Indoor Air Background Concentrations of Volatile Organic Compounds (VOCs) in California Residences

Abstract

A study was performed to measure typical “indoor air background” concentrations of volatile organic compounds (VOCs) in residential buildings in the state of California that were not known to be impacted by subsurface sources. A total of 57 buildings were sampled between February 2023 and February 2024, with one sample collected per building. The data set represents 38 different cities within the state of California. The samples were analyzed using the United States Environmental Protection Agency (EPA) Method TO-15 (EPA 1999) in both full-scan and Selected Ion Monitoring (SIM) modes. A total of 105 individual VOCs were reported for each sample. The overall data set is 5985 individual data points, with concentrations of target compound VOCs reported from less than the laboratory method reporting limit of 0.044 μg/m³ (micrograms per cubic meter) to concentrations up to 14,000 μg/m³. Various VOCs were detected at concentrations above screening levels used in California and elsewhere in the United States, including Benzene, Naphthalene, Tetrachloroethene (PCE), and Trichloroethene (TCE). These compounds are often considered “risk drivers” in vapor intrusion (VI) studies, so their presence in background air at concentrations above screening levels may complicate such studies. Compared with previous, similar studies, this study is more comprehensive with a larger number of VOCs analyzed and with greater analytical sensitivity. Based on the professional backgrounds of the study group which included environmental professionals, regulatory officials, and attorneys, the authors opine that the results of this residential indoor air background study may be biased low relative to the general population. Therefore, the results of the Study are considered to reflect a conservative snapshot of indoor air background VOCs in California residences.

Introduction

For the purposes of this study, indoor air background is considered to be comprised of VOCs present in indoor air due to indoor or outdoor sources and not those due to a subsurface source (such as VI) (EPA 2011, 2015). Indoor air background can be the result of one or more sources such as occupant activities, consumer products, outdoor air pollution, and building materials and furnishings (NJDEP 2018). Individuals in the United States spend most of their time in indoor spaces, with adults typically spending about 16 h per day at residences (EPA 2011); therefore, the inhalation of indoor air presents a potentially significant route of exposure to various chemicals. This activity pattern also is generally true for Californians, who have been reported to spend an average of 87% of their time indoors and an average of 62% of their time indoors at home (Jenkins et al. 1992).

According to EPA's “Total Exposure Assessment Methodology (TEAM) Study (EPA 1987), concentrations of about a dozen common VOCs were 2 to 5 times higher inside homes than outside, regardless of whether homes were located in rural or highly industrial areas.” The study also concluded that people could expose themselves and others to very high pollutant levels while using products containing VOCs, and that elevated concentrations can persist in the air long after the activity is completed. Among the chemicals typically found in indoor air are VOCs, which may be present in residential air due to various sources (NJDEP 2018) including:

• Air emissions from resident activities (e.g., cooking, cleaning, hobbies, occupation);
• Off-gassing of building materials and products stored in or around the building; and
• Infiltration or other entry of outdoor air.

California regulatory guidance identifies the following building materials and household products as specific items to focus on in terms of their contributions to volatile chemicals in indoor air: new carpeting, flooring or furniture, dry cleaned clothes, recent painting, furniture or cabinet re-finishing, spray adhesives or cleaners (Cal DTSC 2012).

There have been numerous previous studies of residential indoor air background concentrations of VOCs. Some of these studies focused on certain regions of the United States (e.g., EPA Region 5, [Clayton et al. 1999]), some focused on certain states (e.g., Massachusetts and New York [Rago et al. 2004; NYSDOH 2006]), and some focused on areas in other countries (e.g., Schleswig-Holstein, Germany [Hippelein 2004]). One study focused on three states (Weisel et al. 2005), but the study was biased toward potential outdoor air source areas, with sampling in Los Angeles, California (near freeways), Elizabeth, New Jersey (near strip malls): and Houston, Texas (near refineries). Studies conducted between 1990 and 2005 in the United States have been compiled (Dawson and McAlary 2009; EPA 2011) and demonstrate that VOCs are commonly identified in indoor air.

Some more recently published indoor air background studies for residential settings include studies undertaken by the Montana Department of Environmental Quality (MTDEQ), the National Institute of Environmental Health Sciences, and the Government of Canada's Chemicals Management Plan. Some more recently published indoor air background studies for nonresidential settings include a study of schools and offices in the United States conducted by a subset of the authors of this study as well as a study of European office buildings. These more recent residential and nonresidential indoor air background studies are described further below:

• The Montana Department of Environmental Quality (MTDEQ) published data for samples collected in 50 nonsmoking Montanan houses and analyzed for 77 VOCs (MTDEQ 2012). The study objectives were to determine “typical” indoor air concentrations of VOCs in nonsmoking Montana residences not impacted by VI. Although MTDEQ indicates that the results of the study are considered a conservative reflection of indoor air background, six VOC analytes (1,2-Dichloroethane, Benzene, C5-C8 Aliphatics, C9-C12 Aliphatics, Ethylbenzene, and Naphthalene) exhibited 95%UTL-50th statistics of typical indoor air concentrations in Montana residences above their respective EPA RSLs.
• The National Institute of Environmental Health Sciences conducted a study of VOCs in residences of children with asthma and published data for samples collected in 126 homes in Detroit, Michigan (Chin et al. 2014). The study examined 56 VOCs and identified concentrations of total VOC concentrations that ranged from 14 μg/m³ to 2274 μg/m³, and a subset of houses with levels of 1,4-Dichlorobenzene, Naphthalene, and Benzene that reached levels commensurate with excess individual cancer risks of 10⁻², 10⁻³, and 10⁻⁴, respectively. The study cited cigarette smoking, vehicle-related emissions, building renovation, solvents, household products, and pesticides as the most important sources of VOCs.
• The Government of Canada's Chemicals Management Plan supported a study to monitor 88 VOCs in 3524 Canadian residential homes as part of the Canadian Health Measures Survey (Li et al. 2019). The study was conducted over a 24-month period in 2012 and 2013 and found higher VOCs in apartments compared to houses. Data were collected monthly to look for seasonal variability in indoor air concentrations. The study also examined VOC patterns in smoking and nonsmoking homes.
• Background indoor air data was studied in 25 schools and 61 office buildings from across the United States from 2013 to 2015 (Rago et al. 2021). Samples were analyzed for up to 105 VOCs by full-scan gas chromatography-mass spectrometry (GC-MS) and up to 58 VOCs by GC-MS in the SIM mode. The study found that one or more samples exceeded the EPA Regional Screening Levels (RSLs) for nonresidential and/or residential indoor air for Acetaldehyde, Acrolein, Benzene, Bromodichloromethane, Carbon tetrachloride, Chloroform, 1,4-Dichlorobenzene, 1,2-Dichloroethane, Chloroform, Ethylbenzene, Naphthalene, and TCE, with concentrations of TCE greater than commonly employed regulatory rapid action levels in two of the office building samples.
• Data for European office buildings also has been published for 19 VOCs and aldehydes (Mandin et al. 2017), which found that concentrations of some VOCs such as α-Pinene and d-Limonene were higher than common VOCs identified in office buildings in other studies, such as Benzene, Toluene, Ethylbenzene, and Xylene.

