Adhesive filters made of various fibers that are coated with adhesive liquid or out to which the particles of lint and dust adhere to. These filters will remove as much as 90% of the dirt if they do not become clogged. The more common filters are of the throwaway or disposable type. These filters should be replaced at least twice each year. The permanent air filter found in most systems is usually made of metal rather than the cardboard type described earlier. These types of filters are cleaned by using warm soapy water.