Genuine AirDoctor 3000 and 3500 Universal Pre-Filter Replacement Filters are thin filters that attach to the front of VOC Carbon filters and are designed to fit both the AirDoctor AD3000 and AD35000 units. Designed to capture larger airborne contaminantst, it is step one of a powerful air cleaner that captures particles at 0.003 microns, which is 100 times smaller than the HEPA standard thus much more powerful than most other premium HEPA filters! With the AirDoctor we provide professional UltraHEPA filtration whether you are looking for home air purifiers that are superb for offices classrooms hospitals, etc. Before replacing the UltraHEPA or carbon/gas trap/VOC filter of your AirDoctor air purifier, turn off and unplug the unit from the electrical outlet and read all safety instructions and important warnings in this owner's manual. Customers who purchased this item also purchased: AirDoctor 3500, AD3500, AD 3500, AirDoctor AD 3500, AirDoctor 5500, AirDoctor AD 5500, AirDoctor 2000, AirDoctor AD2000, AirDoctor 1000