Recent Developments in the Assessment of the Vapor Intrusion Pathway

Defined as the migration of volatile chemicals from the subsurface into the indoor air of overlying buildings, vapor intrusion (VI) has received increased regulatory attention due to its potential for posing unacceptable risks of long-term exposure via inhalation by residents or workers present in those buildings.  The evaluation of VI-related risks is complicated by the presence in indoor air of background levels of volatile chemicals unrelated to VI (e.g., from dry cleaned clothing, cigarette smoke), as well as the spatial and temporal variability of soil gas and indoor air data.  Adding to the challenge are substantial differences existing between VI by chlorinated compounds (e.g., tetrachloroethylene) and VI by petroleum hydrocarbons (e.g., benzene), which are more prone to biodegradation in the vadose zone. 

Currently, 24 states have developed guidance documents to assess and mitigate the VI pathway.  Many other states have incorporated guidelines or developed volatilization criteria in soil gas or groundwater as a first step to determine whether the VI pathway may be complete.  Indoor air screening levels for a given volatile chemical can vary by as much as three orders of magnitude between states.  Likewise, attenuation factors recommended by states to estimate indoor air concentrations from subslab soil gas data range from 0.1 to 0.001. 

US EPA was due to release the final version of its VI guidance at the end of 2012, ten years after its draft VI guidance was first issued.  This final VI guidance document will likely draw from the findings of its recent VI database and background indoor air studies.  It will also place more emphasis on using a multiple line of evidence approach and collecting empirical evidence rather than relying on modeling. 

This presentation provides an overview of recent VI guidance developments and challenges faced when assessing the VI pathway.