As a filter gets dirty, the amount of air that can flow through it is reduced. This is not a problem until less air than the engine needs gets through the filter. Without the proper amount of air, the engine will not be able to produce the power it should, nor will it be as fuel efficient as it should be.
Included in the preventative maintenance plan for all vehicles is the periodic replacement of the air filter. This mileage or time interval is based on normal vehicle operation. If the vehicle is used or has been used in heavy dust, the life of the filter is shorter.
Always use a replacement filter that is the same size and shape as the original.
The shape and size of the air filter element depends on its housing; the filter must be the correct size for the housing or dirt will be drawn into the engine. On today's engines, air filters are either flat or round.
Typical flat air filter element.
Typical round air filter for a late-model vehicle. Courtesy of Ford Motor Company.
When replacing the filter element, carefully remove all dirt from inside of the housing. Large pieces of dirt and stones may accumulate there. It would be disastrous if that dirt got into the cylinders. Also make sure the air cleaner housing is properly aligned and closed around the filter to ensure good airflow of clean air.
(i searched this info on autozone.com)