Nonresidential buildings such as office buildings and schools have been associated with cumulative risks that are 1 to 2 orders of magnitude lower than residential buildings, based in part on lower nonresidential indoor air concentrations and shorter exposure durations (Rago et al. 2017). Nonresidential indoor air background may also differ from residential indoor air background in the types and ranges of VOCs detected (i.e., potentially higher concentrations of halogenated aromatics in commercial buildings and higher concentrations of normal and cyclic aliphatics in schools and residential dwellings) (Cometto-Muñiz and Abraham 2015).

Consumer products as a source of VOCs were examined in a study of post-pandemic indoor air in hotels located in Memphis, Tennessee; Pittsburgh, Pennsylvania: New York City, New York: and Branson, Missouri (Nored et al. 2024). This study reported that concentrations of detected VOCs were below health criteria for customers but exceeded applicable criteria for hotel workers for several VOCs. According to the California Air Resources Board (CARB) (2024b), consumer products are a significant source of VOCs, and in 2015, consumer products in California accounted for about 250 tons per day of organic compound emissions. In California, CARB is charged with protecting the public from the harmful effects of air pollution, including chemicals from consumer products. To assist with this, the California Consumer Product Regulation (CARB 2024c) was originally passed in 1990 (with subsequent updates) and is applicable to any person that sells, supplies, offers for sale, or manufactures for sale or use in California a chemically formulated consumer product. CARB imposes restrictions on the VOC content for several types of consumer products, including cleaners, degreasers, and paint thinners. As a result, measured concentrations of key constituents such as TCE and PCE in the Study herein may be lower than in other states.

State and federal guidance documents may vary greatly in their approaches to VI (Eklund et al. 2024), but nearly all recognize that indoor air background can confound VI investigations. For example, the EPA (EPA 2015) along with California and some other State regulatory agencies, provides guidance for comparing indoor air data to background concentration data as part of the assessment of the VI pathway at contaminated sites. The guidance is not specific regarding the approach for incorporating background indoor air as a line of evidence in VI assessments.

As summarized above, literature background values are available, but many of these studies are older and lack adequate analytical sensitivity. Older data may also reflect data that are no longer representative of current conditions. For example, changes in VOCs and other chemicals in indoor air over the preceding 50 years have been studied (Weschler 2009), which identified major changes in building materials and consumer products used indoors as well as modifications in building operations, with residential and nonresidential buildings potentially being less ventilated than they were decades ago. EPA also noted that time trends in concentrations reported in the indoor air studies evaluated for their compilation suggested that indoor air concentrations measured in North American residences starting in 1990 and later generally were lower than those measured earlier (EPA 2011). EPA cited several references (e.g., [Hodgson and Levin 2003; Zhu et al. 2005; MassDEP 2008]) that indoor air quality has improved over time in the United States and Canada. More recently, the EPA banned many commercial and industrial uses of PCE and TCE (EPA 2024a, 2024b). Although these new EPA rules provide for longer phase-out timeframes for certain applications, these actions are expected to further reduce indoor air background concentrations of TCE and PCE.

It is not always reasonable, however, to conclude that concentrations of certain VOCs present in indoor air background will continue to decrease over time. Some VOCs, such as 1,2-Dichloroethane, were more recently identified in indoor air background (Rago et al. 2021) and attributed to changes in manufacturing processes (Doucette et al. 2010).

Therefore, a need exists for a current indoor air background data set for samples collected from California residences using consistent sampling and analytical methods. These new data may be useful for indoor air quality practitioners for data review, focusing investigations, mitigation decision making, and risk communication. Accordingly, the main objective of this study was to collect indoor air samples from residential buildings across California that are not known to be in the vicinity of subsurface sources of VOCs and therefore should be representative of indoor air background conditions.

Materials and Methods

Study planning, volunteer identification and selection, and access arrangements began in 2023, and subsequently, 57 residential indoor air background samples were collected between February 2023 and February 2024. One sample was collected from each building.

To identify and select potential Study volunteers, the outreach focused on environmental professionals, regulatory officials, and attorneys. Similar to the rationale used in the Nurses Health Study (NHS 2004), this study population was targeted because it was assumed that these volunteers: (1) have the educational background and general awareness of indoor air quality; (2) were likely to be aware if their residence was near a subsurface release; (3) personally certified, prior to their selection as Study volunteers, that they do not have knowledge of releases of oil and/or hazardous materials to the environment that would impact the indoor air of the residential building to be tested; (4) could respond to sampling survey questions more accurately than the general public; and (5) had attained educational and professional backgrounds sufficient to be less likely to compromise the integrity of the sampling procedures. However, since the Study results are from residences occupied by professionals with an environmental science background or focus with associated environmental health awareness, the results of this residential indoor air background study may be biased low relative to the general population.

All Study volunteers completed an Indoor Air Assessment Questionnaire (template provided in Supplemental Information) which gathered general building information (e.g., single-family or multi-family; the age of the building; foundation type; heating source, garage details, consumer product use and storage, hobbies, etc.) and location. Additionally, each potential volunteer received and signed a release acknowledging: (1) they were volunteering to provide their building data for this study; (2) the indoor air sample results may not be reproduceable and represent a “snapshot” in time; (3) the concentrations reported for their building may be the result of temporary or ongoing sources of VOCs which may or may not originate from within the residential building; and, (4) that if they are concerned about the results from their building, a third party should be contacted to assist them with evaluation of their results.

The buildings evaluated in this study included single-family and multi-family structures, ranged from less than 2 to greater than 120 years old, had a variety of foundation types, and had a variety of heating/cooling systems. Table 1 summarizes the selected building characteristics that were reported. Approximately 80% of the buildings included in this study were single-family residences, and the remainder were multi-family residences, townhouses, or condominiums. For comparison, the California housing stock is 65% single-family, 31% multi-family, and 4% mobile/manufactured homes (CHCD 2018).

Table 1. Building Characteristics

Building Characteristic	% Reported
Building type
Single-family	81
Multi-family	4
Townhouse	5
Condominium	11
Foundation type
Slab-on-grade	40
Crawl space	51
Other	9
Garage
Attached	63
Unattached	21
None	16
Heating type
Natural gas	75
Electric	25
Age (years)
<5	4
5–25	11
26–50	37
51–100	39
100–125	11

Sample locations were requested to be placed where building occupants frequently spend time, such as living rooms. In addition, it was recommended that placement near open windows and doors be avoided. A list of the cities where residences were sampled is given in Table 2 and is shown in Figure 1.

Table 2. List of Municipalities Sampled

City	State
Alameda	CA
Albany	CA
Burbank	CA
Carmichael	CA
Caspar	CA
Concord	CA
Cupertino	CA
Davis	CA
Dublin	CA
El Dorado Hills	CA
Elk Grove	CA
Fair Oaks	CA
Fremont	CA
Fullerton	CA
Garden Grove	CA
Granite Bay	CA
Laguna Hills	CA
Livermore	CA
Los Angeles	CA
Mather	CA
Murrieta	CA
Oakland	CA
Palo Alto	CA
Redwood City	CA
Richmond	CA
Rohnert Park	CA
Sacramento	CA
San Diego	CA
San Francisco	CA
San Jose	CA
San Leandro	CA
San Rafael	CA
Santa Barbara	CA
Santee	CA
Simi Valley	CA
Solana Beach	CA
Tustin	CA
Union City	CA

Figure 1

Indoor air sampling locations.

