Until three years ago, public health officials thought that when they had cleaned up spilled toxic solvents in the ground, their work was done. But then they learned about "vapor intrusion."

That is a process where the remaining traces of contaminants -- such as tetrachloroethylene -- form a gas that migrates through the soil into adjacent structures, creating a health hazard. Some of the chemicals are known carcinogens and others could create other health problems.