Vapor intrusion (VI) is one pathway for building occupants to come in contact with chemicals of concern, occurring when vapors from contaminated soil and/or groundwater migrate into the indoor air of overlying structures. The American Society for Testing and Materials (ASTM) Standard Practice for Assessment of Vapor Intrusion into Structures on Property Involved in Real Estate Transactions E 2600-08 has been developed based on an industry need to address the VI pathway. Prospective property owners and investors are concerned with VI because in several cases, sites that have been closed are being re-opened since remediation activities did not consider the VI pathways. Owners also are dealing with liability from tenant or third party lawsuits and devaluation of property due to the stigma associated with VI. Phase I Environmental Site Assessment (ESA) consultants have become concerned with the VI issue based on increasing amount of litigation against environmental professionals who did not consider the possibility of VI when conducting their Phase I ESA. Should VI be within the scope of a Phase I ESA? ASTM has determined it is not, thus the development of the VI standard.

The development process is not finished. The standard is currently under revision by the ASTM Task Group and several changes are anticipated. One major change is the replacement of the term Vapor Intrusion Condition (VIC) with the term Vapor Encroachment Condition (VEC) to clarify the purpose of the screening is focused only on the potential for migrating vapors to reach the subsurface of the target property. Further investigation to assess the possibility of VI into structures on the target property will be beyond the scope of the revised E 2600 document, just as a Phase II further investigates the potential for subsurface contamination is beyond the scope of a Phase I ESA.

Although the standard will be revised, the goal to create a standard practice for evaluating the VI pathway will remain. The standard as it is currently written is a four tiered process. Tiers one and two are screening tools used to identify potential sources of VI and Tiers three and four involve assessment activities in the form of sampling as well as mitigation. Tiers three and four may be taken out of the standard post revisions, so this article will only focus on Tiers one and two.

Tier one is designed to be used as an initial screening tool, relying on the information that has already been collected during the Phase I ESA. In a Tier one, government and historical records are reviewed to identify whether facilities where chemicals of concern (COC) are likely present and are located in close proximity to the property. The size of the area of concern that is defined in the standard (1/10 mile for petroleum volatiles and 1/3 mile for remaining volatiles) reflects plume lengths at the 90th percentile, which have been derived from known plume length data obtained from the National Groundwater Association, dry cleaner trust fund data and the American Petroleum Institute.

The Tier one screening also involves environmental professional judgment. The following factors are what an environmental professional considers when determining whether there is a potential source for vapor intrusion:

• Sensitivity of the target property
• Type of COC
• Groundwater gradient
• Depth to groundwater
• Cleanup status of any contamination on or near the site
• Soil characteristics that could affect permeability
• Presence of conduits or preferential pathways to the indoor air
• Hydraulic or physical barriers to the site or to the  indoor air on the site
• Characteristics of existing or planned structures such as the presence of basements or intrinsically safe construction/design

The Tier two involves a more refined screening process. Information on on-site and near site plumes is collected either through non-invasive known data or through invasive sampling. The plume data is evaluated based on concentration and distance from the site structures.