Removing airborne dust and dirt benefits the people breathing the air and the machinery moving the air. Dirt accumulates on bearings and motor windings and gradually restricts air flow through a heating or cooling coil and settles in ductwork.
Collecting the dust from before it passes through the furnace or air helps to protect the machinery, and the filter should always be installed in the return air duct.
Mechanical or adhesive filters are the least expensive but the least efficient. Constructed of spun fibers shaped to form a mat, they are framed with cardboard to specific sizes. As air is moved across the fibers, dust is attracted and sticks to an oil film or glue sprayed on the fibers. This type of filter collects between 6% and 8% of the dust passing through when new. Efficiency will increase as the filter gets dirtier, but air flow will be restricted as well. Available at many hardware and retail stores, they are inexpensive, and should be replaced monthly if the system is in constant use.
Some manufacturers include a new filter with the equipment that is rubber coated and washable. Coarse fibers are rubberized to bind them together and make them sticky. As air passes through, dust collects on the fibers. A blast from the sink sprayer or garden hose will rinse the dirt away and ready the filter for re-use.