This is a replacement filter for the fit 50 my parents bought me when my fiancé was diagnosed with Hodgkins Lymphoma. When she came home after her stem cell transplant it was important that my bed ridden fiancé be in as germ free environment as possible. I say all of this because while not required by our doctors the Fit 50 with silver filter gave my family and I great piece of mind that my fiancé was continuously being protected from illness as she worked to rebuild her immune system from scratch. A year later I get to tell you not only about my piece of mind over the last year but get to report that my fiancé is now CANCER FREE. I truly believe that your purifier protected the love my life from any airborne illness in her greatest time of need. THANK YOU!