Sampling and Analytical Procedures and Methods

Analytical laboratory services were provided by Eurofins Air Toxics, a commercial analytical laboratory specializing in the analysis of air using EPA and American Society for Testing Materials (ASTM) methods. The laboratory provided individually certified 6-l electropolished stainless steel canisters and mechanical flow controlling devices calibrated to sample at a constant flow rate over a 24-h interval. Laboratory analysis of 105 target compound VOCs was conducted via EPA Method TO-15 in synchronous full-scan and SIM modes (EPA 1999). Method TO-15 was selected for sampling and analysis since it has broad regulatory acceptance for VI studies in the United States and is capable of analysis of large target analyte lists with corresponding low reporting limits.

Method TO-15 VOCs are defined as compounds having a vapor pressure greater than 10-1 Torr at 25°C and 760 mmHg. EPA Method TO-15 documents the sampling and analytical procedures for the measurement of subsets of the hazardous air pollutants (HAPs) listed in Title III of the Clean Air Act Amendments of 1990. For this study, the Method TO-15 full-scan reporting list included 62 target VOCs and the Method TO-15 SIM mode target compound list included 43 target VOCs, for a total of 105 VOCs reported for each sample. A laboratory control sample (LCS) with chromatographic conditions used is shown in Figure 2:

Figure 2

TO-15 LCS chromatogram (typical): Column: Restek RTX-624 60 m × 0.32 mm ID × 1.8 μm film thickness; chromatographic conditions: −40°C for 0.5 min, 40°C/min to 0°C, hold 4 min, 11°C/min to 110°C, hold 5 min, 20°C/min to 220°C, hold 5 min; Flow Rate = 1.4 mL/min, constant flow. Instrumentation: Agilent 7890 GC/5977B MS with Markes Unity-xr/Kori-xr Concentrator with Markes CIA Canister Autosampler.

Data Quality Assessment and Data Usability

Sampling information and analytical laboratory reports were reviewed, including initial and final canister vacuums, Chain of Custody forms, method blanks, LCS recoveries, GC/MS internal standard recoveries, and laboratory report narratives. Initial and final field vacuums were compared with laboratory-reported values, and no qualifications were deemed necessary. Although minor QA/QC nonconformances were noted, the results were not qualified further and were judged to be representative and considered usable for this study.

Data Management and Statistical Analysis

The analytical laboratory provided analytical results via an Electronic Data Deliverable (EDD) for each laboratory report. EDDs were imported into a relational database for processing. This provided a secure platform for managing, checking, and reporting the data. The VOC data sets for 57 samples were then examined using various graphical and statistical testing procedures using ProUCL 5.2.0 and Minitab 17 statistical software. The descriptive statistics were evaluated for the number of observations, number of detects, percentage of nondetects (ND), range of ND concentration, range of detected concentration, mean, percentile (10th, 25th, 50th [median], 75th, 90th, and 95th), variance, standard deviation, and coefficient of variation. For censored (nondetect, or ND) data, methods such as simple imputations and Kaplan-Meier (KM) estimations were used to calculate the descriptive statistics. The ND results were imputed with the whole reporting limit to calculate the percentiles. KM estimation was used to calculate the mean, variance, standard deviation, and coefficient of variation. Exploratory data analysis (Rosner's Test) for the presence of outliers was conducted. Although the test showed the presence of outliers in the data sets (some of which were reporting limits), there was no conclusive evidence or assignable cause to remove these data, and all data were therefore considered in this study. Refer to Supporting Information for these additional summary statistics.

Discussion of Results

Target VOCs were detected in all 57 residential buildings sampled, with 1169 detected results for 54 of 105 analytes.

The results summary Table 3 presented below includes Percent (%) Detection, Range of Reporting Limits, Range of Detected Concentrations, Kaplan-Meier Mean, and the 10th, 25th, 50th, 75th, 90th, and 95th percentile values. Table 3 also includes risk-based screening levels for comparison to the San Francisco Bay Region Water Quality Control Board Environmental Screening Levels (SFRWQCB 2019) for residential indoor air and the lower of the current cancer (1E-06 excess lifetime cancer risk [ELCR]) and noncancer (using EPA hazard quotient [HQ] = 0.1) EPA RSLs for residential indoor air (EPA 2024c). These screening levels were selected since they are default risk-based points of departure commonly utilized in California and throughout the United States, although it is acknowledged that the use of other static screening level bases is also common and reasonable (e.g., HQ = 1 or 1E-05 ELCR) as well as ranges (e.g., 1 < HQ < 3 or IE-04 < ELCR < 1E-06).

