Until the past several years, environmental assessments and investigations focused on soil and groundwater contamination and the potential threat of ingestion and dermal contact associated with that contamination. Little initiative had been placed on the potential for impacts into structures associated with the vapors emanating from subsurface soils and groundwater. Vapor intrusion (VI) occurs as a result of the migration of volatile chemicals from the subsurface into overlying buildings. In the last five years, the U.S. Environmental Protection Agency (U.S. EPA) has placed VI at the top of its leading concerns. The agency issued the “Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils” (U.S. EPA, 2002). The EPA draft VI guidance contains a systematic, tiered approach to evaluating VI. Since that time, more than 20 states have issued similar guidance documents, or at least have pilot programs in place to evaluate and address VI. In most cases, the state guidance documents or programs closely follow the EPA Draft VI Guidance.

Much of the concern associated with VI stems from the fact that the long-term, risk-based, indoor air levels published by the EPA and state agencies for several of the common VOCs, such as perchloroethene (PCE) and benzene, are very low (parts per billion). Further, many of the common compounds associated with VI, such as PCE, trichloroethene (TCE), and benzene are carcinogenic. In most cases, there is no obvious sign there is potential VI, but breathing in low concentrations of these VOCs over a long period of time can result in an increased risk for developing cancer or other chronic ailments. As such, residual soil and groundwater impacts, even at relatively low concentrations, could still pose a threat with respect to VI.

Though commercial and industrial workers of impacted sites are subject to risk, much of the attention given to VI has been associated with residential properties near current or former manufacturing facilities, dry cleaning operations and fueling stations. Activities at these facilities could result in impacted groundwater and soils.  If these impacts migrate off-site, it is possible that adjacent houses and businesses could be affected by VI. In some of the higher profile VI cases, VI was associated with impacted groundwater that migrated greater than a quarter-mile from the source area.

The presence or even the potential presence of VI could thwart property transactions and development deals, slow down the closure process for sites currently in regulatory programs, and add increased burden to the regulated community. Additionally, there have been situations where sites that were closed or were implementing final remedies have been reopened because of potential concerns associated with VI. The VI concern has several ramifications. Developers that build over an impacted area could suffer increased costs, difficulty procuring bank loans and threats of litigation, as commercial and housing residents take legal action over the dangers of living in a contaminated area. For sites undergoing investigation and closure, the use of risk-based approaches that lead to less stringent cleanup standards may become more difficult.

For these reasons, VI has become a significant issue and one that can greatly affect property transactions, site cleanups, and site closures. It is unlikely that the concern over VI will be abated any time soon. Instead, it seems that VI is gaining traction with regulators and becoming an issue of increased importance. Therefore, it is important that this exposure pathway be fully considered and evaluated during the early phases of the project to ensure project success.