Vapor Intrusion

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What is vapor intrusion?
Why is vapor intrusion a concern?
What happens if vapor intrusion is suspected?
What can be done to reduce vapor intrusion and improve indoor air quality?
Related topics
For more information

What is vapor intrusion?
Chemicals that have been spilled or dumped on the ground can pollute soil and groundwater.  One group of chemicals, volatile organic compounds (VOCs), easily evaporates into air.  Petroleum products, dry cleaning solvents, and many industrial chemicals contain VOCs.  VOCs evaporating from the polluted soil and groundwater rise towards the ground surface.  If these vapors come to a basement as they travel to the surface, they may enter through cracks in the foundation, around pipes, or through a sump or drain system.  In this way, the VOC vapors enter buildings and contaminate indoor air.  This process, when pollution moves from air spaces in soil to indoor air, is called vapor intrusion.

Radon also enters buildings from soil through vapor intrusion. Radon is an odorless, radioactive gas that is found naturally in Minnesota soils. 

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Why is vapor intrusion a concern?
If there are high concentrations in indoor air, people may be able to smell chemical vapors, especially those from petroleum. Some people may experience headaches, eye and throat irritation, nausea, or other symptoms.  Some people are more sensitive to these effects than others. The effects are usually temporary, and will go away when the person leaves the building or breathes fresh air.  Fortunately, this situation is rare. 

More often, health officials are concerned about the possible health effects of long-term exposure to low levels of contaminants.  At low levels, there is usually no odor to warn people that contaminants are in the air. 

  • Long-term exposure to some of the VOCs that have been found at vapor intrusion sites may be associated with increased risk of damage to the immune system, nervous system, and to the developing fetus.  Exposure may also lead to an increased risk of developing certain types of cancer.  Even though these risks are usually low, they are avoidable, and health and environmental officials want to identify and take steps to reduce or eliminate vapor intrusion where possible.
  • Radon is the number one cause of lung cancer in non-smokers. We know that 2 in 5 homes in Minnesota have radon levels that pose a significant health risk.   

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What happens if vapor intrusion is suspected?
 Anytime VOCs are in soil or groundwater, there could be vapor intrusion into nearby buildings.  Officials begin an investigation by collecting soil, groundwater and soil gas samples to look for the presence of chemicals and the levels. If chemicals are present near buildings, it may be necessary to collect samples of soil gas beneath the slab or in indoor air.  Sub-slab samples are collected by drilling a small hole into the building slab in order to collect a sample of the soil gas underneath the home or building.  Indoor air samples are typically collected over 24 hours using specialized canisters.

Many factors affect vapor intrusion and indoor air quality, such as the weather or season, type of building construction, and ventilation.  Indoor air sample data can be difficult to interpret. Many of the chemicals that may be present in soil vapor can also be found in common household products, cigarette smoke, and vehicle emissions.  If the chemicals found in soil and groundwater are not detected in indoor air, it lessens the possibility that vapor intrusion can occur, but does not eliminate it.  On the other hand, if the same chemicals found in soil and groundwater are found in indoor air (especially in the basement), it suggests vapor intrusion may be occurring but the air contaminants may also be coming from other sources within the building.  Several indoor air samples taken at different times may be necessary to determine if vapor intrusion is occurring, and if so, the actual chemical concentrations inside the building that are due to vapor intrusion.

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What can be done to reduce vapor intrusion and improve indoor air quality?
If soil and groundwater contamination is found at a site, some sort of cleanup or remedial action may be required.  This could involve digging up and removing contaminated soil, installing a soil vapor extraction (SVE) system, or pumping out and treating the contaminated groundwater.  Any of these actions should help reduce vapor intrusion.  If chemical vapors are found in indoor air at levels that might affect people’s health, it may be necessary to install a system to direct vapors away from indoor air.  These systems are the same systems installed to keep radon from entering homes.  They direct the vapors that collect underneath the foundation to the outside air.  They are relatively inexpensive to operate, simple to design and install, and are a proven solution to radon and vapor intrusion problems.  Usually, when the contamination is the result of a spill or leak, the responsible party or a government agency pays for the installation.

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Related topic

Radon in Minnesota Homes

Volatile Organic Compounds (VOCs) in your home

Trichloroethylene (TCE) and Vapor Intrusion (PDF: 173KB/2pages)

Trichloroethylene (TCE): Screening Values & Measurement
(PDF: 147KB/3 pages)

Investigation into TCE soil vapor in the Como neighborhood of Minneapolis: The Minnesota Pollution Control Agency(MPCA) website has information about public meetings, the study area, the E. Hennepin Ave Site and the former General Mills site, maps, access agreements, contacts and more.

For more information
For questions about chemical leaks or spills, please contact us
For indoor air questions, including testing for radon, contact the Indoor Air Unit 

Printable information sheet: Vapor Intrusion (PDF: 166.66KB/2 pages)

Prepared in cooperation with the US Agency for Toxic Substances and Disease Registry

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Updated Wednesday, July 23, 2014 at 01:45PM