Extractor fans were a function of the Doctor's TARDIS that the Eleventh Doctor was surprised to find existed. They evidently were used to purge toxins from the air inside the TARDIS. They could be turned on with the phrase, "Extractor fans on". The Doctor used them after Mels shot the TARDIS's time rotor and poisonous smoke was emitted, (TV: Let's Kill Hitler) and again after an attempt to navigate to 1938 New York damaged the TARDIS. (TV: The Angels Take Manhattan)
He also used them in an alternate timeline, where the TARDIS had been heavily damaged and the atmosphere of the main control room was, initially, unsafe to breathe until the extractor fans were activated manually via the console. (TV: Journey to the Centre of the TARDIS)