By: Richard J. Wozmak, P.E., P.H., LSP, LEP

Soil and ground water contamination by volatile organic compounds (VOCs), found in petroleum products and solvents, is a common and wide spread occurrence in New England and other areas of the country. When these contaminants are released near buildings, there is the potential for volatilization and intrusion into indoor air. Volatilization of these contaminants can occur from contaminated soil and ground water, and/or from non-aqueous phase liquids such as solvents and petroleum products. With many states now promulgating regulations or policies requiring risk-based cleanup, it is critical that environmental consultants, on behalf of responsible parties, characterize vapor intrusion of VOCs into occupied buildings. In the absence of considering vapor intrusion, one may significantly underestimate the cumulative health risks to occupants of a building. Some states have gone as far as to develop soil vapor, ground water, and indoor air cleanup standards for VOCs that are considered to be protective of human health through the vapor intrusion exposure pathway.

The problem that investigators have in characterizing the vapor intrusion exposure pathway is in trying to distinguish between vapors caused by a release and other “non-release” or background sources that may be present in the building being evaluated. The indoor storage of commonly used products such as cleaners, paints, and petroleum-based products can emit VOCs that are the same or similar in characteristic to those that are emitted from the release being investigated. In some industrial settings, indoor air characterization is complicated by the potential for site-related chemicals released to the environment being the same as those stored and used in the building. To complicate the investigation further, VOCs could also enter a building through ambient air associated with nearby sources such as gasoline stations, landfills, or other industry. These “non-release” sources can cause background contaminant conditions that if not adequately investigated could result in unnecessary cleanup of VOCs that are not associated with the release being investigated. For this article, background is defined as a condition that was present prior to the occurrence of the release being investigated. This article presents a weight-of-evidence approach for developing a sound technical argument  for eliminating from an investigation those VOC sources that may be contributing to indoor air contamination that are not related to the release being investigated. The weight-of-evidence approach is based on the application of one or more investigative approaches. These investigative approaches that can be conducted include:

  • Comparison of VOC concentrations to “generic background” concentrations that have been measured in other buildings;
  • Evaluation of the presence of other VOC sources that may be present in the building;
  • Spatial VOC concentration trends between sub-building environmental media and indoor air;
  • Temporal VOC concentration trends between sub-building VOC media and indoor air;
  • Evaluation of the presence of “indicator” constituents or “markers” that may suggest vapor emissions from other sources, and
  • Vapor intrusion modeling.

The following provides an overview of how a consultant might apply these approaches in indoor air evaluations.

Comparison to Background

Since the mid-1990’s, there has been an increased focus around the United States on measuring indoor air quality. This has resulted in an improved database on typical concentrations of VOCs in indoor air that provide background concentrations from non-release sources. Comparison of indoor air VOC concentrations to published background concentrations may be used to determine if there are environmental media impacts near or under the building that are causing VOC intrusion into the building. If indoor air VOC concentrations are less than or equal to typical background concentrations, it is possible that intrusion of VOCs from a release to environmental media near the building is not occurring. In addition, some state regulations, including Massachusetts, specify that if indoor air concentrations are below regulatory background levels, then further response actions may not be necessary.

Evaluation of Indoor Air VOC Sources

Simple conditions such as the presence of recently dry-cleaned clothes or upholstery, or the presence of new carpets, upholstery, or other textiles can emit VOCs into indoor air. The presence of a nearby high traffic area or industrial facility can also cause ambient air VOC concentrations that can infiltrate a building. A qualitative assessment taking the form of a walk through and around the building, and observing potential sources of the VOCs of concern can provide a line of evidence supporting the potential presence of background sources. Also, interviews with occupants or workers regarding the handling and storage of materials, atypical odors that were observed prior to the release being investigated, and potential outside sources can also provide useful information regarding background sources. All of this information can be critical to the design of subsequent sampling and analytical approaches to reduce the potential of incorrectly linking VOCs detected in indoor air to an environmental release.

Spatial VOC Trends

In a building contaminated by subsurface contamination, the highest VOC concentrations would normally be detected in the basement or lowest level of the building. Samples taken in a spatial gradient away from the source area (e.g., basement) would normally result in lower VOC concentrations with distance from the source area (e.g., first and second floors). Therefore, the results of air samples taken in various parts of a building can be used to identify trends or patterns in contamination. The observed trends or patterns can be evaluated and compared to a typical or expected concentration pattern or trend to determine whether there are anomalies or hot spots in the building from background sources.

The same spatial trend approach can be used to determine the presence of outdoor background sources. For example, an indoor air data set which indicates higher VOC concentrations in upper floors or uniform VOC concentrations among different floors may point to an outdoor or ambient source. In this scenario, the collection and analysis of upgradient ambient air samples can support the presence of an outdoor source.

