What is dirty air?

Dirty air is the turbulent, disturbed airflow left behind a moving race car, and it's one of the most important (and frustrating) aerodynamic phenomena in motorsport. 

As a car moves through the air, its wings, floor, and bodywork are designed to manage smooth, ‘clean’ airflow to generate downforce efficiently. 

Once that air passes over and under the car, it becomes turbulent: lower energy, less organized, and full of eddies and disrupted pressure zones. This turbulent wake trails behind the car, sometimes for a significant distance. 

Any car following behind has to drive through this disturbed air instead of clean, undisturbed air. 

Why it's a problem 

A following car's aerodynamic parts (wings, floor, diffuser) are designed assuming they'll receive clean airflow. When they instead get turbulent air, they can't generate downforce as effectively. 

This causes the trailing car to lose grip, especially in high-speed corners, making it harder to follow closely and even harder to attempt an overtake. The car behind simply can't corner as fast as the car in clean air ahead of it. 

The closer you follow, generally the worse the effect, though the severity depends heavily on the car's aero design (see below). 

Why some cars/series suffer more than others 

Cars that generate most of their downforce from wings (which project turbulence more directly behind them) are more sensitive to dirty air. 

Cars that generate more downforce from the underfloor/ground-effect tunnels are less sensitive, because that airflow is shaped closer to the ground and disperses somewhat differently, leaving a cleaner wake for the car behind. 

It's not always bad 

Disturbed air behind a car also reduces drag for a trailing car. This is the draft, and it can actually help a chasing car close a speed gap on straights, even though the same turbulence can hurt cornering grip when running too close through a corner.