These garages often have high concentrations of volatile organic
compounds (VOCs), carbon monoxide and other combustion pollutants that frequently
impact indoor air quality (IAQ) in adjoining residences. These pollutants derive from the
evaporative and tailpipe emissions of gasoline powered vehicles and equipment stored in
garages. The health impacts are significant. Carbon monoxide emissions from idling vehicles
in garages lead to fatal and sub-lethal poisoning in the USA every year (CDC, 2011).
Benzene exposure from attached garages in three USA cities has been estimated to increase
cumulative exposures to ten times higher than those experienced while commuting in a car in
heavy traffic and to result in excess cancers of 17 per million persons (Hun et al., 2011).
A number of studies have been published on pollutant transport from garages into residences.
Studies up through 2001 have been reviewed by Emmerich et al. (2003). Most of the crosssectional studies have included structures of various ages and home/garage configurations.
Building code regulations now often include requirements that are intended to mitigate
garage impacts on IAQ and/or conserve energy, e.g., gasketing and self-closing mechanisms
on home-to-garage doors. Home configuration also is evolving. Due to generally reduced
parcel sizes, the majority of new homes in the USA now consist of two stories (NAHB, 2006)
frequently with the garage located below living spaces. The analysis reported here is taken
from the California New Homes Study (CNHS) conducted in 2006 – 2007 to assess
ventilation and IAQ in a sample of new, single-family, California (CA), USA homes
(Offermann, 2009). The detailed study data provides an opportunity to investigate home-togarage leakage characteristics and garage pollutant impacts representative of new residential
construction. Specifically, our objectives were to: 1) characterize home-to-garage leakage
areas, home-to-garage coupling factors, and indoor concentrations of garage related VOCs
for this sample of homes; 2) compare leakage and pollutant factors between homes with
garages under living areas and homes with garages to the side of living areas; and 3) discern
if new construction practices produce lower garage impacts in comparison with previously
reported studies.
2 MATERIALS/METHODS
In the CNHS, a total of 108 owner-occupied, single family homes, from 1.7 – 5.5 years old,
were recruited in Northern and Southern CA (Offermann, 2009). The concentrations of 22
volatile organic compounds (VOCs) including formaldehyde and acetaldehyde and ambient
air contaminants were measured simultaneously indoors and outdoors over a 24-hour period.
Indoor measurements were made in the main living/dining/kitchen area. Outdoor
measurements were made at one location for pairs of closely located homes. VOCs were
collected and analyzed by U.S. EPA Method TO-17. Chemicals of concern (COC) plus some
VOCs representative of known indoor sources were selected as targets. VOC sample volumes
were ~14 L yielding method detection limits (MDLs) of 0.1 – 0.4 µg/m3 for all but one VOC.
Sample values were blank corrected using the average of the field blank for each batch of
samples or 0.5 x MDLs, as appropriate.
In a subset of 24 homes, air samples were simultaneously collected over 24 h with flow
controlled, 6-L, evacuated stainless-steel canisters in the garage and adjacent to indoor and
outdoor sorbent tube locations (Jenkins et al., 2011). These samples were analyzed for 60
target VOCs by U.S. EPA Method TO-15. The MDLs for the canister analyses were similar
to those of the sorbent tube analyses. Garage IAQ impact ratios were calculated from the TO15 concentration data as the ratio of indoor-outdoor to garage-outdoor (I-O / G-O) values.
The outdoor air exchange rate was simultaneously measured using passive perfluorocarbon
tracer (PFT) sources and samplers. The PFT sources (PMCH) were placed at 3-5 locations in
each home ~1 week in advance of sampling to allow for their emission rates to equilibrate. A
passive PFT sorbent sampler was co-located with the indoor air sampling equipment. The 24h average outdoor air exchange rate (h-1) was calculated following ASTM E741.
Building envelope air leakage area was determined by depressurization using a multi-point,
blower door test following ASTM E779. The zone pressure diagnostic test of the garage-tohome connection consisted of two home depressurization tests; one with the home door to the
garage closed and one with the door open. From these data, the equivalent leakage area
(EqLA @10 Pa, cm2) was calculated between the garage and the home and between the
garage and outdoors. We also measured the home-to-garage “coupling factor” which is the
ratio of the garage-to-outdoor differential pressure to the home-to-outdoor differential
pressure at -50 Pa. A coupling factor of zero indicates no home-to-garage coupling and a
value of 1.0 indicates total coupling.