Table 3. Summary Statistics for California Residences

Analytes	Percent (%) Detection	Range of Reporting Limits for NonDetects			Range of Detected Concentrations			KM Mean	10th %tile	25th %tile	50th %tile	75th %tile	90th %tile	95th %tile	EPA Residential Air RSL November 2024 HQ = 0.1		CA Indoor Air Direct Exposure Residential ESL July 2019
		Minimum:Maximum			Minimum-Maximum
Volatile organics by TO-15 (μg/m ^3 )
1,1,1,2-Tetrachloroethane	0%	4	:	51				NA	4.56	4.7	5	5.4	9.24	12	0.38	c	0.38	C
1,1,1,2-Tetrafluoroethane	7%	2.5	:	31	4	-	11	2.79	2.8	2.9	3.1	3.3	5.8	11.6	8300	n	NA
1,1-Dichloropropene	0%	2.7	:	34				NA	3	3.1	3.3	3.6	6.1	8.04	NA		NA
1,1-Difluoroethane	35%	1.7	:	9.6	2.3	-	###	89.7	1.8	1.9	2.2	4.2	27.6	129	4200	n	NA
1,2,3-Trichlorobenzene	2%	4.4	:	55	5.5	-	5.5	4.43	4.9	5.1	5.4	5.9	9.94	12.8	NA		NA
1,2,4-Trimethylbenzene	14%	0.6	:	7.3	0.6	-	2.1	0.71	0.65	0.68	0.72	0.79	1.48	2.32	6.3	n	NA
1,2-Dibromo-3-chloropropane (DBCP)	0%	5.7	:	72				NA	6.4	6.7	7.1	7.6	13	16.6	0.00017	c	0.00017	C
1,3,5-Trimethylbenzene	2%	0.6	:	7.3	0.9	-	0.9	0.59	0.65	0.68	0.72	0.78	1.3	1.76	6.3	n	NA
1,3-Butadiene	4%	0.3	:	3.3	0.3	-	0.4	0.27	0.29	0.31	0.32	0.35	0.59	0.78	0.094	c**	NA
1,3-Dichloropropane	0%	2.7	:	34				NA	3.06	3.2	3.4	3.6	6.2	8.12	NA		NA
1-Bromo-2-chloroethane	0%	0.7	:	8.7				NA	0.78	0.81	0.86	0.93	1.6	2.04	0.0063	n	NA
2,2,4-Trimethylpentane	0%	2.8	:	34				NA	3.1	3.2	3.4	3.7	6.24	8.2	NA		NA
2,2-Dichloropropane	0%	2.7	:	34				NA	3.06	3.2	3.4	3.6	6.2	8.12	NA		NA
2-Butanone (Methyl Ethyl Ketone)	58%	1.9	:	22	1.9	-	58	4.18	2	2.1	2.6	4.3	8.22	10.2	520	n	5200	N
2-Chlorotoluene	0%	3	:	38				NA	3.4	3.6	3.8	4.1	6.94	9.16	NA		NA
2-Hexanone (Methyl Butyl Ketone)	0%	2.4	:	30				NA	2.7	2.8	3	3.2	5.5	7.16	3.1	n	NA
2-Methyl butane (Isopentane)	60%	2	:	22	2.1	-	85	5.99	2	2.2	3.5	6.6	10.4	21	NA		NA
2-Phenylbutane (sec-Butylbenzene)	0%	3.2	:	41				NA	3.6	3.8	4	4.3	7.4	9.68	NA		NA
4-Ethyltoluene (1-Ethyl-4-Methylbenzene)	0%	0.6	:	7.3				NA	0.65	0.68	0.72	0.78	1.3	1.76	NA		NA
4-Methyl-2-Pentanone (Methyl Isobutyl Ketone)	28%	0.5	:	6.1	0.6	-	2.8	0.7	0.55	0.58	0.65	0.93	1.64	2.64	310	n	3100	N
Acetone	98%	70	:	70	18	-	330	65.6	25.2	30	47	73	122	188	NA		32,000	N
Acetonitrile	2%	2	:	25	2.7	-	2.7	2.01	2.2	2.3	2.5	2.6	4.5	5.96	6.3	n	NA
Acrolein	75%	1.5	:	17	1.7	-	8.4	3.1	1.7	1.9	2.8	4.2	6.98	8	0.0021	n	NA
Acrylonitrile	0%	1.3	:	16				NA	1.4	1.5	1.6	1.7	2.9	3.8	0.041	c**	NA
Allyl chloride	0%	1.8	:	23				NA	2.1	2.2	2.3	2.5	4.2	5.52	0.1	n	NA
alpha-methylstyrene	0%	2.8	:	36				NA	3.2	3.3	3.6	3.8	6.5	8.56	NA		NA
Benzyl Chloride (alpha-Chlorotoluene)	0%	0.6	:	7.7				NA	0.69	0.71	0.76	0.82	1.4	1.8	0.057	c**	NA
Bromobenzene	0%	3.8	:	48				NA	4.26	4.4	4.7	5.1	8.64	11.3	6.3	n	NA
Butane	79%	1.6	:	8.4	1.6	-	1900	78.1	1.7	3.2	4.8	8.6	68	308	NA		NA
Carbon disulfide	4%	1.8	:	23	3.4	-	3.6	1.87	2.06	2.1	2.3	2.5	4.2	5.44	73	n	NA
Chlorodifluoromethane	2%	2.1	:	26	3	-	3	2.12	2.36	2.5	2.6	2.8	4.8	6.32	5200	n	NA
Cyclohexane	0%	2	:	25				NA	2.3	2.4	2.5	2.7	4.6	6.04	630	n	NA
Cymene (p-Isopropyltoluene)	2%	3.2	:	41	9.7	-	9.7	3.32	3.6	3.8	4	4.3	7.48	11.4	4.2	n	NA
Dibromomethane	0%	4.2	:	53				NA	4.7	4.9	5.2	5.6	9.54	12.5	0.42	n	NA
Dichlorofluoromethane	0%	2.5	:	31				NA	2.8	2.9	3.1	3.3	5.64	7.44	NA		NA
Diisopropyl ether (DIPE)	0%	9.9	:	120				NA	11	12	12	13	22	29.2	73	n	NA
Ethanol	100%				42	-	###	1290	100	220	570	1600	3080	4440	NA		NA
Ethyl acetate	53%	2.3	:	27	2.5	-	38	6.74	2.5	2.6	3.9	8.3	17.2	28.2	7.3	n	NA
Hexane	0%	2.1	:	26				NA	2.3	2.4	2.6	2.8	4.74	6.12	73	n	NA
Isobutane	81%	1.6	:	8.4	1.6	-	310	19.9	1.9	2.4	4.5	7.9	38.4	114	NA		NA
Isopropyl Alcohol (2-Propanol)	70%	6.3	:	73	7	-	1800	76.4	7.06	7.8	16	37	98	218	21	n	NA
Isopropylbenzene (Cumene)	0%	0.6	:	7.3				NA	0.65	0.68	0.72	0.78	1.3	1.76	42	n	NA
Methyl methacrylate	0%	2.4	:	30				NA	2.7	2.8	3	3.2	5.5	7.16	73	n	NA
Methylcyclohexane	0%	2.4	:	30				NA	2.66	2.8	3	3.2	5.4	7.08	9.9	n	NA
Methylene chloride (Dichloromethane)	11%	0.9	:	10	0.9	-	2.1	0.92	0.92	0.96	1	1.1	1.9	2.58	63	n	1	C
N-Butyl alcohol	88%	2.1	:	22	2.3	-	30	7.51	2.78	3.6	4.7	11	19	20.2	NA		NA
n-Butylbenzene	0%	3.2	:	41				NA	3.6	3.8	4	4.3	7.4	9.68	NA		NA
N-Decane	4%	3.4	:	43	14	-	21	3.91	3.86	4	4.3	4.6	8.1	19.4	NA		NA
N-Heptane	0%	2.4	:	30				NA	2.7	2.8	3	3.2	5.5	7.16	42	n	NA
Nonane	0%	3.1	:	39				NA	3.5	3.6	3.8	4.1	7.04	9.24	2.1	n	NA
n-Propylbenzene	0%	0.6	:	7.3				NA	0.65	0.68	0.72	0.78	1.3	1.76	100	n	NA
N-Undecane	2%	3.8	:	47	23	-	23	4.14	4.2	4.4	4.7	5	8.72	21.4	NA		NA
Octane	0%	2.8	:	34				NA	3.1	3.2	3.4	3.7	6.24	8.2	NA		NA
