The migration of volatile contaminants from subsurface soil or groundwater into the indoor air of overlying buildings. A primary exposure pathway and risk driver at chlorinated solvent and petroleum hydrocarbon sites.
Vapour intrusion occurs when VOCs in subsurface soil gas migrate upward through cracks, utility penetrations, and porous building materials into indoor air. Trichloroethene is the canonical concern: chronic indoor inhalation at parts-per-billion levels is associated with cardiac and developmental effects, and TCE drove the U.S. EPA's 2015 Vapor Intrusion Technical Guide and accelerated cleanup pace at hundreds of sites.
VI assessment combines sub-slab soil gas sampling, indoor air sampling, and attenuation modelling. Regulatory screening levels are well below ambient outdoor air concentrations for several solvents, which makes VI one of the most operationally demanding exposure pathways to manage.
A vapor intrusion mitigation system is the engineered control installed when assessment confirms unacceptable indoor risk. The standard design is sub-slab depressurization, adapted from radon mitigation: suction points through the slab connected to a continuously running fan that keeps the sub-slab zone under negative pressure, so soil gas is captured and vented above the roofline instead of entering the building. Passive vapour barriers and membranes are standard in new construction over impacted groundwater; existing buildings typically need active systems. Mitigation buys protection immediately, but it manages the symptom. Durable closure comes from treating the source: the solvent or petroleum mass in soil and groundwater below the building.
Also known as: vapour intrusion, vapor intrusion, VI, soil vapor intrusion, vapor intrusion mitigation, vapor intrusion mitigation system.
Vapour intrusion is the migration of volatile chemicals from contaminated soil or groundwater up into the indoor air of buildings above. Solvents such as TCE and PCE and petroleum vapours are the most common concerns, because chronic inhalation at very low concentrations can affect health.
Most commonly a sub-slab depressurization system: suction points installed through the building slab and connected to a continuously running fan that keeps the soil beneath the building under negative pressure. Contaminated soil gas is captured and vented above the roofline rather than entering indoor air. New construction often adds passive vapour barriers and membranes.
Assessment combines sub-slab soil gas sampling (probes installed beneath the building slab), indoor air sampling over 24 hours or longer, and comparison against regulatory screening levels. Multiple sampling rounds across seasons are often required because vapour intrusion varies with weather and building operation.
It can be, depending on the chemical and concentration. TCE and PCE have indoor air screening levels in the low micrograms-per-cubic-metre range because chronic exposure is associated with cardiac and developmental effects. Confirmed exceedances are treated as priority actions by regulators, which is why mitigation systems are installed quickly once a pathway is verified.
No. A mitigation system protects building occupants immediately, but the contaminant source in soil or groundwater remains. Durable closure requires treating the source itself, through in-situ thermal, chemical oxidation, or excavation, so the mitigation system can eventually be retired.
Chlorinated Solvents (TCE, PCE, TCA, DCE, VC): treatment approach
Man-made chlorinated aliphatic hydrocarbons — tetrachloroethylene (PCE), trichloroethylene (TCE), 1,1,1-trichloroethane (TCA), cis-1,2-DCE, and vinyl chloride — that formed the industrial degreasing and dry-cleaning solvent market for four decades and left the densest DNAPL groundwater plumes in North America.
The regulatory metric for thermal destruction performance, expressed as the percentage of a target contaminant destroyed across the system. Common permit thresholds range from 99.99 percent for general hazardous organics to 99.9999 percent (six nines) for PCBs, dioxins, and PFAS.
The duration that gas, vapor, or solids remain inside the heated zone of a thermal treatment unit; together with temperature and turbulence, one of the three governing parameters of complete contaminant destruction.
Liquid that has percolated through contaminated soil, waste, or stockpiles, dissolving and carrying contaminants downward to groundwater or to a collection system. The principal migration pathway for PFAS, dissolved metals, salts, and hydrocarbons from impacted sites.
Laboratory method that oxidizes PFAS precursors in a sample into measurable perfluoroalkyl acids, then quantifies the difference against a pre-oxidation baseline. Reveals the hidden precursor mass that targeted PFAS analysis alone misses.