In the CNHS pilot study, the transport of garage air contaminants into the indoor air was
measured for three homes by introducing a second PFT (p-PDCH) during the 24-hour air
contaminant measurement period (Offermann, 2009). Two sources of this PFT were placed at
a central location in the garage. Their emission rates were temperature corrected by
calculation. The percent of garage air contaminant emissions entering the home was
determined from the ratio of the calculated emission rate of the garage PFT entering the home
to the calculated emission rate of the garage PFT. The emission rate of the garage PFT
entering the home was determined as the product of the concentration of this PFT in the home
and the outdoor airflow rate entering the home, i.e., the product of the outdoor air exchange
rate from the first PFT measurements and the indoor air volume of the home.
The measured parameters were not normally distributed as determined by a Shapiro-Wilk
test. A nonparametric Mann-Whitney rank sum (U) test was used to assess the differences
between median values for the garage under living vs. the garage to the side comparisons.
3 RESULTS
Characteristics and field measured parameters for study of 105 new homes
For this analysis, three homes were excluded because they lacked a garage (1 home) or had
incomplete measurements. For homes tested more than once, only the results of the first test
were considered. The 105 homes had from two to four bedrooms (median = 4) with floor
areas ranging from 119 to 470 m2 (median = 250 m2). Sixty-nine homes (66%) were twostory construction. All had attached garages. In 63 homes (60%, all two story), the garages
were situated below a living space (Garage Under). In the others, the garages were located on
the side (Garage Side). The occupant surveys indicated that 90+% of the garages were used
regularly for vehicle parking. Field observations confirmed that all garage/home passageways
were gasketed and had automatic door closers as required by the CA building code.
Distributions of the home outdoor air exchange rates, equivalent leakage areas (EqLAs, cm2)
between living spaces and garages, and garage-to-home coupling factors are presented in
Table 1. The median air change rate was 0.26 h-1. High rates measured in a few homes were
associated with open windows and doors.
Table 1. Distributions of outdoor air exchange rates; home-to-garage leakage areas and
coupling factors; and indoor minus outdoor (I-O) concentrations of benzene, toluene and
xylenes for 105 new homes
Parameter
Units
10%
25%
50%
75%
90%
Max
Air exchange rate
1/h
0.13
0.18
0.26
0.47
0.95
6.5
Leakage area*
cm2
45
70
112
170
280
620
Coupling factor*
-0.010
0.022
0.039
0.079
0.133
0.26
Benzene, I-O
µg/m3
<0.17
<0.18
0.33
1.46
3.3
19
3
Toluene, I-O
µg/m
1.6
3.7
6.9
15.0
40
114
3
Xylenes, I-O
µg/m
<0.3
1.7
3.3
8.8
22
79
*No. observations: leakage area, n=101; coupling factor, n=91, all others, n=105
Of the 60 compound TO-15 target list, just 39 VOCs were above their MDL in the garage
samples. The 10 most abundant compounds with median garage concentrations exceeding 10
µg/m3 were; ethanol (300 µg/m3), toluene (68 µg/m3), xylenes (55 µg/m3), acetone (50
µg/m3), 2-propanol (21 µg/m3), benzene (13 µg/m3), hexane (12 µg/m3), ethylbenzene (11
µg/m3), 2-butanone (11 µg/m3), and 1,2,4-trimethylbenzene (11 µg/m3). The hazard quotient
as calculated from the ratio of the median garage concentrations and their health exposure
guidelines (i.e., the OEHHA CRELs or if not available then 1% of the Cal/OSHA PELs) only
exceeded 0.01 for three compounds; benzene, toluene, and xylenes (BTX). As a result, the
analyses in this paper are focused upon BTX.
The correlations of co-located indoor measurements of BTX by Methods TO-17 (sorbent tube
sampling) and TO-15 (canister sampling) had correlation coefficients >0.93. On average, the
TO-15 concentrations were typically 15% higher than TO-17 values for benzene and xylenes
and 2% lower for toluene. For the co-located outdoor air samples, correlation coefficients
were <0.3 with TO-15 concentrations exceeding TO-17 values by as much as a factor of two.
Also, since this paper focuses on garages as an indoor source of air pollutants, the
concentration distributions of BTX are shown as indoor minus outdoor (I-O) values (Table
1). Median outdoor concentrations by TO-17 were: benzene <0.3 µg/m3 (max 2 µg/m3),
toluene 1.2 µg/m3 (max 6 µg/m3), and xylenes 1.2 µg/m3 (max 4 µg/m3).
