Air filters are used to remove contaminants from the air such as dust, pollen, mold spores and bacteria. They can be used in internal combustion engines, gas compressors, diving air compressors and ventilation systems.

Engines usually require paper, foam, or cotton filters. Some man made environments like buildings and planes use foam filters, pleated paper, or spun fiberglass filters. Another type of filter uses a static electrical charge to catch dust particles.

Cotton air filters are considered to be the best because of technological advantages, durability, air flow and filtration.

Paper filters are usually used in automobiles because they can be tossed away and they are relatively inexpensive.

Synthetic web formations are also used for air filtration and are made from polyester fiber. They can withstand high temperatures (about 250F) and are used for industrial, commercial, or residential application. Cotton and polyester can be blended together or with other fibers to resulting in a wide range of performance characteristics. Sometimes polypropylene is used to enhance chemical resistance because of its lower temperature tolerance. In many types of HEPA (High Efficiency Particulate Air Filter) filters, synthetic fibers known as micro-fibers are used.

Spun fibreglass is used for in-duct filters for home air heating and air conditioning systems. These come in many densities and sizes, are inexpensive and can be thrown away. The less dense the higher the airflow, but they don't remove as much dust as the denser ones. The denser filters remove more dust but are more expensive and don't let air pass through as easily. They can also become more loaded with dust and dirt but they are still cheaper to use than the pleated-paper filters of the same size.

Cotton is used for engine filters because it has excellent filtration and air flow but filters particles much smaller than the actual holes. Oiled layers of cotton gauze are used for high performance air filters. Dust particles caught in the filter are held suspended in the cotton gauze and become part of the filter medium which increases filtration but doesn't obstruct airflow to an engine. They outlast paper filters but are more expensive. However, over a service interval of 50,000 miles the disposable paper filter will have already been replaced several times.

The combustion air filter and the cabin air filter are the two main types of air filters used in automobiles. The combustion air filter prevents separate particle of matter from entering the engine's combustion chambers. When the oil is changed, this filter is usually changed but may be changed at longer or shorter times, depending on the use and condition of the vehicle.

Older vehicles that have carburetors or throttle body fuel injection usually use a cylindrical air filter which is 14 inches in diameter and 3 inches tall. These are positioned above the carburetor or throttle body and secured in place with a metal lid.

Another pleated-paper filter is the cabin air filter that is placed in the outside-air intake for the vehicle's passenger compartment and come in different shapes and sizes. If the cabin air filters are improperly removed and reinstalled it can lead to water leaks and in rare cases, a cracked windshield. So if you don't know what you're doing it's best to get a professional to replace them.