So what do you know about vapor intrusion?

No, it is not somebody’s cheap cologne wafting into your space and causing your allergies to act up. 

It is a relatively new issue that arises in hazardous site cleanups and brownfield redevelopment.  After the source of the contamination is removed, the groundwater (http://en.wikipedia.org/wiki/Groundwater; http://www.groundwater.org/) remains.  Groundwater is found at different depths in different climates. The groundwater can also be sampled for contamination, and if it is contaminated, environmental engineers do testing to determine how far the groundwater is contaminated (called the plume).   Contaminated groundwater can be inadvertently pumped to the surface by wells or sump pumps.   In addition, it may create vapors that can enter buildings through pathways such as sewer pipes, sump pits, floor cracks or water pipes. ( http://www.vaporintrusionupdate.com/; http://dhfs.wisconsin.gov/eh/air/pdf/VI.pdf)