Engineering and Expeditionary Warfare Center

Vapor Intrusion

Schematic of the vapor intrusion process from the NAVFAC EXWC RITS 2011a VI Tool



Vapor Intrusion (VI) is the migration of volatile contaminants from the subsurface into indoor air. It is important to determine if VI is occuring and causing an unacceptable health risk in buildings near soil or groundwater impacted with volatile compounds. Actions should be taken to mitigate VI risks as appropriate.



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Characterization - Technologies

This page provides a very brief introduction to types of sampling technologies that may be required for VI investigations. Initially, the potential for VI is usually determined by sampling groundwater and/or soil gas. If elevated concentrations of VOCs are detected in these media in proximity (usually defined as 100 ft) of a building, additional sampling may be required to determine if VI is occurring and presents a significant health risk to the building occupants. Because the VI is often a complex pathway, VI investigations may require samples from several environmental media to serve as multiple lines of evidence to support site decisions. These samples may include: soil gas samples taken near the building, sub-slab soil gas samples (immediately beneath the building), indoor air samples, and outdoor air (sometimes referred to as "ambient air") samples. (More detailed information on the assessment process can be found in this portal under “Guidance” and under “VI Assessment Tool.”)

Soil Gas: Soil gas samples are collected to characterize the nature and extent of vapor contamination in the soil in a given area. Often, they are collected before sub-slab vapor and/or indoor air samples to help identify buildings or groups of buildings that need to be sampled. Soil gas samples are used to determine the potential for vapors to accumulate beneath buildings.

Sub-slab Soil Gas: Sub-slab samples are collected to characterize the nature and extent of vapor contamination in the soil immediately beneath a building with a slab or beneath the basement floor. Sub-slab vapor and outdoor air samples are usually collected at the same time as indoor air samples to help determine where volatile chemicals may be coming from (indoor sources, outdoor sources, and/or beneath the building).

Indoor air: Indoor air samples are collected to characterize the nature and extent of vapors within a building. They provide a direct measure of the contaminants a person may inhale. They are also compared to sub-slab vapor and outdoor air results to help determine where volatile chemicals may be coming from (indoor sources, outdoor sources, and/or beneath the building). Indoor air samples are generally collected from the lowest-level space in a building, such as a basement. They are often collected during cold weather when the building is heated and doors and windows are typically kept closed. The collection time usually is representative of daily exposure, typically 24 hrs for residential settings and 8 to 12 hrs for an occupational setting.

Outdoor air: Outdoor air samples are collected to characterize site-specific background air conditions. Outdoor air results are used to evaluate the extent to which outdoor sources, such as automobiles, lawn mowers, oil storage tanks, gasoline stations, commercial/industrial facilities, and so forth, may be affecting indoor air quality. However, the presence of a contaminant in the outdoor air does not necessarily mean that the contaminant will be present in indoor air.


Characterization – Building Characteristics

The construction and use of a building can affect the health risks VI presents, the sampling approach selected, and the cleanup levels required. Therefore, one of the initial steps in VI assessment is often a building survey to collect needed information about the building(s) being investigated for VI. These surveys generally include questions about:

-----Use of the building (commercial or residential),
-----Number of people (adults and children) using the building,
-----Number of floors,
-----Age of the structure,
-----Presence of a basement or crawl space,
-----Specifics of the heating and venting system,
-----Presence of any sumps or utilities in the foundation that may increase the vapor movement into the building or prevent the use of certain sampling techniques.
-----Chemicals stored inside the building and recent work on the building that may have involved paints, cleaning supplies, bug sprays, carpeting, or other sources of VOCs.
-----Other information relevant to determining the potential for vapor intrusion to occur or determining background contaminants in indoor air.



Vapor Intrusion Assessment




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     Policy & Guidance            Publications             Videos            Related Sites    



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Policy and Guidance

Topics Title and Description

EPA Guidance


OSWER Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (June 2015)

This technical guide presents current technical recommendations of the EPA based on the current understanding of vapor intrusion into indoor air from subsurface vapor sources. One of its main purposes is to promote consistency in assessing the vapor intrusion pathway.3 At the same time, it provides a flexible science-based approach to assessment that accommodates the different circumstances (e.g., stage of the cleanup process) in which vapor intrusion is first considered at a site and differences among pertinent EPA programs.

