[crmalcolm]

https://www.mclaren.com/racing/f1-playbook/dirty-air/

As air passes over a Formula 1 car’s surfaces it produces a wake of turbulent air that hampers the aerodynamic flow of cars directly behind it. This wake – nicknamed ‘dirty air’ – can be of benefit to a following car on the straight, as the car in front is effectively punching a hole in the air and doing more work.

Dirty air does, however, hamper the efficiency of the following car’s own aerodynamic surfaces, reducing downforce, making it slower in the turns, and limiting the effectiveness of the cooling system.