Some Air Purifiers have an independently controlled ionizer, which, when turned on, releases negative ions into outgoing filtered air. Ions are tiny particles that carry a positive or negative charge. These ions exist naturally around us, in the air, water and ground. Both positive and negative ions are colorless, odorless and completely harmless. Negative ions help the air purification process by attaching themselves to very small airborne particles in the room. You may also note after extended use, that dust may have collected around the grills or front panel. This is from the ionization affect caused by the negative ions exiting from the air outlet. This is additional evidence of the air cleaning effectiveness of negative ions. The dust can be easily removed with a clean, damp cloth or soft brush. Finally, using your ionizer may result in an occasional popping or cracking sound. This is a normal sound, generally caused by particles of dust that are interrupting the flow of ions, causing a small build-up of ions that, when discharged, cause the popping or cracking sound. NOTE: It is important to replace the HEPA filter at the recommended intervals in order to maintain optional air quality.