Vapor Intrusion



NAVFAC Guidance for Communicating Vapor Intrusion at Environmental Restoration Sites (October 2014)

This guidance, prepared by Naval Facilities Engineering Command (NAVFAC) and Navy and Marine Corps Public Health Center (NMCPHC), is designed to provide Navy Environmental Restoration Program (ERP) Remedial Project Managers (RPMs) with recommendations for notifying and relaying information regarding vapor intrusion (VI) investigations to Base personnel and potentially affected occupants of industrial, office, or residential buildings.


Assessing Protectiveness at Sites for Vapor Intrusion – Supplement to the Comprehensive Five-Year Review Guidance, OSWER Directive 9200.2-84 (November 2012)

The purpose of this supplemental guidance is to provide recommendations for assessing the protectiveness of a remedy for vapor intrusion at private and federal facility Superfund sites during the five-year review process described in the Comprehensive Five-Year Review Guidance (EPA, 2001). It also provides recommendations for assessing protectiveness at sites where a vapor intrusion remedy has not been implemented and: 1) the vapor intrusion pathway was never adequately characterized; or 2) changes in site conditions since the last five-year review have potentially led to a complete vapor intrusion pathway.



NAVFAC Interim Final Guidance for Environmental Background Analysis Volume IV: Vapor Intrusion Pathway (April 2011)

Guidance on assessing potential background sources as a part of the assessment of the vapor intrusion pathway.


Vapor Intrusion - Review of Best Practices, Knowledge and Data Gaps, and Research Opportunities for the U.S. Department of Navy Vapor Intrusion Focus Areas, SPAWAR Systems Center Pacific (May 2009)


DoD VI Handbook 

DoD Vapor Intrusion Handbook (January 2009)



Navy/Marine Corps Policy on Vapor Intrusion, CNO Letter 5090 Ser N453/8U158104 (April 2008)

This Navy policy provides direction on the evaluation and remediation of the vapor intrusion (VI) pathway in the Environmental Restoration Program. Topics covered include determining whether to evaluate the VI pathway for a site, planning and implementing a VI pathway evaluation, addressing background chemical issues, evaluating risk for human health exposures associated with the VI pathway, evaluating remedial alternatives, and considering previously transferred property.


ITRC Vapor Intrusion Pathway: A Practical Guideline (January 2007)



ITRC Vapor Intrusion Pathway: Investigative Approaches for Typical Scenarios - a Supplement to Vapor Intrusion Pathway: A Practical Guideline (January 2007)



Navy Policy on the Use of Background Chemical Levels CNO letter 5090 Ser N45C/N4U732212 (January 2004)

This policy clarifies the Navy’s position on consideration of background chemicals as it applies to the Navy Environmental Restoration Program and clarifies the Navy’s interpretation of U.S. EPA’s “Role of Background in the CERCLA Cleanup Program, April 2002.”



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Topics Title and Description     



DoD Vapor Intrusion Handbook Face Sheet Update – Matrix for Selecting Vapor Intrusion Investigation Technologies (July 2019)

Provides a matrix that facilitates the selection of the most effective technologies for investigating the Vapor Intrusion pathway at DoD facilities.

Background Chemicals

DoD Vapor Intrusion Handbook Fact Sheet Update - Determining the Influence of Background Sources on Indoor Air Concentrations in Vapor Intrusion Assessment (September 2017)

Describes field and calculation methods for determining the influence of background sources and for developing background investigation strategies.

Tracers, Surrogates, Indicator Parameters

DoD Vapor Intrusion Handbook Fact Sheet Update -  Use of Tracers, Surrogates, and Indicator Parameters in Vapor Intrusion Assessment (September 2017)

Describes how tracers, surrogates, and indicator parameters can be used to guide VI and background investigations.

Real-Time Monitoring 

DoD Vapor Intrusion Handbook Fact Sheet - Real Time Monitoring for Vapor Intursion Assessment (2017)     

Real-time monitoring involves the collection of environmental data and reporting of sampling results in real time (i.e., within a very short timeframe on the order of seconds to minutes). Examples of vapor intrusion data that can be measured and reported in real time include: indoor air and soil vapor concentrations, building pressure differentials and ventilation rates, and tracer gas concentrations.

