A diesel engine uses about 500m3 of air each hour, but even one spoonful of dust can cause irreparable damage. The air filter must allow all that air into the engine with minimal restriction but still keeping out 99.99% of the dust.
Blocked air filters mean a restricted airflow which reduces the engine power and increases the fuel consumption leading to a build-up of carbon deposits on overhaul components. Particles entering the combustion chamber because of inefficient filtering can cause wear of the valves and valve seats, leading not only to poor performance but also to damage to the turbochargers and piston rings, which are expensive to repair.