Pentane	28%	1.8	:	22	2.2	-	15	2.63	2	2.1	2.3	3.3	5.8	10	100	n	NA
Propylene (Propene)	28%	2	:	25	2.4	-	100	5.57	2.3	2.4	2.6	3.8	12	19.4	310	n	NA
Tert-Amyl Methyl Ether (TAME)	0%	9.9	:	120				NA	11	12	12	13	22	29.2	NA		NA
Tert-Butyl Alcohol (tert-Butanol)	2%	7.2	:	90	19	-	19	7.42	8.06	8.4	8.9	9.6	16.4	23	520	n	NA
Tert-Butyl Ethyl Ether (ETBE)	0%	9.9	:	120				NA	11	12	12	13	22	29.2	35	c	NA
tert-Butylbenzene	0%	3.2	:	41				NA	3.6	3.8	4	4.3	7.4	9.68	NA		NA
Tetrahydrofuran	2%	1.7	:	22	23	-	23	2.07	1.96	2	2.2	2.3	4.04	9.68	210	n	NA
Vinyl acetate	0%	2.1	:	26				NA	2.3	2.4	2.6	2.8	4.74	6.12	21	n	NA
Vinyl Bromide (Bromoethene)	0%	2.6	:	32				NA	2.9	3	3.2	3.4	5.9	7.68	0.19	c**	NA
Volatile organics by TO-15 (SIM) (μg/m ^3 )
1,1,1-Trichloroethane	4%	0.1	:	1.6	0.2	-	0.8	0.14	0.14	0.15	0.16	0.17	0.29	0.72	520	n	1000	N
1,1,2,2-Tetrachloroethane	0%	0.2	:	2				NA	0.18	0.19	0.2	0.22	0.37	0.48	0.048	c	0.048	C
1,1,2-Trichloroethane	0%	0.1	:	1.6				NA	0.14	0.15	0.16	0.17	0.29	0.38	0.021	n	0.18	C
1,1-Dichloroethane	0%	0.1	:	1.2				NA	0.11	0.11	0.12	0.13	0.22	0.28	1.8	c	1.8	C
1,1-Dichloroethene	0%	0	:	0.6				NA	0.05	0.06	0.06	0.06	0.11	0.14	0.41	n	73	N
1,2,3-Trichloropropane	0%	0.4	:	4.5				NA	0.4	0.42	0.44	0.48	0.81	1.07	0.031	n	0.31	N
1,2,4-Trichlorobenzene	2%	0.9	:	11	4.5	-	4.5	0.95	0.99	1	1.1	1.2	2.04	4.56	0.21	n	2.1	N
1,2-Dibromoethane (Ethylene Dibromide)	0%	0.2	:	2.3				NA	0.2	0.21	0.22	0.24	0.41	0.54	0.0047	c	0.0047	C
1,2-Dichlorobenzene	2%	0.4	:	4.4	1.2	-	1.2	0.37	0.4	0.41	0.44	0.47	0.82	1.36	21	n	210	N
1,2-Dichloroethane	67%	0.1	:	1.2	0.1	-	6.9	0.58	0.12	0.13	0.23	0.5	1.14	2.58	0.11	c**	0.11	C
1,2-Dichloropropane	4%	0.3	:	3.4	0.3	-	0.6	0.28	0.31	0.32	0.34	0.36	0.62	0.81	0.42	n	0.28	C
1,2-Dichlorotetrafluoroethane (CFC 114)	0%	0.2	:	2.1				NA	0.18	0.19	0.2	0.22	0.37	0.49	NA		NA
1,3-Dichlorobenzene	0%	0.4	:	4.4				NA	0.4	0.41	0.44	0.48	0.8	1.07	NA		NA
1,4-Dichlorobenzene	28%	0.2	:	1.8	0.2	-	16	1.02	0.16	0.17	0.19	0.32	1.11	4.24	0.26	c	0.26	C
1,4-Dioxane	0%	0.4	:	5.3				NA	0.48	0.5	0.53	0.57	0.96	1.26	0.56	c**	0.36	C
Benzene	86%	0.2	:	2.4	0.2	-	1.8	0.63	0.28	0.41	0.6	0.8	1.18	1.44	0.36	c**	0.097	C
Bromodichloromethane	4%	0.4	:	5	0.6	-	0.9	0.43	0.44	0.46	0.49	0.53	0.91	1.18	0.076	c	0.076	C
Bromoform	0%	0.6	:	7.6				NA	0.69	0.71	0.76	0.82	1.4	1.8	2.6	c	2.6	C
Bromomethane (Methyl Bromide)	0%	2.3	:	29				NA	2.6	2.7	2.8	3.1	5.2	6.92	0.52	n	5.2	N
Carbon tetrachloride	95%	0.8	:	1.9	0.4	-	1	0.55	0.47	0.49	0.52	0.6	0.75	0.88	0.47	c*	0.47	C
Chlorobenzene	0%	0.3	:	3.4				NA	0.31	0.32	0.34	0.36	0.62	0.81	5.2	n	52	N
Chloroethane	0%	0.2	:	2				NA	0.18	0.18	0.19	0.21	0.35	0.47	420	n	10,000	N
Chloroform (Trichloromethane)	93%	0.3	:	1.4	0.2	-	7.1	1.12	0.27	0.37	0.69	1.2	2.58	3.84	0.12	c*	0.12	C
Chloromethane (Methyl Chloride)	7%	1.3	:	15	1.7	-	1.9	1.34	1.4	1.4	1.5	1.6	2.8	3.66	9.4	n	94	N
cis-1,2-Dichloroethene	0%	0.1	:	1.2				NA	0.1	0.11	0.12	0.12	0.21	0.28	4.2	n	8.3	N
cis-1,3-Dichloropropene	0%	0.3	:	3.4				NA	0.3	0.31	0.33	0.36	0.61	0.8	NA		NA
Dibromochloromethane	2%	0.5	:	6.3	1.1	-	1.1	0.51	0.57	0.59	0.63	0.67	1.14	1.52	NA		NA
Dichlorodifluoromethane (CFC-12)	98%	3.6	:	3.6	2.1	-	3.2	2.62	2.4	2.5	2.6	2.8	3	3.12	10	n	NA
Ethylbenzene	86%	0.1	:	1.3	0.1	-	4.2	0.46	0.15	0.22	0.3	0.52	0.98	1.34	1.1	c*	1.1	C
Hexachlorobutadiene	2%	0.6	:	7.9	2.3	-	2.3	0.66	0.71	0.74	0.78	0.84	1.44	2.54	0.13	c	0.13	C
m, p-Xylenes	93%	0.3	:	2.6	0.3	-	9.8	1.21	0.38	0.55	0.69	1.2	2.64	3.56	NA		NA
Methyl Tert-Butyl Ether (MTBE)	0%	0.4	:	5.3				NA	0.48	0.5	0.53	0.57	0.97	1.26	11	c*	11	C
Naphthalene	68%	0.1	:	0.8	0.1	-	1.9	0.17	0.07	0.08	0.12	0.18	0.3	0.52	0.083	c**	0.083	C
o-Xylene	89%	0.1	:	1.3	0.1	-	3.5	0.5	0.17	0.22	0.34	0.51	1.08	1.58	10	n	NA
Styrene	40%	0.3	:	3.2	0.3	-	40	1.16	0.3	0.31	0.35	0.59	1.44	1.8	100	n	940	N
Tetrachloroethene	19%	0.2	:	2	0.2	-	1.6	0.25	0.18	0.19	0.2	0.29	0.67	1.01	4.2	n	0.46	C
Toluene	96%	1.3	:	2.8	0.4	-	9.6	1.96	0.84	1	1.5	2.6	3.6	3.94	520	n	310	N
trans-1,2-Dichloroethene	4%	0.5	:	5.9	0.6	-	0.6	0.48	0.53	0.55	0.58	0.63	1.1	1.4	4.2	n	83	N
trans-1,3-Dichloropropene	0%	0.3	:	3.4				NA	0.3	0.31	0.33	0.36	0.61	0.8	NA		NA
Trichloroethene	2%	0.1	:	1.6	0.2	-	0.2	0.13	0.14	0.15	0.16	0.17	0.29	0.38	0.21	n	0.48	C
Trichlorofluoromethane (CFC-11)	95%	1.8	:	4.2	1	-	51	2.23	1.2	1.2	1.3	1.4	1.8	2.22	NA		NA
Trifluorotrichloroethane (Freon 113)	25%	0.5	:	5.7	0.5	-	0.6	0.53	0.52	0.54	0.57	0.6	1	1.38	520	n	NA
Vinyl chloride	5%	0	:	0.4	0	-	0.1	0.03	0.03	0.04	0.04	0.04	0.07	0.11	0.17	c*	0.0095	C

