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Filter media are the portion of a filtering system that separates unwanted particles from the substance being filtered. The type of material used as a filter medium is dependant upon the material being filtered. In general, there are two common types of filters, each using a different set of media. These types are liquid and gas filters. However, the term filter also refers to many other types of specialized separation devices, such as optical filters (used in photography, astronomy and scientific instrumentation) and audio filters.

In an air filter, the most common filter media are paper, cotton and foam. Paper provides an inexpensive, though less durable medium. It is typically used in automobile applications. Foam is used as a lightweight refractory material for a multitude of industrial applications. Though often less porous than other filter media, they offer stability, thermal conductivity and the ability to isolate specific materials. Cotton is recognized as an exceptional air filtration medium because it allows for high airflow while providing the ability to filter minute particles.

Perhaps one of the most popular commercial air filters is the HEPA, or high-efficiency particulate air, filter. With an excellent capability to remove airborne particulates, HEPA filters owe their existence to atomic-bomb research, where they were used to filter radioactive contaminants from the air.

Liquid filters remove impurities, contaminants and valuable materials from fluids. Filter media for liquids include fiber, granular beds, woven fabrics and metal screens. As you might expect, the best choice depends on the nature of the particles and fluids being separated. It also depends on whether you intend to remove impurities or to recover materials within the fluid. Metal screens are effective when removing larger particles from a fast-moving liquid, while woven fabrics are useful for removing much smaller materials from slower fluids. Some filtration systems allow you to switch filter media based on your current requirements.

The most important traits of a filter medium are its strength, stability, resistance to wear and stretch, absorption, permeability and cost. Disposable oil and air filters are relatively easy to replace, and thereby make up for their short lifespan with cost-efficiency. More expensive filters can often provide longer life or more effective filtration.