Definitions for "Dirty air"
The rear wing of the car in front tends to push the air higher, creating a very turbulent low-pressure area directly behind the car. At high speeds, downforce can be disrupted by following closely behind another car. A car following closely often will suffer understeer as a result of being in this "dirty air."
The turbulence created by the passing of a car. Another car following closely in this air finds its aerodynamics disrupted. With downforce cut drastically, the car behind has less effective steering, less grip and may spin off.
Air turbulence behind or beside the car often creating a loss of downforce