• Note: Bold indicates a value greater than the EPA RSL or the CA ESL.
• NA, not available; RSL, EPA regional screening level, November 2024; HQ, hazard quotient; n, noncancer; c, cancer; c*, n RSL < 100X c RSL; c**, n RSL < 10X c RSL; ESL, CA Environmental Screening Level, July 2019; N, noncancer; C, cancer; μg/m³, microgram per cubic meter.

Indoor air background concentrations of target compound VOCs ranged from less than the laboratory method reporting limit of 0.03 μg/m³ (for Vinyl Chloride) to concentrations up to 14,000 μg/m³ (for Ethanol). Hydrocarbons, ketones, halomethanes, alcohols, and chlorofluorocarbons (Freons) were the most reported of the 54 analytes detected, with hydrocarbons comprising approximately one-half of the top 20 compounds detected:

• 100% frequency of detection: Ethanol;
• 90% to 99% frequency of detection: Acetone, Dichlorodifluoromethane (CFC-12), Toluene, Carbon Tetrachloride, Trichlorofluoromethane (CFC-11), Chloroform, m/p-Xylene; and
• 60% to 89% frequency of detection: o-Xylene, n-Butyl Alcohol, Benzene, Ethylbenzene, Isobutane, Butane, Acrolein, Isopropyl Alcohol, Naphthalene, 1,2-Dichloroethane, and 2-Methyl Butane (Isopentane).

Some VOCs have historically been reported ubiquitously in ambient air monitoring at similar levels and may therefore be present due to infiltration of outdoor air. Although outdoor air data was not collected in this study, nor was its data compared to available public data sources on a city-specific basis, CARB has a statewide outdoor air monitoring program that includes 36 monitoring locations for select VOCs (CARB 2024a). Between 2017 and 2023, the statewide 90th percentile for Carbon Tetrachloride ranged from 0.44 to 0.63 μg/m³; Dichlorodifluoromethane (CFC 12) was consistently reported at 2.1 μg/m³; Trichlorofluoromethane (CFC 11) was consistently reported at 1.1 μg/m³. For these three analytes, this study's 90th percentile results were reported at concentrations that are within approximately 116% and 138% of the statewide CARB data reported between 2017 and 2023.

Many of the VOCs shown in Table 3 may originate from indoor sources and activities, with potential sources of 1,2-Dichloroethane from plastics (Doucette et al. 2010; Gorder and Dettenmaier 2011) and Chloroform arising from tap water (Nuckols et al. 2005; Eklund and Rago 2024) and bleach-containing household products (Odabasi 2008). Ethanol has been cited as being among the most abundant organic compounds in indoor air (Nazaroff and Weschler 2024), and has been cited as an exhalation product of humans (Tang et al. 2016; Wang et al. 2022).

Benzene and other hydrocarbons are widely associated with vehicle emissions for outdoor air contributions, and cars and gasoline stored in attached garages are possible indoor sources (Graham et al. 2004; Batterman et al. 2007; Zielinska et al. 2012). When comparing the Study statistics of attached garage (n = 36), unattached garage (n = 12), and no garage (n = 9), the range of concentrations of Benzene and naphthalene was slightly higher in homes with attached garages, but the frequency of detections was highest in homes with an unattached garage or no garage (see Supporting Information).

The database population included 51 VOCs that were not detected in any of the samples (0% frequency of detection). Table 4 lists the VOCs that were ND in all 57 indoor air samples. These VOCs may be compounds that are uncommon to consumer products and materials such as halopropanes and haloaromatics but also include compounds of common environmental interest that may be useful as indicators of VI such as cis-1,2-Dichloroethene, 1,1-Dichloroethane, and 1,1-Dichloroethene. Other compounds that were not detected included Hexane, Cyclohexane, Heptane, and Methyl tert-butyl ether (MTBE).

Table 4. Compounds Not Detected in Any Indoor Air Samples

Analytes
Volatile Organics by TO-15	Volatile Organics by TO-15 (SIM)
1,1,1,2-Tetrachloroethane	1,1,2,2-Tetrachloroethane
1,1-Dichloropropene	1,1,2-Trichloroethane
1,2-Dibromo-3-chloropropane (DBCP)	1,1-Dichloroethane
1,3-Dichloropropane	1,1-Dichloroethene
1-Bromo-2-chloroethane	1,2,3-Trichloropropane
2,2,4-Trimethylpentane	1,2-Dibromoethane (Ethylene Dibromide)
2,2-Dichloropropane	1,2-Dichlorotetrafluoroethane (CFC 114)
2-Chlorotoluene	1,3-Dichlorobenzene
2-Hexanone (Methyl Butyl Ketone)	1,4-Dioxane
2-Phenylbutane (sec-Butylbenzene)	Bromoform
4-Ethyltoluene (1-Ethyl-4-Methylbenzene)	Bromomethane (Methyl Bromide)
Acrylonitrile	Chlorobenzene
Allyl chloride	Chloroethane
alpha-methylstyrene	cis-1,2-Dichloroethene
Benzyl Chloride (alpha-Chlorotoluene)	cis-1,3-Dichloropropene
Bromobenzene	Methyl Tert-Butyl Ether (MTBE)
Cyclohexane	trans-1,3-Dichloropropene
Dibromomethane
Dichlorofluoromethane
Diisopropyl ether (DIPE)
Hexane
Isopropylbenzene (Cumene)
Methyl methacrylate
Methylcyclohexane
n-Butylbenzene
N-Heptane
Nonane
n-Propylbenzene
Octane
Tert-Amyl Methyl Ether (TAME)
Tert-Butyl Ethyl Ether (ETBE)
tert-Butylbenzene
Vinyl acetate
Vinyl Bromide (Bromoethene)

Selected Study Comparison

To further explore the results from this study, we compared the results to those presented in the EPA Indoor Air Background Compilation (EPA 2011) and those presented in the MTDEQ study (MTDEQ 2012). The EPA compilation was selected for comparison to this study because it includes a large number of samples at various United States locations (including California) and is considered the most robust database available of VOCs identified in indoor air background. The MTDEQ study was selected as it is considered a conservative dataset for which to compare this study's results, with MTDEQ citing “Montana's low population, rural conditions, small manufacturing base, and fewer contaminant sources of VOCs than locations predominantly included in other background studies.” For the sake of comparison, the same 18 VOCs common to VI evaluations and indoor air background evaluated by EPA were retained for this evaluation.

