Combined air filters work by drawing in unclean surrounding air and passing it through a combined air filter medium with fine narrow or progressively smaller openings. The unwanted particles that are larger than the openings are trapped inside or by the progressively denser filter media and are separated from the passing air.

Air leaves the filter with reduced particulate content. The efficiency of most commercial particulate filters increases, goes up. or improves as the combined air filter loads up, reducing the opening in the filter media. A filter's efficiency, on day one, can be 50% of what it is when replaced. As the particles in Combined air filters increase, their filtration efficiency, as particles form a layer in the upstream side of the filter media; aids in screening other particles.

Combined air filter medium is replaced regularly or cleaned to prevent a filter from clogging or blinding and restricting airflow. While a combined air filters efficiency increases as it loads up, so does its resistance to the airflow, which can have a negative effect on cooling and heating costs.