High Volume Sampling 

DoD Vapor Intrusion Handbook Fact Sheet - High Volume Soil Gas Sampling for Vapor Intrusion Assessment (2017)

High volume sampling (HVS) is a method for assessing vapor concentrations and distributions in the subsurface, and is particularly well suited to sub-slab soil vapor sampling as part of a vapor intrusion (VI) assessment. The technique involves removing a large volume of gas from below the concrete floor slab, and monitoring the organic vapor concentrations and pneumatic response for analysis and interpretation of the vapor distribution between and beyond the point(s) of suction. The HVS method provides more information than traditional discrete sub-slab soil gas sampling, and is faster, less expensive and less disruptive (especially in large buildings).  



DoD Vapor Intrusion Handbook Fact Sheet - Use of Building Pressure Cycling for Vapor Intrusion Assessment (2017)

Vapor intrusion (VI) can be challenging to assess using conventional discrete indoor air and sub‐slab sampling because of spatial and temporal variability in volatile organic compound (VOC) concentrations and background sources of VOC vapors. Building Pressure Cycling is an investigation technique that manipulates building air pressure and ventilation to promote or inhibit VI. The difference between indoor air VOC concentrations measured under positive and negative pressures induced at similar building ventilation rates provides a measure of the contribution from subsurface sources. Additionally, Building Pressure Cycling can be used to identify the presence and contribution of background sources of VOCs to indoor air within a building. VOCs detected in indoor air under positive pressure conditions indicate the presence of background sources.  




Indicator Parameters 

DoD Vapor Intrusion Handbook Fact Sheet - Use of Tracers, Surrogates, and Indicator Parameters in Vapor Intrusion Assessment (2017)

VI assessments can be improved using tools that help guide investigations or clarify the processes or mechanisms affecting vapor transport. Tracers are substances that migrate similarly to the volatile organic compounds (VOCs) of interest for VI. Indicators are parameters or variables that are associated with the potential for VOC exposures through VI. Surrogates are variables with a quantitative relationship to the target VOCs for a VI study, sufficient to be useful as a substitute for directly measuring the target compounds. 

passive sampling 

DoD Vapor Intrusion Handbook Fact Sheet -  Passive Sampling for Vapor Intrusion Assessment (2017)  

Passive sampling is an alternative to USEPA Methods TO‐15 and TO‐17 for monitoring VOC vapor concentrations and has some potential advantages for vapor intrusion assessments.  This fact sheet is intended to provide a high‐level overview and offer reference to resources that detail the application and analysis of passive sampling for vapor intrusion. 

Decision Framework 

NESDI 476: A Quantitative Decision Framework for Assessing Navy Vapor Intrusion Sites Fact Sheet (April 2016)

A fact sheet provides a brief introduction to the Navy's vapor intrusion (VI) assessment framework. This framework provides a practical tool to evaluate site and building specific data and to interpret results for future VI-related actions for Navy industrial buildings. It can be used be Navy Remedial Project Managers (RPMs) to prioritize buildings for further assessment, assess the likelihood of VI occurrence in individual buildings, and plan for the long-term stewardship of current and future buildings.

Passive sampling 

Technical Memorandum: Passive Sampling for Vapor Intrusion Asessment (July 2015)

This technical memorandum was prepared for NAVFAC Remedial Project Managers, contractors, and other stakeholders to provide an overview of the use of passive samplers for vapor intrusion (VI) applications. It describes the basics of passive sampler theory and design, the available types of passive samplers, the advantages and limitations of passive samplers, and important considerations when implementing a passive sampling program.  In addition, the results from two VI case studies at Department of Defense sites are highlighted.

decision framework 

NESDI 476: A Quantitative Decision Framework for Assessing Navy Vapor Intrusion Sites (June 2015)

This project developed a quitative decision framework to improve decision making and site management practices for Navy industrial/commercial vapor intrusion (VI) sites.  

site characterization 

Vapor Intrusion - Innovative Site Characterization Methods Fact Sheet (February 2013)

This fact sheet provides an overview of several emerging and innovative methods for the characterization of indoor air at potential vapor intrusion (VI) sites. These methods include passive sampling, use of a portable gas chromatography/mass spectrometry (GC/MS) instrument, use of building pressure control techniques, hydrocarbon fingerprinting, compound specific isotope analysis (CSIA), and radon sampling

Conceptual site model 

CSM Checklist - Vapor Intrusion (October 2012)

This checklist summarizes key elements to consider in building a CSM for a VI site.  


new buildings 

Vapor Intrusion Mitigation in Construction of New Buildings Fact Sheet (August 2011)

This fact sheet provides an overview of VI mitigation methods that can be integrated during the construction of new buildings, along with important factors to consider when selecting and designing these systems.


