The Winix Filter NK305 is suitable for the Winix NK300 and NK305 air purifier. This filter is a complete filter cartridge consisting of a pre-filter, carbon filter and HEPA filter. The carbon filter and pre-filter ensure that the large dust particles are removed from the air and that odors are absorbed. The HEPA filter removes fine dust particles from the air. When you have to change the filter of your air purifier depends on the use. On average you replace the filter after a year.