The same model of air filter, under the same rated air flow rate, has the same structural resistance but different total resistance, mainly due to the different resistance of the filter material, resulting in different total resistance. The resistance of the filter material is caused by the head-on resistance of the fibers when the airflow passes through the fiber layer.

The main technical indicators closely related to filter paper and resistance:
(1) Breathability is the primary performance indicator to consider for filter paper. The higher the air permeability of filter paper, the better the air permeability, and the lower the original resistance. Conversely, the opposite is true. There is an inverse qualitative relationship between breathability and resistance.
(2) The thickness of filter paper can limit the maximum filtering area of the filter, which means that increasing the effective filtering area of filter paper also has a significant impact on the resistance of the filter. This is because increasing the effective filtering area reduces the airflow velocity passing through the unit area, resulting in a decrease in the resistance of the air filter. In order to increase the filtration area, the air filter element is made into various shapes, with the most commonly used being cylindrical.
(3) If the filtering material is non-woven fabric, it is mainly made of chemical fibers. The characteristics are low resistance, high filtration efficiency, and large ash storage capacity, but poor stiffness and strength. To compensate for this deficiency, filter paper is used as a backing to form a composite material of filter paper and non-woven fabric. Comparing the fiber diameter can also distinguish the magnitude of resistance. When the fiber is fine, there is more fiber per unit volume, and the resistance generated by the airflow around the fiber is greater. Conversely, when the fiber is coarse, the resistance generated is smaller.