Vapor Intrusion and Vapor Encroachment in a Phase 1 ESA
Vapor intrusion is contamination migrating from beneath a property into a building above it; vapor encroachment is the risk of that happening from a contaminated property nearby. ASTM E1527-21 requires evaluating both.
Vapor intrusion and vapor encroachment describe two related but distinct risks, and the difference matters when you're reading a Phase 1 ESA report. Vapor intrusion happens when volatile chemicals migrate from contaminated groundwater or soil sitting under a building into the building itself, an on-site issue. Vapor encroachment is about off-site sources: whether a contaminated property nearby could send vapor onto, or into a building on, the subject property.
ASTM E1527-21 requires a Phase 1 ESA to evaluate vapor migration risk from both directions, including the potential for impacts caused by the historical use of adjacent and nearby properties, not just the subject property's own history.
How the screening actually works
For off-site vapor encroachment specifically, industry practice generally follows a tiered approach: a Tier 1 screening searches regulatory databases for known or suspected contaminated sites with volatile hazardous substances or petroleum products within a defined radius of the subject property, commonly cited around a third of a mile for non-petroleum releases and a tenth of a mile for petroleum releases. If that screening raises a concern, a Tier 2 review digs into more detailed records and can include actual soil gas or indoor air sampling.
The screening reaches one of a few conclusions: a vapor encroachment condition exists, likely exists, can't be ruled out, or can be ruled out based on the available information. A finding of "exists" or "likely exists" is treated the same way any other Recognized Environmental Condition would be, as a reason to consider further investigation.
Why this is worth understanding as a buyer
Vapor risk is genuinely one of the harder findings to fully resolve without direct testing, since it depends on conditions you can't see from the surface on a neighboring property you don't control. If a Phase 1 ESA flags a vapor concern tied to a nearby site, ask specifically what that means for your intended use of the building, particularly if it involves regular occupancy.
Sources
Get 3 quotes for your Phase 1 ESA
Tell us about your property and timeline and we will pass your details to Phase 1 ESA providers serving your area.