Existing Buildings 

Vapor Intrusion Mitigation in Existing Buildings Fact Sheet (May 2011)

This fact sheet reviews methods that can be used to mitigate VI in existing buildings, along with important considerations for selecting and designing an appropriate mitigation system.



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topics Title and Description
Portable GC/MS 

Investigating Vapor Intrusion : Portable GC/MS Video, NAVFAC EXWC (June 2013)      

The purpose of this video is to provide a quick overview of how to use a gas chromatograph mass spectrometer (GC/MS) technology and to show the value of real-time data collection for vapor intrusion sites.


Understanding Vapor Intrusion: A Guide to Key Concepts and Principles (Video), NMCPHC


site investigation 

Understanding Vapor Intrusion: A Step-by-Step Guide to the Site Investigation Process (Video), NMCPHC




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T2 Tools (Technology Transfer (T2) Informational Tools)



  T2 Tools  

Vapor Intrusion Tool

This tool explains the use of ZVI as a reducing agent to transform contaminants like chlorinated ethenes and energetics into non-toxic end products.

Print/Text Version



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Related Sites

Title and Description

United States Environmental Protection Agency (EPA) Clu-In Vapor Intrusion Web Page



EPA Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air, OSWER Publication 9200.2-154 (June 2015)



EPA Technical Guide for  Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites, EPA 510-R-15-001 (June 2015)



EPA Assessing Protectiveness at Sites for Vapor Intrusion - Supplement to Comprehensive Five-Year Review Guidance, OSWER Directive 9200.2-84 (14 November 2012)



Interstate Technology Regulatory Council (ITRC) Vapor Intrusion Page



ITRC Petroleum Vapor Intrusion Page



Johnson and Ettinger (1991) Model for subsurface Vapor Intrusion into Buildings (

Concerns have been raised about the potential for subsurface contamination in either soil or ground water adversely impacting indoor air quality. In September 1998, EPA developed a series of models for estimating indoor air concentrations and associated health risks from subsurface vapor intrusion into buildings. These models were based on the analytical solutions of Johnson and Ettinger (1991) for contaminant partitioning and subsurface vapor transport into buildings


EPA Mid-Atlantic risk Assessment Regional Screening Table


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Frequently Asked Questions (FAQs)

What is vapor intrusion?

Answer: Vapor intrusion (VI) refers to the migration of chemicals in gaseous (vapor) state from underground sources into the indoor air of an overlying building. Chemicals may be released into soil and groundwater in a variety of ways such as spills, past disposal practices, or leaking underground storage tanks. These chemicals can be transported by groundwater and those that are sufficiently volatile may evaporate into the soil gas and travel upward through the soil. In undeveloped areas, the contaminated vapors disperse into the air. However, if the soil is covered by a building, the vapor can enter the building through cracks, sumps, or other openings in the foundation. This movement of contaminants into a building is referred to as VI. In some cases, these vapors can increase to concentrations that may be harmful to human health.


What chemicals are associated with vapor intrusion?

Answer: Only contaminants that are volatile are a concern for VI. A class of chemicals known as volatile organic compounds (VOCs) is the most common cause of VI. Gasoline contains several VOCs, including benzene, toluene, ethylbenzene, and xylene (known together as BTEX) that are often associated with VI. Other common VI sources are chlorinated VOCs, which are used in degreasing operations, dry cleaning and other industrial operations. These chemicals include tetrachloroethene (PCE) and trichloroethene (TCE). Also, both PCE and TCE break down into other VOCs (dichloroethene isomers and vinyl chloride) which can be a VI concern. Elemental mercury and hydrogen sulfide are examples of volatile inorganics that can be a concern for VI; however, most metals (e.g., lead, chromium, cadmium) are not volatile and thus, not a concern for VI.


How do vapors get into buildings?

Answer: When contaminated vapors are present in soil gas directly next to or beneath the foundation of a building, VI is possible. When the building is under negative pressure due to forces such as the ventilation system or convection caused by warm air rising, vapors can be drawn into the building through cracks, utilities penetrations, or other openings in the foundation. Usually, VI is more likely to be significant in the winter months when furnaces are operating and windows are closed. Also, vapors can travel through the permeable gravel used to backfill utility line installations, and be drawn into nearby buildings.


