Air quality index factor (4)
I believe the answer is:
smog
'air quality index factor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for smog that I've seen before include "Smoky fog" , "Pollutant fog" , "Pea-souper" , "Pollution haze/fog" , "Polluted atmosphere" .)