Coarse filtration is used in first stage filtration, especially filtering pollutants with a particle diameter of 10 µm and above, by retaining large particles, dust and dirt in the air. These filters are usually made of fiber, sponge, polyurethane or other coarse materials and allow clean air flow by trapping large particles in the air. Coarse filtration is an initial stage used to improve air quality and ensure that finer particles are captured in further filtration stages.