The History and the State of Vapor Intrusion in 2019

February 8, 2019

Environmental regulations for vapor intrusion (VI) have been in a constant state of flux since recognition of this potential exposure pathway decades ago. Sites such as Love Canal, where vapor intrusion modeling was part of the risk assessment, launched VI into the full attention of federal and state regulatory agencies in the late 1970s and early 1980s. Characterization of subsurface gases was included in the 1986 Resource Conservation and Recovery Act (RCRA) Corrective Action Directive guidance document and, by the mid-1990s, some states (Massachusetts, Michigan, Connecticut, and Rhode Island) required the VI exposure pathway be evaluated during environmental site assessments. In 2002, the U.S. EPA published its “Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from Groundwater and Soils,” bringing VI and associated exposure pathways to the forefront of the regulatory community. This document remained in draft form for years until the U.S. EPA issued the updated version: “Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air” in 2015. It is interesting to note that VI was not recognized as a component of an ASTM International (ASTM) environmental site assessment (ESA) until 2010, when ASTM introduced its Vapor Encroachment Screening Standard (E2600‑10), and in 2013, when VI was incorporated into the Phase I ESA Standard (E1527-13).

These federal guidance documents provided individual states a foundation from which to develop their own individual guidance documents, while the associated ASTM Standards often are the baseline for the due diligence community. According to a recent publication in the journal Remediation (2018; 28:23–25), Eklund et al. document that, as of December 2018, 42 states have issued draft or final VI guidance documents. The remaining eight states (Georgia, Mississippi, South Carolina, Arkansas, Oklahoma, Texas, North Dakota, and Wyoming) without individual guidance documents continue to adopt the federal guidance or regulate VI on a case-by-case basis. Of the 42 states with VI guidance documents, screening levels for volatile chemicals vary widely depending on the cancer risk parameters (10–5 or 10–6), attenuation coefficients, and chemical data (toxicity and exposure factors) adopted by each state. One of these chemicals, trichloroethene (TCE), has drawn recent attention from regulators due, in part, to prior health studies which have indicated the potential for fetal heart malformations in rat populations exposed to ingested TCE. These findings have been extrapolated to the inhalation exposure pathway, prompting 11 states (Arkansas, California, Colorado, Connecticut, Massachusetts, Michigan, Minnesota, New Hampshire, New Jersey, New York, and Ohio) and the U.S. EPA Region 9 to adopt trigger concentrations for short-term TCE exposure.

Example of soil gas mapping
Example of soil gas mapping

This emphasis on potential short-term TCE exposures to susceptible populations, on the order of hours to days, creates a focus on possible imminent threat scenarios in an industry historically governed by chronic exposure scenarios typically evaluated over years or decades. Trigger concentrations vary but currently hover around 2 micrograms per cubic meter (µg/m3) for residential buildings and 8 µg/m3 for commercial buildings (Eklund et al. 2018). In addition, in response to the short-term exposure concerns, some states have adopted corresponding actionable response guidance (e.g. accelerated, urgent, or imminent) depending on measured TCE concentrations that dictates responses in hours to days of discovery. Reportedly, additional studies are being conducted by state agencies and the U.S. EPA as well as by industry stakeholders to confirm or refute TCE’s adverse health effects; some suggest similar chemicals, such as tetrachloroethene (PCE), may also become the focus of short-term exposures.

State and federal agencies and their published documents provide guidance for assessment strategy and sampling techniques, applicable screening and regulatory levels, and mitigation when risks exceed thresholds. The quantity and variety of VI guidance available, as well as varied handling and interpretation by due diligence professionals, often results in confusing and contradictory information, leaving assessors, property owners, attorneys, and even some regulators scratching their heads in confusion. Agility and a willingness to be comfortable with constant change has been the norm for many consultants in the VI assessment industry. Rapid advances in field-based screening tools and instruments, as well as expanding use of chemical-specific isotope analysis (CSIA), add to the resources and complexity of the exposure pathway. In response to elevated risks, several mitigation options may be available for consideration, and should be tailored to the site-specific conditions. Detailed evaluation and mitigation planning is especially important for adapting sub-slab depressurization systems for use in large and/or complex multi-family, commercial, and industrial structures.

CEC’s experts in our VI Technical Advisory Group stay at the forefront of these regulations, advances, and technologies to assess the risks effectively and accurately and to provide practical advice to our clients. Stay tuned and stay agile as we all continue to monitor and respond to the state of VI throughout the year. If you have any questions related to VI, please contact Jim Zentmeyer, chair of CEC’s VI Technical Advisory Group, at or at (513) 483-3506.

Author: Adam Flege

About the Author

CEC Staff

Headquartered in Pittsburgh, Civil & Environmental Consultants, Inc. (CEC) provides comprehensive market-oriented consulting services to advance client strategic business objectives. CEC is recognized for delivering innovative design solutions and integrated expertise in air quality, civil engineering, ecological sciences, environmental engineering and sciences, manufacturing infrastructure services, survey/geospatial, waste management, and water resources.

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