Polluted mist (4)
I believe the answer is:
smog
'polluted mist' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for smog that I've seen before include "Man-made atmospheric pollution" , "Pollutant fog" , "Fog and smoke mix" , "Pollutant haze" , "Bad visibility caused by emissions from chimneys" .)