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EPA's Region 6 Office

Serving: Arkansas, Louisiana, New Mexico, Oklahoma, Texas, and 66 Tribal Nations

Vapor Intrusion

Vapor intrusion occurs when vapors and gases from contaminated soil and groundwater seep into indoor air spaces and pose potential health problems.  Some of the most commonly asked questions about this important health issue are discussed below.

What is vapor intrusion?

When volatile chemicals or petroleum products are spilled on the ground or leak from underground storage tanks, they can give off gases, or vapors, that can migrate from the subsurface into overlying buildings (residences, commercial or industrial buildings, or other enclosed structures).  Common products that can cause vapor intrusion include but are not limited to; gasoline or diesel fuel, dry cleaning solvents, and industrial de-greasers.  The vapors move through the soil and seep through cracks in slabs or basement floors and walls, openings for utility lines where pipes and electrical lines go through the foundation, crawl spaces below floors, and other openings.  Vapor intrusion is a concern because vapors can build up to a point where the health of residents or workers in those buildings could be at risk.  Some vapors such as those associated with petroleum products have a gasoline odor, others are odor-free.

Vapor intrusion is now a standard consideration during investigations conducted under the Resource Conservation and Recovery Act (RCRA); Underground Storage Tanks (UST) Program; and the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA/Superfund).

What contaminants can cause vapor intrusion?

Chemicals that readily evaporate and form vapors are called “volatile chemicals”.  Volatile chemicals include volatile organic compounds (VOCs), many of which may be harmful.  Subsurface sources of volatile chemicals may include contaminated soil and groundwater, or other buried wastes.  Some of the typical organic compounds found at vapor intrusion sites include solvents and degreasers such as:  Tetrachloroethylene (PCE), Trichloroethylene (TCE), 1,1-Dichloroethane, 1,1-Dichloroethylene (1,1-DCE), 1,2-Dichloroethane (1,2-DCA), Chloroethene (vinyl chloride).  Typical petroleum-related VOCs include:  Benzene, Toluene, Ethylbenzene, and Xylene.

How do subsurface vapors migrate into indoor air?

This figure depicts the typical migration pathways of volatile chemicals from contaminated soil and groundwater into buildings.

vapor intrusion into house

Three conditions must exist for environmental contaminant vapors to reach the interior of buildings:  vapors from contaminated soil or groundwater must migrate to the subsurface near the building foundation, entry routes into the building must be present (cracks, openings or permeable materials), and there must be driving forces (e.g., stack and wind effects) present that can move the vapors through these entry routes (ASTM, 2005).  Heating, ventilation or air-conditioning systems may also create a negative pressure within a building that can draw soil vapor inside.

What types of homes does vapor intrusion affect?

Vapor intrusion can affect any type of home or commercial building (e.g., buildings with basements, slab-on-grade foundations, or pier and beam/crawl-space foundations).

How could I be exposed to chemicals through soil vapor intrusion and what are the health concerns?

People can be exposed to soil vapor contaminated with volatile chemicals when vapors from beneath a building are drawn through cracks and openings in the foundation and mix with indoor air.  Inhalation is the route of exposure, or the way in which the volatile chemicals actually enter the body, once in the indoor air.  "Current exposure" occurs when vapor intrusion is documented in an occupied building.  "Potential exposures" are when volatile chemicals are present, or are accumulating as vapors beneath a building, but have not yet affected indoor air quality.

In general, exposure to a volatile chemical does not necessarily mean that health effects will occur.  Whether or not a person experiences health effects depends on several factors such as:  the type of chemical(s) present, the levels of the chemical found, the length of exposure, a person’s sensitivity to the chemical, and the overall health of the exposed person.  At high concentrations some temporary symptoms may include eye and respiratory irritation, headaches and nausea.  Over many years, low-level exposures to certain chemicals may raise the lifetime risk of some cancers or other chronic diseases.  Some vapors are flammable and can present risks of explosion.

Can vapors found in my home come from household sources?

