By David Shea, P.E.

Environmental due diligence for potentially contaminated properties should include an assessment of vapor intrusion risk, among other environmental concerns. Vapor intrusion is defined by U.S. EPA as the migration of hazardous vapors from any subsurface contaminant source, such as contaminated soil or groundwater or contaminated conduits, into an overlying building or structure through any opening or conduit. [1] The ultimate concern with vapor intrusion is the potential for human exposure to hazardous vapors in indoor air. Figure 1 illustrates potential vapor intrusion pathways from subsurface contamination.

Vapor intrusion has been a recognized human exposure risk since the 1980s, with concerns centered on radon entry into homes and schools. Radon is a colorless, odorless, radioactive gas that is naturally occurring in soil in many parts of the United States. The awareness of radon intrusion foreshadowed concerns associated with properties containing subsurface vapor-forming chemicals (VFCs). Some common VFCs responsible for vapor intrusion are: perchloroethylene (PCE), often associated with dry cleaner releases; trichloroethylene (TCE), often found at industrial sites; and benzene, often connected to hydrocarbon releases. Since these VFCs and others may also be found in consumer products, distinguishing vapor intrusion from indoor sources of air quality concerns can be an investigation challenge. 

Most states, in addition to U.S. EPA, have published technical guidance for assessing and mitigating vapor intrusion. [2] Potential vapor migration (i.e., the movement of hazardous vapors in the subsurface) is included in the scope of an ASTM E1527-21 Phase I Environmental Site Assessment, [3] while ASTM E2600-22 is a guide for evaluating vapor encroachment, defined as the presence of contaminated vapors at a subject property due to an on-site or off-site release. [4]

Conventional Assessment Approach Takes Time

Despite the long-standing awareness of vapor intrusion risk and its inclusion in the scope of property transaction due diligence, as well as the thousands of properties that have been identified with potential vapor intrusion concerns, little has changed or improved over the past two decades in the methods typically used to investigate and test most of these sites. The conventional vapor intrusion assessment approach is to collect samples of soil vapor from either below a building (termed sub-slab vapor samples) or from outside the building footprint (termed exterior soil vapor samples), oftentimes in combination with indoor air samples. The samples are then sent to a laboratory for analysis of volatile organic chemicals (VOCs). The steps are often repeated to address data gaps or new questions arising from the initial results. With laboratory wait times of several weeks, followed by data analysis and reporting, this can be a drawn-out process if multiple sampling events are required to address uncertainties and data gaps – an undesirable situation when human health, environmental impact, and business interests in the property are in question. 

Furthermore, vapor intrusion is an inherently highly variable and structure-specific phenomenon as reflected in highly variable VOC concentrations in indoor air over periods ranging from hours to months. [5] A single, conventional sampling event is unlikely to provide an accurate representation of the vapor intrusion risk. Thus, many states require multiple sampling events across different seasons, including the winter in the northeast when vapor intrusion is more likely to occur due to the stack effect (i.e., when indoor-outdoor temperature differences drive indoor convection currents that favor drawing soil vapor into a structure). As a result, a vapor intrusion investigation that adequately assesses the indoor air exposure risk can take months to years using conventional methods.

Adaptive Investigations Using Real-Time Analysis 

To accelerate and improve the efficiency of vapor intrusion assessments, a leading approach is to employ field instruments that can analyze samples of soil vapor and indoor air in real time as opposed to waiting for laboratory results. The Interstate Technology Regulatory Council (ITRC) recently published a Fact Sheet on the use of real-time monitoring for vapor intrusion assessment. [6] Real-time sample analysis supports adaptive, high-confidence investigations, whereby the initial results guide investigators, while they are still in the field, on where to collect supplemental samples to address questions that arise from the initial results. Near-continuous, real-time analysis at stationary locations over a day or more can reveal underlying variability in VOC concentrations.