The percentage of the garage air contaminant emissions entering the three CNHS pilot homes
as determined from the PFT sources installed in the garage, were 2.6% (Home P1), 10.1%
(Home P2) and 9.8% (Home P3). The corresponding home-to-garage coupling factors were
0.010, 0.012 and < 0.001. These coupling factors compare to the median of 0.039 observed in
the full study. The lower percentage of garage emissions entering home P1 may be attributed
to this home having just one garage wall adjacent to the home, whereas home P2 had one and
one-half walls and home P3 had two walls and the garage ceiling adjacent to the home.
Impact of garage location on home/garage parameters and indoor BTX emission rates
We looked for associations between garage location (Garage Under vs. Garage Side) and
home-to-garage EqLAs and coupling factors (Table 2). While there was a trend of higher
EqLAs for Garage Under homes, the median values were not significantly different at a 5%
probability of no difference (P = 0.059). However, the median coupling factor for Garage
Under homes was significantly higher (P < 0.001). We also looked for associations between
garage location and I-O BTX concentrations and BTX emission rates. An emission rate
(mg/h) was calculated as the product of the I-O concentration (mg/m3) and the home outdoor
air flow rate (m3/h) from the PFT measurement and home volume. There was no apparent
association with concentrations. But, Garage Under homes had significantly higher (P <0.05)
median emission rates of benzene and xylenes. The lack of an association for toluene
emissions is indicative of other indoor toluene sources.
Table 2. Comparisons of home-to-garage leakage areas and coupling factors and emission
rates of benzene, toluene and xylenes between new homes with attached garages under and to
the side of living spaces
Parameter
Units
Garage
n
25%
50%
75%
P*
Under
61
80
117
188
Leakage area
cm2
0.059
Side
40
68
90
137
Under
57
0.028
0.062
0.094
Coupling factor
-<0.001
Side
35
0.015
0.025
0.039
Under
63
0.059
0.129
0.410
Benzene ER
mg/h
0.009
Side
42
0.038
0.063
0.176
Under
63
0.92
1.54
2.2
Toluene ER
mg/h
0.454
Side
42
0.75
1.17
2.9
Under
63
0.48
0.99
2.0
Xylenes ER
mg/h
0.011
Side
42
0.24
0.56
1.04
*Probability of no significant difference between medians by Mann-Whitney rank sum test
Garage BTX concentrations and their impact on indoor concentrations
Three homes with incomplete data were excluded from the garage sub study. Garage minus
outdoor (G-O) BTX concentrations (range and median) for the 21 homes were: benzene 0.70
– 80 µg/m3 (median = 12.4 µg/m3); toluene 14.5 – 640 µg/m3 (median = 66 µg/m3); and
xylenes 5.1 – 570 µg/m3 (median = 53 µg/m3). The median G-O BTX concentrations were
from 10 – 37 times higher than their respective median I-O concentrations for the full study
(Table 1). Garage impact ratios (I-O / G-O) were calculated for benzene and xylenes (Table
3) to show the impact of garages on the indoor concentrations of these compounds. Toluene
was omitted due to the apparent presence of other sources. The median impact ratios were:
benzene 0.050 and xylenes 0.076. At the 90th percentile, the impact ratio was 0.250 for both
compounds. The median EqLA and the median coupling factor were significantly higher for
the Garage Under vs. the Garage Side homes in this subset of homes (Mann-Whitney rank
sum test, P <0.05, comparison not shown). However, there were no significant differences
with garage location for the benzene and xylenes garage impact ratios.
Table 3. Impact ratios of garage concentrations of benzene and xylenes on their respective
indoor concentrations for 21 new homes with simultaneous garage and indoor measurements
Parameter
10%
25%
50%
75%
90%
0.004
Benzene
0.016
0.050
0.105
0.250
0.002
Xylenes
0.013
0.076
0.142
0.250
Impact ratios calculated as I-O / G-O.
4 DISCUSSION
Comparison of BTX concentrations with exposure guidelines
We compared the I-O BTX concentrations for the full study to Chronic Reference Exposure
Levels (CRELs) for non-carcinogenic health effects (OEHHA, 2008). These guidelines are:
benzene 60 µg/m3, toluene 300 µg/m3, and xylenes 700 µg/m3. The median I-O
concentrations are <1 – 2% of the CRELs and the 90th percentile values are 3 – 13% of the
CRELs. The median G-O concentrations are 8 – 22% of the guidance values, and the
maximum G-O concentrations approach or exceed guidance. Even though the impact ratio of
the garage in a typical home is less than 0.10 (Table 3), the garage can significantly increase
an occupant’s inhaled dose of BTX. Following the Batterman et al. ( 2007) Table 7 example
and adding the median outdoor concentration to the I-O and G-O concentrations, the inhaled
dose of benzene associated with the garage (0.9 h exposure) somewhat exceeds the indoor
dose (13.5 h exposure); the garage doses of toluene and xylenes are below but within a factor
of two of their indoor doses. Cancer risk also was considered. In the larger study, 63% of the
homes exceeded the CA Proposition 65 No Significant Risk Level for benzene of 0.65 µg/ m3
(13 µg/20 m3/day) (Offermann, 2009).