• The EPA Indoor Air Background Compilation (EPA 2011) summarized indoor air background data compiled from studies that were completed between 1990 and 2005. The compilation is based on EPA's review of 18 indoor air studies conducted since 1981, wherein the 15 most recent (1990 through 2005) indoor air quality studies were selected for further evaluation of reported summary statistics. The EPA compilation included 2898 indoor air background samples for states across the US (Arizona, California, Colorado, Illinois, Massachusetts, Minnesota, New Jersey, New York, Ohio, Texas) and Ottawa, Canada, in residences that were not expected or known to be located over contaminated soil or groundwater or that have effective VI mitigation systems in place. This report was also used in EPA's VI database in the development of the generic groundwater and sub-slab vapor attenuation factors (EPA 2012).
• The MTDEQ publication (MTDEQ 2012) considered the 50 study houses as ones that represent a cross section of “typical” residential building use (in the absence of smoking) in both urban and rural Montana. The study states the MTDEQ position that “the range of indoor air concentrations for VOCs typically found in Montana is more narrow and falls at or below the low end of nationwide background ranges.”

The comparison between the three data sets is summarized in Table 5. Several compounds (Carbon tetrachloride, 1,2-Dichloroethane, Chloroform, and Freon-113) show an increase in percent detected for the California data set versus the EPA value. For 1,2-Dichloroethane, that is potentially due to air emissions from molded plastic items as previously discussed. A few compounds show a decrease in detection frequency, such as 1,1,1-Trichlorethane, MTBE, Methylene chloride, PCE, and TCE. That is potentially a function of these compounds being used less in California than in the past. These decreases may also be the result of state or federal regulatory actions such as CARB restrictions or those recently enacted by EPA under the Toxic Substances Control Act (EPA 2024a, 2024b). The petroleum hydrocarbons (i.e., Benzene, Toluene, Ethylbenzene, and Xylenes) showed little or no change in the frequency of detection and these compounds were ubiquitous in this study and other indoor air studies.

Table 5. Comparison of Summary Statistics for Background Indoor Air Concentrations of Common VOCs Measured in this study, North American Residences between 1990 and 2005 (EPA Data), and Montana Study (all concentrations expressed in μg/m³)

Compound	Number of Studies	Number of Samples	Total Percent Detects	Reporting Limit Range	Range of 50th %tile	Range of 75th %tile	Range of 90th %tile	Range of 95th %tile
This study
Benzene	1	57	86%	0.22–2.4	0.6	0.8	1.18	1.44
1,1,1-Trichloroethane	1	57	4%	0.13–1.6	0.16	0.17	0.294	0.722
1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113)	1	57	75%	0.5–5.7	0.57	0.6	1	1.38
1,1-Dichloroethane	1	57	0%	0.096–1.2	0.12	0.13	0.22	0.28
1,1-Dichloroethene	1	57	0%	0.047–0.59	0.058	0.063	0.11	0.14
1,2-Dichloroethane	1	57	67%	0.11–1.2	0.23	0.5	1.14	2.58
Carbon tetrachloride	1	57	95%	0.82–1.9	0.52	0.6	0.748	0.882
Chloroform	1	57	93%	0.26–1.4	0.69	1.2	2.58	3.84
cis 1,2-Dichloroethene	1	57	0%	0.094–1.2	0.12	0.12	0.21	0.278
Ethylbenzene	1	57	86%	0.12–1.3	0.3	0.52	0.976	1.34
m/p-Xylene	1	57	93%	0.28–2.6	0.69	1.2	2.64	3.56
Methyl tert-butyl ether (MTBE)	1	57	0%	0.42–5.3	0.53	0.57	0.97	1.26
Methylene chloride	1	57	11%	0.86–10	1	1.1	1.9	2.58
o-Xylene	1	57	89%	0.14–1.3	0.34	0.51	1.08	1.58
Tetrachloroethene	1	57	19%	0.16–2	0.2	0.29	0.67	1.008
Toluene	1	57	96%	1.3–2.8	1.5	2.6	3.6	3.94
Trichloroethene	1	57	2%	0.13–1.6	0.16	0.17	0.29	0.38
Vinyl chloride	1	57	5%	0.03–0.38	0.038	0.04	0.071	0.106
EPA data
Benzene	14	2615	91%	0.05–1.6	ND—4.7	1.9–7.0	5.2–15	9.9–29
1,1,1-Trichloroethane	9	1877	53%	0.12–2.7	ND—5.9	ND—7	ND—68	3.4–28
1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113)	3	600	38%	0.25–3.8	ND—0.5	ND—1.1	ND—1.8	ND—3.4
1,1-Dichloroethane	2	682	1%	0.08–0.25	ND	ND	ND	ND
1,1-Dichloroethene	2	475	13%	0.01–0.25	ND	ND—0.37	ND—0.8	0.7
1,2-Dichloroethane	7	1432	14%	0.08–2.0	ND	ND—0.08	ND—0.4	ND—0.2
Carbon tetrachloride	6	1248	54%	0.15–1.3	ND—0.68	ND—0.72	ND—0.94	ND—1.1
Chloroform	11	2278	69%	0.02–2.4	ND—2.4	ND—3.4	ND—6.2	4.1–7.5
cis 1,2-Dichloroethene	3	875	5%	0.25–2.0	ND	ND	ND	ND—1.2
Ethylbenzene	10	1484	86%	0.01–2.2	1–3.7	2–5.6	4.8–13	12–17
m/p-Xylene	10	1920	93%	0.4–2.2	1.5–14	4.6–21	12–56	21–63.5
Methyl tert-butyl ether (MTBE)	4	502	55%	0.05–1.8	0.025–3.5	0.03–11	0.03–41	71–72
Methylene chloride	8	1724	79%	0.12–3.5	0.68–61	1.0–8.2	2.0–510	2.9–45
o-Xylene	12	2004	89%	0.11–2.2	1.1–3.6	2.4–6.2	5.5–16	13–20
Tetrachloroethene	13	2312	63%	0.03–3.4	ND—2.2	ND—4.1	ND—7	4.1–9.5
Toluene	12	2065	96%	0.03–1.9	4.8–24	12–41	25–77	79–144
Trichloroethene	14	2503	43%	0.02–2.7	ND—1.1	ND—1.2	ND—2.1	0.56–3.3
Vinyl chloride	4	1484	9%	0.01–0.25	ND	ND	ND—0.04	ND—0.09
Montana study
Benzene	1	50	98%	0.16–0.32	0.9	2.5	8.4	12
1,1,1-Trichloroethane	1	50	4%	0.054–0.54	<0.84	<0.93	<1.0	<1.7
1,1,2-Trichloro-1,2,2-trifluoroethane (Freon 113)	1	50	0%	0.50–3.2	<0.83	<0.93	<0.98	<1.3
1,1-Dichloroethane	1	50	0%	0.081–0.40	<0.83	<0.93	<0.98	<1.3
1,1-Dichloroethene	1	50	0%	0.040–0.40	<0.83	<0.93	<0.98	<1.3
1,2-Dichloroethane	1	50	100%	0.081–0.40	0.17	0.48	0.82	1.2
Carbon tetrachloride	1	50	8%	0.63	<0.85	<0.93	<1.0	1.5
Chloroform	1	50	36%	0.49	<0.93	1.7	2.7	3.6
cis 1,2-Dichloroethene	1	50	0%	0.079–0.40	<0.83	<0.93	<0.98	<1.3
Ethylbenzene	1	50	94%	0.087–0.43	0.78	2.1	4	6
m/p-Xylene	1	50	66%	0.17–0.43	2.7	6.6	8.9	24
Methyl tert-butyl ether (MTBE)	1	50	0%	0.36	<0.83	<0.93	<0.98	<1.3
Methylene chloride	1	50	38%	0.69	<0.93	1.5	9	29
o-Xylene	1	50	42%	0.087–0.43	<0.94	2.4	5.2	7.6
Tetrachloroethene	1	50	84%	0.14–0.68	0.099	0.24	2.3	2.8
Toluene	1	50	98%	0.38	8.3	25	49	60
Trichloroethene	1	50	44%	0.11–0.54	<0.048	0.096	0.58	1.3
Vinyl chloride	1	50	2%	0.026–0.26	<0.042	<0.046	<0.049	<0.064

• μg/m³, microgram per cubic meter; ND, not detected.