Rapid on-site testing also allows for the analysis of many more samples, more than 100 per day depending on the instrument, at lower cost per sample than conventional laboratory analysis. Lots of quick, inexpensive data offers opportunities to identify patterns and variability in the results, which are likely to lead to faster insights into the sources and pathways of the vapor intrusion risk and inform potential mitigation solutions. Real-time analysis can also help identify indoor sources of VOCs by allowing for targeted sampling near suspected indoor sources, as well as facilitate more sophisticated investigation methods, such as building pressure cycling to distinguish vapor intrusion from indoor sources of VOCs. [7] 

Instruments and analyzers capable of providing real-time results range from total VOC analyzers, such as hand-held photoionization detectors (PIDs), to equipment capable of analyzing and reporting individual VOCs, such as portable gas chromatographs (GCs), which are often coupled with various detectors or mass spectrometers (MS). This equipment is available for rental or purchase from manufacturers or vendors of environmental instruments. Figure 2 shows some examples of real-time analyzers deployed in the field.

The accuracy and detection limits of field analyzers for individual VOCs continues to improve, which is important because certain VOCs, such as PCE and TCE, have very low indoor air screening level concentrations established by U.S. EPA and most states. The instruments are sufficiently rugged so that they can be moved between sampling locations by hand or on a cart. Alternatively, the instruments can be set up in a stationary location in the field and the samples brought to the instrument for analysis using a sampling syringe or sampling bag.

Overcoming Limitations

Purchasing or renting field instruments for vapor intrusion assessment is typically a cost adder to the conventional approach of collecting samples and sending them to a laboratory for analysis. However, the cost per sample decreases with every sample analyzed using real-time instruments; thus, the overall cost of the investigation is likely to be lower as a result of obtaining more data in a single field event, and by reducing or eliminating the need for multiple field campaigns to obtain sufficient data to appropriately evaluate the risk. A recent vapor intrusion assessment of a 10,000-square-foot industrial building using real-time analysis was completed at about a third of the cost of the conventional approach. The cost savings were achieved by completing the real-time investigation in one mobilization as compared to the conventional approach, which would likely have required three or more sampling events to identify the vapor intrusion pathway.

Another potential limitation is that field instruments can be susceptible to loss of accuracy or malfunction that is difficult to diagnose and trouble-shoot without specific expertise. To ensure consistent data quality, real-time analysis should include calibration checks at the beginning and end of each day at least, or after obtaining a high-level reading. This requires having a gas standard on hand with a known VOC concentration to test the accuracy of the instrument within the concentration range of interest. 

Real-time analysis of soil vapor and indoor air samples using field instruments may be considered by regulators to be “screening” data that does not completely replace conventional sampling for vapor intrusion and other environmental investigations. Nevertheless, most regulators have embraced real-time vapor intrusion assessments because of how quickly health risks can be evaluated and mitigated if warranted. Even though conventional laboratory sample results are still likely to be required by regulators to support final decisions and approvals, the number of such samples and associated costs can be drastically reduced by doing most of the investigation using real-time analysis. 

Closing

Like investigations of soil or groundwater contamination, vapor intrusion assessments can potentially prolong schedules for mitigation and reuse of contaminated properties. Fast-track assessment using real-time analytical instruments is a way to streamline vapor intrusion investigations as compared to conventional approaches that rely on collecting samples, oftentimes over multiple site visits, for laboratory analysis. Limitations of real-time analysis include the cost and reliability of using field instruments, but these are typically outweighed by rapid, higher resolution assessments that can lead to lower overall investigation costs.

References

[1] U.S. EPA, OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air, June, 2015.

[2] Eklund, B. et al., Overview of State Approaches to Vapor Intrusion: 2023 Update, Groundwater Monitoring & Remediation, February 7, 2024.

[3] ASTM E1527-21, Standard Practice for Environmental Site Assessments: Phase I Environmental Site Assessment Process, December 21, 2021.

[4] ASTM E2600-22, Standard Guide for Vapor Encroachment Screening on Property Involved in Real Estate Transactions, May 18, 2022.

[5] Holton C. et al., Temporal Variability of Indoor Air Concentrations under Natural Conditions in a House Overlying a Dilute Chlorinated Solvent Groundwater Plume, Environmental Science & Technology, Vol. 47(23), 2013.

[6] ITRC Vapor Intrusion Toolkit, Real-Time Monitoring Fact Sheet, January 2026.

https://itrcweb.org/vapor-intrusion-toolkit. 

[7] Guo, Y. et al., Development and Validation of a Controlled Pressure Method Test Protocol for Vapor Intrusion Pathway Assessment, Environmental Science & Technology, Vol. 54(12), 2020.