Definition
Vapor intrusion is the process by which volatile chemicals from contaminated soil or groundwater migrate upward as vapor and enter the indoor air of overlying buildings through cracks in foundations, utility penetrations, or other openings.
It is a significant environmental risk because contamination may affect occupant health, trigger regulatory action, and create liability exposure for property owners.
How It Occurs
- Historical spills or leaks release petroleum or solvents into soil/groundwater.
- Volatile compounds evaporate and form subsurface vapors.
- Pressure differences draw vapors into buildings.
- Indoor air concentrations may accumulate over time.
Common Chemicals Involved
- Benzene (often associated with petroleum releases)
- Trichloroethylene (TCE) and Perchloroethylene (PCE) (dry cleaner solvents)
- Other volatile organic compounds (VOCs)
Why It Matters to Investors
Vapor intrusion can exist even when soil contamination appears limited. Because it directly affects indoor air quality, it may lead to:
- Tenant complaints or health claims
- Mandatory mitigation systems
- Regulatory oversight and monitoring requirements
- Disclosure obligations during sale
- Reduced asset liquidity
How It’s Evaluated
If a Phase I Environmental Site Assessment (ESA) identifies potential vapor risk, a consultant may recommend further testing:
- Soil gas sampling
- Indoor air sampling
- Sub-slab vapor sampling
Mitigation Options
- Sub-slab depressurization systems (similar to radon systems)
- Sealing foundation cracks and penetrations
- Improved building ventilation
- Long-term monitoring programs
Example
A multifamily property sits adjacent to a former dry cleaner site. Although no direct contamination is documented on the subject parcel, a Phase I flags potential vapor migration. Indoor air sampling detects elevated VOC levels, requiring installation of a sub-slab depressurization system before refinancing.
Common Pitfalls
- Ignoring adjacent properties: contamination can migrate across parcel lines.
- Assuming “no soil impact†means no risk: vapor can travel independently of visible contamination.
- Underestimating timeline: regulatory review and mitigation installation can delay transactions.
- Weak disclosure planning: failure to document mitigation may complicate future sales.