Toxic gas, Rn (5)

I believe the answer is:

radon

(Other definitions for radon that I've seen before include "Noble gas, Rn" , "Inert gas - adorn (anag)" , "Element, symbol Rn" , "Natural gas considered a hazard to health" , "A radioactive gas, symbol Rn" .)

I've seen this clue in The Guardian.

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