Home inspection concern (5)

I believe the answer is:

radon

(Other definitions for radon that I've seen before include "hazardous substance" , "Gaseous element, Rn" , "Inert gas - adorn (anag)" , "Radioactive gas, linked to granite" , "A radioactive gas, symbol Rn" .)

I've seen this clue in The New York Times.

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