Vapor Intrusion

Although the United States Environmental Protection Agency (USEPA) published guidance in 2002, the subject of vapor intrusion continues to be an emerging issue in risk assessment. Vapor intrusion is the term used to describe the exposure to contaminants in soil or groundwater via inhalation of vapors in indoor spaces. For volatile chemicals, inhalation exposure may occur as a result of migration of vapors from the subsurface soil or groundwater through the foundation of a building and into the indoor space. In their guidance on vapor intrusion, the USEPA described a three-tiered evaluation process: primary screening, secondary screening, and site-specific pathway assessment. The diagram below illustrates the migration of vapor from the source to a point of potential human exposure.

A tool that is commonly used to calculate risks and develop health protective screening levels for indoor air due to vapor intrusion is the USEPA version of the Johnson and Ettinger model (J&E model). The J&E model allows for the use of USEPA recommended default assumptions or site-specific parameters when available. In addition to USEPA default values, many states have developed their own default parameters when investigating the vapor intrusion pathway. For example, the USEPA recommends a default parameter of 0.25 air exchanges per hour for buildings. The California Department of Toxic Substances Control (DTSC) on the other hand, uses a value of 0.50 air exchanges per hour for residential buildings as a default parameter.

In conjunction with the J&E model, site geology and site-specific soil gas studies are often used when performing a risk assessment. Passive and active soil gas sampling techniques aid in the evaluation of various vapor intrusion scenarios that are possible for a single site. The Interstate Technology Regulatory Council (2007) addresses the complexity of vapor intrusion investigations and provides guidance for investigative approaches for typical scenarios.

The factors that may influence a vapor intrusion site and add complexity include:

• Structural differences
• Ambient sources
• Detection limits
• Indoor air sources
• Regional toxicity values
• Site geology

In recent years, governmental regulators have placed an emphasis on the vapor intrusion pathway and its impact on human health for both residential and industrial/commercial scenarios. Trichloroethylene has drawn particular interest due to its presence at many historically contaminated sites. Trichloroethylene has been detected at approximately 60 percent of National Priority List sites. In fact, the USEPA has reopened some previously closed sites where trichloroethylene is a primary chemical of concern, based on the vapor intrusion pathway. The growing awareness of vapor intrusion has caused interested parties both in the regulatory community and private sectors to re-evaluate sites that have a known history of contamination and/or remedial actions associated with them.

Use of generic procedures developed by the USEPA to identify vapor intrusion concerns identifies a high percentage of sites as being of interest. For example, cancer risk-based vapor intrusion screening levels for benzene, trichloroethylene, tetrachloroethylene, and other chemicals in groundwater are well below their respective drinking water standards. These overly protective screening procedures may result in the identification of a site as a vapor intrusion problem when there is none. Ultimately, investigation of vapor intrusion sites may require monitoring of volatile organic chemicals soil vapor in basements, crawlspaces, and the indoor air of residences and businesses.

CTEH has the capabilities to provide a variety of services for evaluating the vapor intrusion pathway, including guidance and support for soil gas work plans, on-site air monitoring, interpretation of air sampling results, human health risk assessments, review of historical human health risk assessments, development of risk-based cleanup levels, interaction with regulatory agencies and participation in public meetings CTEH has an extensive amount of experience when evaluating and communicating the results of indoor air studies and possible health risks to homeowners and businesses. For more information regarding vapor intrusion, please contact Casey Harbison at 501-801-8500 or by email at charbison@cteh.com