Yes.  Volatile chemicals are also found in many household products and can be a source of indoor air problems.  Some examples of these products include:  paints, paint strippers and thinners, mineral spirits, glues, solvents, cigarette smoke, mothballs, new carpet, stored fuel, air fresheners, cleaning products, dry cleaned clothing, etc. 

How does EPA test for vapor intrusion?

The process of investigating vapor intrusion typically requires more than one set of samples to be collected to determine the extent of vapor contamination.  EPA uses several testing methods, either separately or in combination with one another.  Soil, soil gas and ground water samples collected near a contaminated site are used to evaluate whether or not there is the potential for vapor intrusion to exist.  Further tests take place where vapor intrusion appears to be a possibility.  Canisters or other air sampling methods can be used to collect samples of ambient (outdoor) air, soil vapor/gas, indoor air, and air from beneath the foundation of a building or within crawl-spaces of peer and beam structures.

Soil vapor samples are collected to generally characterize the nature and extent of vapor contamination in a given area. 

Sub-slab or crawl-space samples are collected to characterize the nature and extent of vapor contamination in the soil immediately beneath a building with a slab foundation or a basement, or for pier and beam foundations, vapors that are collecting within the open crawl-space between the ground and the floor of the home.

Indoor air samples are collected to characterize the nature and extent of air contamination within a home or building.  They are compared to sub-slab or crawl-space samples and outdoor samples to help determine where the volatile chemicals may be coming from (indoor sources, outdoor sources and/or from soil gas below the building).

Outdoor air samples are collected to characterize site-specific background conditions for volatile organics in ambient air.  Outdoor air results are used to evaluate the extent to which outdoor sources such as automobiles, lawn mowers, gas stations, commercial/industrial facilities, etc. may be affecting indoor air quality.

vapor intrusion inpection

The left and center pictures depict a technician drilling a hole in a foundation to collect soil gas from below the slab using both summa canister and tedlar bag collection devices.  The picture on the right shows a technician placing a summa canister in a crawl-space to collect soil gas accumulating between the ground and the floor of the building.

What solutions are available to prevent or correct the problem?

Ultimately, the best long-term solution is to clean up the source of subsurface contamination that is producing the vapors.  Often, soil and groundwater cleanups can take years to complete.  So, the most common short-term solution is to install vapor control/mitigation systems, which remove soil vapors below basements, foundations or within crawl spaces before they can enter homes. 

Vapors are vented outside and above the home where they become dispersed and harmless.  In addition, any cracks in the building foundation should be sealed to help prevent vapors from entering the home.  Once the source of vapors (contaminated soil or groundwater) is eliminated, the mitigation systems should no longer be necessary for chemical vapors.  However, vapor controls remain valuable for reducing exposures to naturally occurring radon gas (which can present much higher risks).

subsurface systems

Pictured above is a schematic of a typical active (fan driven) soil depressurization system.  This system provides a pathway for the vapors collecting below the foundation to vent to the outside air, instead of migrating into the building.

EPA Region 6 RCRA Corrective Action Contacts:

For more information, please contact:  Richard Ehrhart, RCRA Corrective Action Coordinator, Corrective Action / Waste Minimization Section (6PD-C), at (214)-665-6765, or by email at ehrhart.richard@epa.gov; or David Vogler, RCRA GPRA Coordinator, RCRA State and Tribal Oversight Section (6PD-O), at (214) 665-7428, or by email at vogler.david@epa.gov.

Other Sources of Information on Vapor Intrusion:

EPA Region 6 RCRA Corrective Action Program Vapor Intrusion Policy (PDF), (3 pp, 21K, About PDF) October 2010

EPA Region 6 Vapor Intrusion Frequently Asked Question (FAQs)

EPA Region 6 RCRA Training on Vapor Intrusion (February 2010)
   Part 1 - Vapor Intrusion an Overview (PDF) (53 pp, 2580K, About PDF)
   Part 2 - Vapor Intrusion Assessment and Mitigation Strategies (PDF) (113 pp, 7234K, About PDF)

EPA OSWER Vapor Intrusion Web Page

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