Vapor Intrusion Testing

Vapor intrusion is a pathway of potential exposure to volatile and semi-volatile contaminants (collectively referred to here as VOCs or vapors) that migrate from the subsurface to the air inside occupied buildings. In short, it described that the indoor breathing air gets contaminated as a result of being in proximity to soil or groundwater releases of hazardous chemicals. Generally speaking, VOCs that have been released or spilled into the subsurface display a preference to evaporate into air spaces, or voids, in the soils. These vapors can then disperse and travel through the soils to nearby buildings. This particular pathway has gained increasing attention in the past decade because of several highly publicized cases where volatile contaminants were detected in indoor air samples at concentrations higher than human health risk-based quality standards. To date, the practical experience with assessing vapor intrusion has demonstrated that this pathway can be very challenging to assess for several reasons. The risk-based quality standards for indoor air are very low, so extra care is required to avoid positive bias from contaminated sampling equipment. Several common contaminants are typically found in indoor air in residences at concentrations at or above risk-based quality standards due to emissions from consumer products, building materials and even outdoor air sources, so the relative contribution from the subsurface is often difficult to resolve. Indoor air concentrations vary with the weather and building ventilation rate, and concentrations of VOCs in soil gas and groundwater samples show spatial and temporal variability. All of these issues contribute to a relatively high level of uncertainty in the assessment of vapor intrusion.

The sampling and analysis necessary for site-specific assessment of subsurface vapor intrusion are challenging, because of the low quality standards, multiple alternative sources of vapors at these low concentration levels, and the potential for sampling bias and variability.

In the US, several studies of background concentration of contaminants in indoor air have recently been compiled. This compilation identified several contaminants (benzene, tetrachloroethene, chloroform, carbon tetrachloride, and methyl tert-butyl ether) whose average concentrations are similar to the range of common risk-based quality standards. Therefore, there will very likely be issues to resolve regarding the relative contribution of subsurface and indoor sources for vapor intrusion investigations at sites where any of these contaminants are present in the subsurface. Occupant's habits and commercial products vary considerably from house-to-house, so control properties may not provide relevant background data. Therefore, it is generally not recommended to include indoor air quality monitoring in homes outside of the study area in order to assess background concentrations. Alternate approaches have been used to assess indoor air background concentrations. These alternate approaches include:

• Qualitative comparison to published data for background (indoor) and ambient (outdoor) air quality.

• Use of a tracer compounds to select homes with no (or insignificant) subsurface impacts. Tracer compounds are compounds that are present in the subsurface and indoor air at concentrations clearly higher than background levels. This may include compounds that are not common in consumer products and building materials (e.g., 1,1-dichloroethene, cis-1,2-dichloroethene), naturally occurring radon, or other contaminants.

• Evaluation of indoor air concentrations prior to and after operation of a sub-slab venting system, or under conditions where the building pressure is manipulated to be greater than and less than soil gas pressures.

• Evaluation of the ratio of indoor air to subsurface concentrations for a large data set including a wide range of subsurface concentrations.

Alfa Chemistry is professional in air quality analysis and we can provide accurate and reliable tests results within the time constraint. We have conducted ambient air analysis, Benzene/Formaldehyde VOC Monitoring, indoor air quality analysis and mold, asbestos, radon analysis. Please feel free to contact us if you have any concern or would like us to perform any air quality testing. Check our instruments list to see our capabilities.