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Environmental Health Division
Commonly Asked Questions at
Vapor Intrusion Investigation Sites
Contaminants in indoor air resulting from vapor intrusion are often estimated using the concentrations in a sub-slab sample multiplied by an attenuation factor (AF).
An AF accounts for the reduction in concentration that occurs when vapors enter a building and mix with indoor air. The Minnesota Pollution Control Agency (MPCA) is using a very protective AF of 0.03, (or about 3% of the soil vapor found in the sub-slab will be in indoor air). The amount of soil vapor that enters a building is different for every building, but this AF overestimates indoor air concentrations in most homes.
EPA recommended the AF of 0.03 after looking at data from numerous homes across the country (USEPA, 2015). 95 percent of the homes studied had (basement) indoor air concentrations less than or equal to 0.03 times the sub-slab concentration, and 50 percent of the homes had (basement) indoor air concentrations less than or equal to 0.003 times their sub-slab concentration. An example of the use of these AFs to predict what may be in indoor air is described below:
Estimated Basement Air Concentrations Due to Vapor Intrusion (µg/m3)
|Concentration of TCE/PCE in sub-slab (µg/m3)||Predicted TCE/PCE concentration in the basement indoor air: 95th percentile attenuation factor of 0.03 (USEPA, 2015)||Predicted TCE/PCE concentration in the basement indoor air: median attenuation factor of 0.003 (USEPA, 2015)|
The amount of soil vapor that actually enters a building can be influenced by many factors. For more information, see the MPCA webpage Understanding Your Vapor Intrusion Test Results (PDF).
In order for there to be a health concern, contaminated vapor has to get into the indoor air of a building at relatively high levels AND people need to be present to breathe the contaminated vapor over time.
- Health risks from vapor intrusion are typically low. Sub-slab concentrations that are above sub-slab screening values may result in contaminant concentrations in basement indoor air above Intrusion Screening Values, but this does not mean that health effects will occur.If you are concerned about your health, please contact MDH to discuss your situation.
- MDH is most concerned about people who spend most of their time at home or in their basement (basement bedroom, home office, etc.). Indoor air contamination from vapor intrusion is typically the highest in the lowest level of a building. The first floor air generally has less contamination from soil vapor than the basement and the second floor air generally has less than the first floor.
- MDH is less concerned about vapor intrusion in the warmer months of the year. Vapor intrusion is highest in the during the winter months. Heated indoor air can cause house depressurization which tends to draw air up through the home. Sub-slab soil vapor levels can rise over the course of the winter because vapors don’t escape through frozen ground. In addition, there is much less fresh air brought into the home in the winter.
If soil vapors are found under your home or building that could indicate a concern, a mitigation system can be installed to remove vapors from beneath the foundation and vent the vapors to the outside air. These are the same systems commonly used to keep radon from entering homes. They are relatively inexpensive to operate, simple to design and install, and are a proven solution to radon and vapor intrusion problems.
In your home:
Levels of chemicals found in indoor air from vapor intrusion are typically low. The level is similar to what you may expect if there are products in the buildings that contain those same chemicals. Volatile Organic Compounds (VOCs) are commonly found in indoor air inside homes. For example, freshly dry cleaned clothes can bring PCE inside a home. TCE and PCE have been found in solvents and cleaners that people may store in their basement. See the MDH webpage for more information about Volatile Organic Compounds in Your Home.
In outdoor air:
VOCs can also be found at low levels in outdoor air. For example, long-term averages for TCE and PCE in outdoor air in Minneapolis are approximately 0.1 µg/m3.
In the workplace:Exposures in the workplace (occupational exposures) to VOCs may be much higher. They can be hundreds to tens of thousands times higher than residential soil vapor exposures.
Comparison of TCE/PCE values
|Average Outdoor Air Concentrations in Minneapolis*||0.1||0.1|
|Average Indoor Air Concentration in three Twin Cities communities in 1999 (Sexton, et al., 2004)||0.5||2.9|
|MPCA Residential Intrusion Screening Value||2.1||3.4|
|MPCA Workplace Intrusion Screening Value||7||33|
|Level in air you can smell||150,000||7,000|
|American Conference of Governmental Industrial Hygienists occupational standard||55,000||170,000|
|MN OSHA occupational standard||270,000||170,000|
* MPCA Ambient Air Monitoring Network, 2008-2014
Municipal drinking water usually comes from deep wells or surface water. Vapor intrusion is often associated with contamination of shallow groundwater or soil.
- Municipal drinking water is routinely tested for contamination to ensure that drinking water meets standards.
- If you use a private well for drinking water and your property is undergoing a vapor intrusion investigation, contact us for more information.
MDH does not recommend that people have a medical test for VOCs when living on sites where vapor intrusion may be occurring. Levels of chemicals found in indoor air from vapor intrusion are typically low. It is unlikely that tests will be able to detect VOCs or their metabolites at concentrations people may be exposed to from vapor intrusion.
- There is not a medical test to determine if people have been exposed to VOCs in the past. VOCs do not accumulate in the body.
- VOCs and their breakdown products from high current exposures (such as occupational exposures) may be found in blood or urine.
Instead, if environmental sampling indicates vapor intrusion may be occurring, MDH recommends installation of a vapor mitigation system to reduce or eliminate exposure.
MDH does not know how vapor intrusion investigations affect property values; however, installation of a mitigation system can be a positive addition to a home.
Minnesota state law now requires a home seller to disclose in writing to the buyer any knowledge the seller has of radon concentrations in the dwelling. MDH strongly recommends that all homebuyers have an indoor radon test performed prior to purchase or taking occupancy, and recommends having the radon levels mitigated if elevated radon concentrations are found. Thus, having a mitigation system in place is a positive attribute when selling a home in Minnesota, especially if radon levels are elevated prior to installing the system. See the MDH webpage for more information about Radon and Real Estate in Minnesota.
Prepared in cooperation with the US Agency for Toxic Substances and Disease Registry