The data presented in the comparison table suggest that the Study data are generally comparable to the EPA compilation and MTDEQ study data at lower percentiles and diverge with relatively higher concentration ranges of petroleum VOCs reflected at the upper percentiles for the EPA compilation and the MTDEQ study. The Study's lower results may also be a result of generally decreasing trends of indoor air VOC concentrations over time (e.g., Hodgson and Levin 2003; Dawson and McAlary 2009; Weschler 2009; EPA 2011).

Although not included in the comparison above, it is noteworthy that the results from this study differ from the results recently reported for Canadian homes (Li et al. 2019). The study also identified “higher concentrations of the majority of measured VOCs in apartments compared to houses; and of several tobacco-related VOCs (benzene, styrene, naphthalene, 2-butanone, 2-methyl-1,3-butadiene, 2-furancarboxaldehyde, 2,5-dimethylfuran, benzofuran and phenol) in smoking homes.” The Canadian study detected 55 VOCs at a frequency >50%, including various compounds not included in the California study (e.g., Limonene, Decamethylcyclopentasiloxane, Hexanal, Nonanal, α-Pinene). Hydrocarbons such as Hexane, Heptane, and Cyclohexane were detected at >99% frequency in Canadian houses, but were not detected in any of the California houses. The absence of these common hydrocarbons in these Study data supports the notion that indoor air background is building-specific and should be evaluated in the context of indoor air background ranges, and with multiple lines of evidence.

Summary and Conclusions

The Study generated 5985 new indoor air background data points for residential buildings located in 38 cities in California. Indoor air background concentrations of target compound VOCs ranged from less than the lowest laboratory method reporting limit of 0.044 μg/m³ to concentrations up to 14,000 μg/m³. VOCs such as hydrocarbons, ketones, alcohols, chlorofluorocarbons, and some chlorinated solvents were identified ubiquitously in indoor air background. A review of the Study results indicates that one or more samples exceeded either the California screening level for residential air or the EPA regional screening level for residential air or both. Compounds with one or more exceedances of screening levels are shown in Table 6 and include compounds such as Benzene, Naphthalene, 1,2-Dichloroethane, Carbon tetrachloride, Tetrachloroethene, and Trichloroethene. Although indoor and outdoor air background are considered the sources of the VOCs detected (and not VI) based on Study volunteer certifications and associated sampling questionnaires, the exceedances are significant because many of these VOCs are typical “risk drivers” for VI investigations.

Table 6. Compounds Exceeding Screening Levels in Any Indoor Air Sample

Analytes
Volatile Organics by TO-15	Volatile Organics by TO-15 (SIM)
1,3-Butadiene	1,2,4-Trichlorobenzene
Acrolein	1,2-Dichloroethane
Cymene (p-Isopropyltoluene)	1,2-Dichloropropane
Ethyl acetate	1,4-Dichlorobenzene
Isopropyl Alcohol (2-Propanol)	Benzene
Methylene chloride (Dichloromethane)	Bromodichloromethane
	Carbon tetrachloride
	Chloroform (Trichloromethane)
	Ethylbenzene
	Hexachlorobutadiene
	Naphthalene
	Tetrachloroethene
	Trichloroethene
	Vinyl chloride

The role of background in the Comprehensive Environmental Response Compensation and Liability Act (CERCLA) has been addressed (EPA 2002). Cleanup levels are not generally set at concentrations below natural background levels. Similarly, for contaminant concentrations due to anthropogenic sources, CERCLA normally does not set cleanup levels below anthropogenic background concentrations. Reasons for this CERCLA approach include cost-effectiveness, technical practicability, and the potential for recontamination of remediated areas by surrounding areas with elevated background concentrations. However, regulatory agencies are generally not specific regarding the approach for incorporating background indoor air as a line of evidence in VI assessments. Appropriate regulatory consideration of indoor air background levels is therefore recommended when developing and updating screening levels.

When VI pathway determinations appear to be complicated by potential contributions of indoor air background VOC sources, the VI practitioner may also need to employ more sophisticated investigation strategies such as techniques and methods evaluated by the United States Navy (NAVFAC 2011). These may include comparison with published indoor air background values and ranges, calculation of constituent ratios between media and/or compounds, and field measurement methods such as differential pressure monitoring, building pressure cycling, tracer compound analyses, and continuous monitoring using field instrumentation (Beckley, McHugh, Gorder, et al. 2013a; Kram et al. 2019). In addition, environmental forensic analytical techniques may be employed (Plantz et al. 2008; Beckley, McHugh, Kuder, et al. 2013b; Beckley et al. 2016).

Understanding indoor air background data is a critical aspect of VI pathway evaluations and stakeholder communication. It can allow regulators and practitioners to focus on investigations and mitigation decision making. Since indoor air background is building-specific and since commercial product formulations can and do change over time, ranges of background concentrations are considered more applicable than “bright line” values such as upper fence values or medians.

Acknowledgments

The authors thank Lila Beckley for the support and technical review of the draft manuscript. The authors also want to thank the state and local officials, environmental consultants, and attorneys who volunteered to participate in the study.

Study Design and Funding

The study was designed and implemented by Gina Plantz and Rich Rago of Haley & Aldrich. The study was funded entirely by Haley & Aldrich, Inc., and Eurofins Air Toxics.

Conflict of Interest

The authors declare no competing interests.

Biographies

• Gina Plantz, Senior Principal, Haley & Aldrich, Inc., 426 17th Street, Suite 700, Oakland, CA 94612; [phone].

• Kelly Chatterton, corresponding author, is a Technical Expert at Haley & Aldrich, Inc., 465 Medford Street, Boston, MA 02129; [phone]; [email protected]

• Rich Rago, Senior Technical Expert, Haley & Aldrich, Inc., 3 Bedford Farms Drive, Bedford, NH 03110; [phone].

• Bart Eklund, Senior Technical Expert, Haley & Aldrich, Inc., 3 Bedford Farms Drive, Bedford, NH 03110; [phone].

• Heidi Hayes, Technical Director, Eurofins Environment Testing Northern California, LLC; 180 Blue Ravine Road, Suite B, Folsom, CA 95630; 916-365-5294.

• Monica Tran, Senior Project Manager, Eurofins Environment Testing Northern California, LLC; 180 Blue Ravine Road, Suite B, Folsom, CA 95630; 916-605-3408.

Open Research

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.