Comparison with previous garage studies
Graham et al. (2004) measured home-to-garage leakage areas (EqLAs) for a sample of 25
Canadian homes ranging up to ~40 years old and with various home/garage configurations.
The average EqLA was 124 cm2. Sheltair (2004) made additional measurements in Canadian
homes and combined these with the 25 Graham et al. homes. The average EqLA for these 42
homes was 107 cm2. These average home-to-garage leakage areas compare to the median of
112 cm2 observed in this study of new homes. Batterman et al. (2007) calculated
garage/indoor (G/I) concentration ratios using average values for a sample of 15, new to 70year old, homes in Michigan, USA. When converted to impact ratios used in this paper (i.e.,
I/G), the values are benzene 0.044 and xylenes 0.068. Based on these literature reports, the
new CA homes have about the same EqLAs and home-to-garage leakage rates and impact
factors as sets of older North American homes.
5 CONCLUSIONS
In this study, the typical impact of garages on indoor concentrations of benzene and xylenes
was 5 – 8%. Although precise comparisons are not possible, it appears that the impact of a
typical attached garage on indoor exposures to BTX for this sample of new CA homes is
similar to central values reported for samples of multi-age homes in North America. This
indicates there has been little progress on isolating garages from living spaces. We
established that homes with garages under living spaces typically have higher garage-tohome coupling factors than homes with garages to the side. There were no discernable
concentration differences; however, the emission rates of benzene and xylenes, the best VOC
indicators of a gasoline source, were higher in homes with garages under living spaces.
Efforts should be made to develop and validate construction practices that minimize the
garage-to-home leakage areas, particularly for two-story homes with garages situated under
spaces.
ACKNOWLEGEMENT
Financial support for the field study was provided by the California Energy Commission,
Contact 500-02-023, and the California Air Resources Board (CARB), Contract 04-310.
6 REFERNCES
Batterman, S, Jia, C. and Hatzivasilis, G. 2007. Migration of volatile organic compounds
from attached garages to residences: A major exposure source, Environ. Res. 104, 224240.
Centers for Disease Control and Prevention (CDC). 2011. Morbidity and Mortality Weekly
Report 60(30), 1014-1017.
Emmerich, S.J, Gorgain, J.E, Howard-Reed, C. 2003. Air and pollutant transport from
attached garages to residential living spaces – Literature review and field tests, Intl. J.
Ventilation 2(3), 265-276.
Graham, L.A, Noseworthy, L, Fugler, D, et al. 2004. Contribution of vehicle emissions from
an attached garage to residential indoor air pollution level, JAWMA 54(5), 563-584.
Hun, D.E, Corsi, R.L, Morandi, M.T. and Siegel J.A. 2011. Automobile proximity and indoor
residential concentrations of BTEX and MBTE, Building Environ. 46, 45-53.
Jenkins, P.L, Johnson, R.D, Phillips, T.J. and Offermann, F.J. 2011. Chemical Concentrations
in New California Homes and Garages. In: Proceedings Indoor Air 2012, Austin TX,
USA.
National Association of Home Builders (NAHB). 2006. Housing facts, figures and trends.
Report NAHB Public Affairs and NAHB Economics. 18 pages.
Offermann, F.J. 2009. Ventilation and Indoor Air Quality in New Homes. California Air
Resources Board and California Energy Commission, PIER Energy-Related
Environmental Research Program. Collaborative Report CEC-500-2009-085,
http://www.arb.ca.gov/research/single-project.php?row_id=64697.
OEHHA. 2008. Acute, 8-hour and Chronic Reference Exposure Levels (chRELs). California
Office of Environmental Health Hazard Assessment.
Sheltair. 2004. Garage performance testing. Final Report, Canada Mortgage and Housing
Corporation, 40 pages.
Disclaimer: The opinions expressed in this paper are those of the authors and not necessarily
those of CARB.