Sub-basement soil gas VOC concentrations can also be useful when using the spatial gradient approach. In the case where the contaminant source is from a release to ground water or sub-basement soil, it is unlikely that the soil gas VOC concentrations would be lower than the basement VOC concentrations. This type of spatial concentration pattern may indicate the potential for a background source in the basement.

Temporal VOC Trends

Identifying increasing or decreasing VOC concentration trends or patterns over time at specific locations in the building and in sub-building soil gas can be useful in identifying the potential presence of a background source. A good example would be a VOC release below a basement slab where remediation resulted in residual VOCs remaining in the soil. The collection and analysis of soil vapor along with basement indoor air over time could show that VOC concentrations are decreasing in the sub-slab soil vapor and either increasing or remaining constant in the basement. This type of temporal VOC concentration trend could be used to support the presence of a background VOC source in the basement. Another example would be the migration of VOC-contaminated ground water under a building that is decreasing in concentration over time; however, the indoor air concentration is increasing or remaining constant. In this case, the temporal VOC concentration trend in ground water could be compared to that in indoor air to possibly support a background source in the basement.

Indicator Compounds and Markers

Indicator compounds and markers can be used to evaluate the source and nature of indoor air VOCs. Fuels such as gasoline or heating oil are made up of many compounds that give the fuel a unique identity. Industrial solvents often have additional compounds present, such as the stabilizer 1,4-dioxane, that can give the solvent its own unique identity. In some instances, by comparing the compounds present in  the source material to what is detected in the soil vapor or indoor air, a determination may be made regarding the nature or the source of indoor air VOCs. For example, petroleum-based paints or concrete sealants may have VOCs similar to the source being investigated, but may also have compounds unique to the paint or sealant. The detection of these unique “indicator compounds” in indoor air may indicate the presence of a background source (in this example, recently applied paint or sealant).

Another approach includes the monitoring of naturally occurring volatile compounds in the environment or “markers.” Radon, a natural volatile constituent in soil gas in the Northeast United States, may be used to evaluate if subsurface or building characteristics are conducive to vapor intrusion. For example, if radon is detected in sub-slab soil vapor, but not in the basement, it may provide evidence to support that intrusion of VOCs released under the building is not occurring, and that VOCs detected in the building are from a background source. The basis for this argument is that if radon vapor is not migrating into a basement, then factors and conditions that influence VOC vapor intrusion are not present, independent of the presence of subsurface VOC contamination.

Vapor Intrusion Modeling

If it is demonstrated that background sources are contributing to indoor air VOC levels along with the source being investigated and, the background source is the same as the release source (e.g., a solvent release at an industrial facility that still uses the solvent), indoor air sampling may be impracticable or considerable measures may be necessary for the proper sampling and analysis. In the former case, vapor intrusion modeling may provide an alternative, where simulated indoor air concentrations are used in place of indoor air sampling to represent the vapor intrusion exposure pathway associated with the release being investigated.

The U.S. Environmental Protection Agency (EPA) developed a series of models for estimating indoor air concentrations and associated health risks from subsurface vapor intrusion into buildings based on the analytical solutions developed by Johnson & Ettinger. These models are the most commonly used and are widely accepted by the regulatory community. The analytical solutions were developed as a screening level tool, and consequently, are based on a number of simplifying assumptions on contaminant distribution and occurrence, subsurface characteristics, and building construction characteristics. As a result, many regulatory authorities require clean justification for the use of models instead of actual data, and typically require that the model input  parameters and values be appropriately and/or conservatively justified for the specific application.


Assessment of health risks for VOC releases near and under occupied buildings will likely require the characterization of vapor intrusion into buildings. Incorrectly linking indoor air VOC concentrations with subsurface contaminant releases can lead to unnecessary response actions, unnecessary costs, and potentially improper assignment of liability. Consultants need to have available assessment methods to properly characterize the nature of VOCs detected in indoor air.

There are many physical and chemical factors and conditions that can influence and complicate evaluation of VOC vapor intrusion, including chemical properties of contaminants being evaluated, soil properties, type and condition of basement floor, and the type and operation of heating or other air exchange systems in the building. In addition, some of these factors and conditions can change seasonally or over a longer period of time. If an objective of the vapor intrusion study  is to determine if indoor air VOCs are or are not from the release being investigated, application of more than one of the above approaches may be needed to provide a weight-of-evidence in supporting the technical argument. For example, the use of literature-based background concentrations to support a background argument may not be adequate if the site has a lower actual background level in comparison to literature values. Other approaches may be necessary to support the background argument. This article offers six approaches for consideration. The approaches used will depend upon the unique characteristics of the site and the level of scientific certainty needed to support the technical argument.

Published in “Environmental Law and Policy” a periodic publication of Morrison, Mahoney, LLP Environmental Practice Group, Summer 2006