According to the EPA, vapor intrusion occurs when chemicals migrate from any subsurface source, such as contaminated soil or groundwater, into an overlying building.
In very basic terms, this happens when chemicals in the ground emit vapors and pressure and temperature differences pull them through the foundation into indoor air.
The most common chemicals of concern are chlorinated solvents like trichloroethylene (TCE) and tetrachloroethylene (PCE), and petroleum hydrocarbons such as benzene.
Where does it come from?
Former dry cleaners, gas stations, manufacturing facilities, and landfills are sites where residual contamination can persist in soil and groundwater for decades. As urban infill and brownfield redevelopment accelerate, more new buildings are going up in proximity to land used for such past purposes. Remediation must take place before construction can start, and vapor barriers are often required to prevent the migration of any contamination into new structures.
To try and understand how these chemicals may behave, vapor intrusion pathways have become a routine consideration in environmental due diligence, including vapor encroachment screening under ASTM E2600.
Why is below-grade construction the pressure point?
The foundation is where the building literally meets the contaminant source: Slabs, elevator pits, sump openings, utility penetrations, and cold joints all provide entry routes. Buildings also tend to run at slightly negative pressure relative to the ground beneath them, which actively draws vapors inside.
According to EPA, occupants breathing contaminated vapors can face both acute and chronic health risks, and TCE in particular carries short-term exposure concerns.
What does the guidance say?
The technical framework comes primarily from EPA's 2015 Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (OSWER 9200.2-154) and its companion guide for petroleum sites. ITRC's new Vapor Intrusion Toolkit, which consolidates and updates its earlier VI-1, PVI-1, and VIM-1 documents, now represents the current state of practice for investigation and mitigation. Many states layer their own screening levels and requirements on top. We explored state-level regulation in a two-part blog series last year. (Here's Part II of the series.)
Why address it during construction?
For new below-grade construction, the most reliable and least expensive time to manage vapor intrusion is before the slab is poured. Products like EPRO’s Geo-Seal line of vapor intrusion mitigation systems can suit a wide range of site conditions and are engineered for preemptive protection on lower risk sites, or multi-layered protection for eliminating vapor intrusion for brownfields or any environmentally impaired site.
Contamination conditions rarely announce themselves. Understanding vapor intrusion pathways early, at the Phase I stage rather than after occupancy, keeps a manageable design detail from becoming a liability. If your project involves a known or suspected contaminated site, reach out and we will figure out a solution, together!