Are there other sources of indoor air contamination?

Answer: Yes. VOCs and other volatile chemicals are found in many household products including paints, paint strippers, thinners, cigarette smoke, aerosol sprays, mothballs, air fresheners, new carpeting or furniture, hobby supplies (glues and solvents), stored fuels, and recently dry cleaned clothes. These products can be a source of VOCs found in indoor air and can make it difficult to determine how much contamination is attributable to VI. Also, contaminants present in ambient outdoor air can impact the quality of indoor air.


Why is vapor intrusion a concern?

Answer: Exposure to volatile chemicals resulting from VI can cause an increased risk of adverse health effects. In most cases, the VI concern is the potential for an increased risk of long-term or chronic effects such as cancer that might result from long-term exposure to very low concentrations of contaminants. Whether or not a person experiences any health effects depends on several factors, including the concentration of the chemical, the frequency and duration of exposure, the toxicity of the chemical and the individual's sensitivity to the chemical. Rarely, very high concentrations may occur and cause immediate acute effects such as dizziness, burning of eyes, or even risk of explosion, requiring immediate evacuation of the building.


How is vapor intrusion investigated?

Answer: In most cases, the potential for VI can be verified or ruled out by collecting soil gas samples and/or ground water samples near the contamination site. In cases where the contaminant plume is near or below a building, it may be necessary to sample soil gas immediately next to or beneath the building. In some cases, indoor air samples are collected inside the building; however, sampling the indoor air will not necessarily confirm that VOCs detected inside the building are the result of VI, since a variety of VOC sources from household or industrial activities can be present within the building itself. Also, indoor air quality can vary considerably from day to day.


How can you tell if VOCs are from chemicals in the building or from vapor intrusion?

Answer: To determine if indoor air contaminants are resulting from VI rather than other sources within or outside the building, it is important to compare data from a variety of sources such as groundwater and soil gas collected near the building, sub-slab soil gas samples collected beneath the foundation, indoor air samples and/or ambient outdoor air samples. For example if a particular VOC is present in indoor air but not in the subsurface soil gas or groundwater, it is unlikely that VI is the source of that contaminant. The Navy has developed a new guidance document, Guidance for Environmental Background Analysis, Volume IV: Vapor Intrusion Pathway (add link when available) that discusses procedures for assessing background indoor air concentrations that are not attributable to VI.


What happens if soil vapor contamination or soil vapor intrusion is identified during investigation of a site?

Answer: Depending on the investigation results, additional sampling may be performed to confirm that risk levels attributable to VI exceed acceptable limits for a particular contaminant and to verify questionable results. Once it is confirmed that the risk due to VI is above acceptable levels, mitigation is usually implemented to minimize exposures to indoor air contamination while cleanup of the source of the vapors is occurring.


How can vapor intrusion problems be corrected?

Answer: If VI is adversely affecting the air in a building, mitigation measures can be implemented to address the problem. Most mitigation measures are aimed at preventing the contaminated vapors from entering the building. These measures include sealing cracks in the foundation and penetrations by utility lines; covering sumps; adjusting the building heating, ventilation and air-conditioning systems; and/or installing a sub-slab depressurization (SSD) system. If these measures are not effective or if mitigation is needed while other methods are being adjusted, air filters may be used to remove contaminants from the indoor air. Sealing alone is usually not sufficient, particularly if the risk from VI is significant. When new buildings are being constructed in areas where there is potential for VI to be significant, a vapor barrier is often installed beneath the foundation along with a passive venting or active SSD system.


What is a sub-slab depressurization system?

Answer: A sub-slab depressurization (SSD) system is one of the most reliable mitigation measures to address VI. An SSD system is essentially the same type of system used to mitigate radon gas, and thus has been widely used and tested. The system prevents soil gases from entering the building by creating a negative pressure area below the building and venting the sub-slab vapors to the outside. In existing buildings, any cracks or openings in the slab are sealed prior to installation. Then, a hole is drilled through the slab at the suction points and piping is installed to convey the vapors to the outside. One or more blowers are used to apply a low amount of suction below the foundation and the vapors are vented outside, usually above the roof line and away from any air intakes. The system is operated until the subsurface source is cleaned up and the indoor air concentrations attributable to VI are decreased to acceptable risk levels when the system is not operating.


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