The potential for Vapor Intrusion (VI) to migrate into structures is an important environmental issue with more than 500,000 suspected contaminated sites. Background contamination, preferential pathways, the invasive nature of sampling and other factors make Vapor Intrusion difficult to evaluate. The variability of screening criteria developed by state, regional and federal agencies also contribute to the complexity of an assessment. VI screening criteria can be designed for groundwater, sub-slab (shallow) soil gas, exterior (deep) soil gas, indoor air, emergency action or notification. These categories can be further divided into residential, non-residential, industrial and occupational.

Significant differences exist between the occupational exposure standards, such as the OSHA Permissive Exposure Limits (PELs) and action levels or guidance adopted by state or federal environmental and health regulatory agencies to address exposures resulting from VI. The assumptions used to formulate occupational standards are very different from those used in developing VI standards or guidance, yielding radically different criteria for acceptable exposures and the need for mitigation. A significant lack of consensus also exists between regulatory agencies. Each agency has its own set of parameters and procedures to develop screening criteria, some based on policy, others promulgated in law. Finally, even within a particular agency, the application of VI criteria can vary given the situation, depending if the contaminant concentrations represent an acute exposure risk requiring immediate action such as building evacuation or a chronic exposure risk requiring long term mitigation.

The development of adequate VI screening criteria represents a challenge for regulators trying to focus resources on the protection of the public health. It is becoming increasingly apparent that not all No Further Action (NFA) decisions provide the same level of confidence through time. Some NFAs will likely need to be revisited, even where VI was previously addressed. The increase of third party purchasers and landowners interested in protecting themselves, coupled with the increasing role of for-profit environmental insurance, VI assessments are no longer only for regulators and the responsible parties. There is a need for protection of innocent purchasers of properties that were not the source of the contamination for VI but located near contaminated properties that are potential sources of VI. The recently formed ASTM VI Task Group (E50.02.06) is evaluating the need to include some level of VI assessment into the due diligence requirements for property transfer.

Because of the complexity and uncertainties associated with this evolving issue, engineering review including VI assessment has become critical in the underwriting process for environmental insurance.

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