Crossword Genius

Unfresh air (4)

Ross

I believe the answer is:

smog

(Other definitions for smog that I've seen before include "Fog polluted by smoke" , "Polluted haze" , "Fog and smoke mix" , "Bad visibility caused by emissions from chimneys" , "Thick polluted mist" .)

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