Replacement Honeywell 50300 - 50399 Series Air Purifier Pre-Filter - Compatible Honeywell HRF-APP1 PreFilter Replacement Honeywell 50300 - 50399 Series Air Purifier Filter Replaces Honeywell HRF-APP1 PreFilter Can be cut to make 1-8 pre-filters depending on model. Filters large particles such as dust, lint, fibers & pet fur. For best results, change air filter at least once per season or more often depending on air quality and use. Essential for Allergy Sufferers. On Sale for a Limited Time!