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What is Vapor Intrusion?
Vapor intrusion is the migration of volatile chemicals from the subsurface into overlying buildings.
Soil vapor, or soil gas, is the air naturally found in the pore spaces between soil particles. Pressure and temperature differences between the subsurface and buildings cause soil vapor to enter buildings through conduits such as cracks in the sub–slab, electrical and plumbing lines, or other openings in the foundation. Heating, ventilation and/or air–conditioning systems may also create negative pressures inside a building that results in soil vapor being drawn up into the building.
Soil vapor becomes contaminated when chemicals, including petroleum products and solvents that are leaked or spilled on the ground seep into underlying soil and groundwater. When certain chemicals including volatile organic compounds (VOC’s), select semi–volatile organic compounds, and some inorganic chemicals, such as elemental mercury and hydrogen sulfide are released into soil or shallow groundwater, these chemicals evaporate (volatilize), producing contaminated vapors that travel up through the soil. When these contaminated vapors enter a building, indoor air may be affected. A conceptual drawing illustrating the vapor intrusion process is shown below.
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What Chemicals Might Enter My Home or Business Through Vapor Intrusion?
  Petroleum products and solvents are the most common volatile chemicals that are found in soil vapor and which can move through soil and into buildings. Gasoline, diesel fuel, and home heating oil are examples of petroleum products. Examples of solvents include tetrachloroethene (a common solvent used in the dry cleaning industry, also known as “Perc” or PCE), and trichloroethene (a common solvent used for cleaning and degreasing, also known as TCE). Chemical odors from vapor intrusion may or may not be noticeable to residents, and are largely dependent upon the contaminant type and relative concentration. Typically odors from petroleum products are noticeable at lower concentrations than those associated with solvent vapors.
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What Other Sources of Volatile Chemicals Can Make Indoor Air at My Home or Business Unhealthy?
 Volatile chemicals are found in many household products. Paints, paint strippers and thinners, glues, solvents, and gasoline powered equipment such as lawnmowers are examples of products and equipment that contain volatile chemicals that can affect indoor air quality. Dry cleaned clothing and cigarette smoke also contribute volatile chemicals to indoor air. Attached as Table VI–1, is a summary of several studies conducted in residential homes which document the “background” level of several volatile chemicals in homes that were not influenced by vapor intrusion concerns. As can be seen in Table VI–1, many volatile chemicals are typically found in the average home.
Indoor air quality can also be affected by outdoor air that contains volatile chemicals. Volatile chemicals occur in outdoor air when they are released from various industries, dry cleaners, gasoline stations and vehicles.
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What are the Health Concerns with Vapor Intrusion?
 The health effects from chemical exposures vary based on the individual exposed and the chemical involved. When a person is exposed to contaminated indoor air (for example, when levels are high enough to cause a strong petroleum odor), some people will experience eye and respiratory irritation, headache, and/or nausea. These symptoms are temporary and should go away when the person is moved to fresh air. Usually, health officials are most concerned about low level chemical exposures over many years, as this may raise a person’s lifetime risk for developing cancer. This low level chemical exposure may or may not be noticeable to the exposed person.
The United States Environmental Protection Agency (EPA) and several states have developed guidelines to evaluate incremental increases in exposures and risks from subsurface contaminants that may be intruding into indoor air. These guidelines provide a mechanism to evaluate whether contamination is present at levels that may pose a significant risk to human health or the environment. Table VI–2 presents indoor air and sub–slab vapor screening levels and action levels for many of the common volatile chemicals found at VOC and solvent impacted properties. This table includes screening levels prepared by the U.S. EPA and several states including California, New Jersey, Pennsylvania, New York, and Massachusetts. The screening levels are for guidance information only, and have not been adopted into regulations.
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What Happens if Vapor Intrusion is Suspected Near My Home or Business?
 When vapor intrusion is suspected, it should be investigated by the party responsible for the contamination. In addition, property owners may also request independent investigation by government agencies or outside consultants to provide unbiased verification of the responsible parties’ findings.
Investigation’s typically involves first testing soil, groundwater, and soil vapor at test points outside homes and businesses. This testing helps to determine if volatile chemicals might pose an indoor air quality problem, without a need to disturb residents or businesses. At many contaminated sites, this initial testing is sufficient to determine that volatile chemical levels are low and are not considered a problem. At other sites, these levels in soil, groundwater, or soil vapor are high enough to cause concerns about indoor air quality in nearby homes or businesses.
When potential risks to human health and the environment is identified as described above, additional investigation is often necessary. This investigation typically involves collecting samples from inside homes or businesses. The samples are needed to determine if the volatile chemical levels in buildings are making indoor air unhealthy and whether vapor intrusion might be responsible. Typically, these samples include sub–slab samples collected from beneath the home’s or business’ lowest floor and indoor air samples. Occasionally, outdoor air samples are also collected to evaluate possible effects of outdoor air quality on indoor air quality.
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What Happens if a Vapor Intrusion Problem is Identified?
 If soil vapor contaminated with volatile chemicals are detected in your home at levels that pose a health concern, several actions can be taken together or individually. These action items include:
- Additional sampling to verify results and define the extent and magnitude of soil vapor contamination.
- Implement a monitoring program to collect samples on a recurring basis to determine if conditions change over time.
- Seal any cracks in the buildings foundation or at the pipeline and utility penetrations.
- Adjust the heating, ventilation, and air conditioning systems to provide a positive pressure to prevent infiltration of subsurface vapors.
- Install a sub–slab depressurization system beneath the building.
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What Else Can I Do to Improve Indoor Air Quality?
There are other sources of indoor air problems. Consider these tips to improve air quality:
- Do not buy more chemicals than you need at a time. Be aware of what products contain VOCs.
- Store unused chemicals in appropriate tightly–sealed containers and place in a well–ventilated location, preferably away from the living space in your home.
- If you smell a chemical odor that does not seem to be from an indoor source, contact your local health department. For very strong odors, your local fire department can determine if there is a fire hazard.
- Keep your home properly ventilated. Fresh air will help prevent both build up of chemicals in the air and mold growth.
- Fix all leaks promptly, as well as other moisture problems that encourage mold growth.
- Make sure all major appliances and fireplaces are in good condition. Have them checked annually by a professional.
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