A furnace filter has a very important job. It’s job is to protect the blower fan from all of the hair, dust and other dirt the return duct brings in.
Furnace filters are rated using MERV– Minimum efficiency reporting value. These ratings range from 1-16. A higher rating does mean that the filter will be able to remove more dirt however it also means that less air will be flowing through and can force your furnace to work harder. For most homes a rating between 8-11 is recommended.