An air ioniser is a device which uses high voltage to ionise, or electrically charge, molecules of air. These machines can be designed either to generate specifically charged ions (all positive or all negative), or to create both polarities indiscriminately. However, most commercial air purifiers are designed to generate negative ions. Negative ions are particles that temporarily contain an extra electron, causing the entire molecule to possess a negative electrical charge. Similarly, a positive ion is deficient by one electron and has an overall positive charge. Unfortunately, the high electric fields used to create the air ions can also generate ozone (an energetic allotrope of oxygen), and NOx. Both are toxic. Even in relatively low concentrations, they can irritate lung tissues, cause chest pain, coughing, throat irritation, and can worsen the conditions of persons suffering from asthma.
Ionic air purifiers use an electrically charged plate to produce negative gas ions that particulate matter sticks to (in an effect similar to static electricity). Many ionisers are sold as air purifiers, but in this regard they are very inefficient. They will clean the air to a small degree, by charging dust and smoke particles which will then be attracted to a neutral or positively charged surface. Heavier combined particles may precipitate (fall) out of the air should two smaller particles of different charge clump together.
Criticisms of ionisers as air purifiers include:
- Cleaning range, as portable units are typically built to clean one room only.
- All the affected airborne particles ultimately wind up on surfaces close to the ioniser, making the area immediately surrounding the ioniser dirty and reducing the unit efficiency as the high voltage emitter is slowly covered in dirt. Overall cleaning efficiency roughly on par with the static charge from the front of a powered-up CRT TV screen.
- Companies or individuals who sell (or support the use of) air ionizing devices claim additional, less easily substantiated effects, including:
- Generation of ozone as a beneficial byproduct.
- Creation of an ionic wind, caused by the repulsion of similarly charged ions near the electrodes.
- Negative ions producing a general feeling of well-being (and positive ions producing a feeling of decreased well-being), an effect duplicated in thunderstorms, waterfalls, cathode ray tubes found in televisions and computer monitors.
