By: Bart Eklund
Bart Eklund is the Global Practice Leader for Vapor Intrusion at AECOM. He has been involved in VI evaluations for over 350 sites. He is an internationally recognized expert in air quality issues.
Experience has shown that where vapor intrusion (VI) proves to be an actual problem, the risk drivers typically are chlorinated volatile organic compounds (VOCs). Trichloroethylene (TCE) and tetrachloroethylene (PCE) are the main VOCs of interest. Where petroleum hydrocarbons are an issue, the primary risk driver typically is benzene.
There are other compounds that are sometimes detected in soil gas and/or indoor air that can be problematic. These compounds may not be a true VI issue, but their presence can complicate the VI evaluation process and make it difficult to achieve consensus among the various stakeholders. The compounds that most often prove to be difficult to correctly interpret are discussed below.
Problematic Compounds in Soil Gas
Two compounds that are sometimes detected in soil gas samples are 1,3-butadiene and acrolein. Both are organic compounds with double bonds and therefore both are highly reactive. Because of their reactivity, they have potential health effects and relatively low screening levels. But because of their reactivity, they have relatively short half-lives in the environment. These and other double-bonded organics are not generally present in soil gas. If they are found in soil gas, it is either an artifact of the sampling process or an artifact of the analysis.
1,3-butadiene is sometimes detected and invariably its presence is associated with direct push sampling through a tight formation. It is believed that the friction from drilling heats up o-rings or other components and results in emissions of 1,3-butadiene. The chemical does not persist in the soil and therefore is not found during any follow-up testing.
Acrolein is sometimes detected in canisters, especially if the samples contain high levels of certain polar compounds (e.g., acetone, methyl ethyl ketone [MEK]). The longer the canister hold time between sample collection and analysis, the more acrolein that may be detected. The increase in concentration is more pronounced if the relative humidity in the canisters is low, which suggests that active sites on the canister wall are acting as a catalyst (water vapor competes for these active sites, so there will be less catalytic activity with higher relative humidity within the canister). More detail can be found in Dann and Wang (2007) and Shelow, et al. (2009).
If 1,3-butadiene or acrolein are detected in soil gas samples, they generally should be assumed to be false positives. These two compounds should be viewed very skeptically in any VI evaluation.
Problematic Compounds in Indoor Air
Indoor air samples generally contain numerous VOCs, with both outdoor air and consumer products contributing to what is detected. Among the compounds often detected are common solvents such as acetone, isopropyl alcohol (2-butanol), and 2-butanone (MEK). Another solvent sometimes detected is carbon disulfide. The sources generally are consumer products and these compounds are unlikely to be significant in terms of risk. Deleting these compounds from target analyte lists may simplify risk communication with stakeholders. Furthermore, it should be recognized that any total VOC values will tend to be dominated by such non-VI-related compounds.
Several compounds that may be detected in indoor air at concentrations approaching or exceeding screening levels merit further discussion. 1,4-dichlorobenzene (p-dichlorobenzene) is a potential carcinogen that is present in various cleaning products and deodorizers such as urinal cakes. This compound is often detected in indoor air samples collected at public or industrial buildings.
Chloroform is a trihalomethane found in chlorinated municipal water supplies. Federal drinking water standards limit total trihalomethanes (i.e., chloroform, bromodichloromethane, dibromochloromethane, and bromoform) to 80 µg/L (US EPA, 2009). It is released from water use indoors, such as showering and clothes washing (McKone, 1987)(Shepherd, et al., 1996). The USEPA reviewed various studies and reported that chloroform was detected in 73% of 2,210 indoor air samples, with a median value of 1.0 mg/m3 and a maximum value of about 11 mg/m3 (USEPA, 2008).
Chloroform is ubiquitous in indoor air and often found in soil gas samples due to irrigation using chlorinated water, leaks from water lines, etc. Therefore, the evaluation of potential vapor intrusion for chloroform is more difficult than for many other VOCs. At the Hill Air Force Base site, for example, chloroform was detected in background indoor air samples at 0.68 to 6. 8 mg/m3 and it also was detected in groundwater samples. The concentrations in indoor air did not decrease after application sub-slab venting systems, indicating that vapor intrusion was not the source of the measured indoor values (Kiefer, et al., 2005).
Naphthalene is another problematic compound that is often detected in indoor air. It is contained in consumer products such as mothballs and certain insect repellants. It also is a component of asphalt and particulate matter (dust) that enters a building in air or tracked in via footwear may contain naphthalene. Naphthalene may also be present in crawl space samples or sub-slab soil gas samples due to past pest treatments. For sites with groundwater plumes containing constituents of gasoline, fuel oil or other petroleum fuels, naphthalene may be a concern. In such cases, however, naphthalene will be one of a number of petroleum VOCs that are present. If only naphthalene is found in the shallow subsurface and indoor air at concentrations that exceed screening levels, the source of the naphthalene is unlikely to be VI.
In general, if VI evaluations show that 1,4-dichlorobenzene, chloroform and/or naphthalene are the only compounds showing exceedances, engineering controls are unlikely to be an appropriate follow-up action. Non-VI sources should be assumed to be potentially significant.
Dann, T., and D. Wang. Canada’s Experience with Acrolein Measurements Using Canisters – Preliminary Results. Presented at NESCAUM Monitoring and Assessment Committee Meeting, Troy, NH. April 24-25, 2007.
Kiefer, K., M. Jones, M> Shibata, H. Olsen, S. Steinmacher, and J. Case. Dealing with Confounding Background Indoor Air Concentrations. Presented at A&WMA Symposium on Air Quality Measurement Methods and Technology. April 19-21, 2005.
McKone, T.E. Human Exposure to Volatile Organic Compounds in Household Tap Water: The indoor Inhalation Pathway. Environ. Sci. Technol., Vol 21, pp1194-1201. 1987.
Shelow, D., J.Rice, M. Jones, L. Camalier, and J. Swift. Acrolein Measurements. Presentation at 2009 National Ambient Air Monitoring Conference, Nashville, TN. 2009.
Shepherd, J., R. Corsi, and J. Kemp. Chloroform in Indoor Air and Wastewater: The Role of Residential Washing Machines. J. Air & Waste Manage. Assoc., Vol. 46, pp631-642. 1996.
USEPA. U.S. EPA’s Vapor Intrusion Database: Preliminary Evaluation of Attenuation Factors. Draft. USEPA, OSW, Washington, DC. March 4, 2008.
US EPA. Drinking Water Contaminants. Accessed October 15, 2009. Available at: