What is vapour intrusion risk and should I worry about it buying near a former industrial site in Ontario?

Vapour intrusion is the process by which chemical vapours from contaminated soil or groundwater migrate upward through soil and into the indoor air of a building sitting above or near the contamination, most often associated with volatile substances like those from fuel or solvent spills. Unlike surface contamination you might see or smell, vapour intrusion can affect indoor air quality inside a home even when the contamination source is some distance away and completely invisible from the surface.

Buying near a former industrial site, gas station, or dry cleaner is a reasonable trigger for asking about this specifically, since these are common sources of the volatile compounds most associated with vapour intrusion. A standard Phase 1 assessment will identify whether this risk is plausible based on nearby historical uses and known contamination, and if so, a Phase 2 assessment can include specific vapour or soil gas sampling designed to measure whether vapour intrusion is actually occurring or likely. This is a more specialized and less commonly requested test than basic soil or groundwater sampling, so buyers with real reason for concern should ask their environmental consultant directly whether it's included in their scope of work.