Cyclones are less efficient when the dust density is:

The efficiency of cyclones, which are used for dust collection and air cleaning, is significantly affected by the density of the dust particles being collected. When the dust density is low, the cyclone struggles to effectively separate lighter particles from the air due to insufficient mass for the centrifugal forces to act upon.

In a cyclone, the collection efficiency relies on the inertia of the particles. Higher density particles are better able to resist the air flow and are more likely to be thrown towards the wall of the cyclone for collection. Conversely, with low dust density, there may not be enough particle mass to contribute to the inertial forces, making it more difficult for the cyclone to capture and remove those particles efficiently from the airflow.

In contrast, higher density dust allows for greater inertia, helping improve the separation process. Therefore, cyclones operate optimally when the dust density is at an appropriate level, rather than too low, where their design features cannot be effectively utilized.