An air ionizer (or negative ion generator) is a device that uses high voltage to ionize (electrically charge) air molecules. Negative ions, or anions, are particles with one or more extra electrons, conferring a net negative charge to the particle. Cautions are positive ions missing one or more electrons, resulting in a net positive charge. Most commercial air purifiers are designed to generate negative ions.
The Geophysical Institute of the University of Alaska Fairbanks has published a paper reporting that cosmic rays normally create around 1000 negative ions and positive ions per cubic centimeter of outdoor air, the concentration being higher at higher altitudes and also near the sea. But indoor city environments may typically have half that concentration. Controlled studies have reported greater subjective well-being in an artificially negatively-ionized environment, though the reason for this is unknown.
The high electric potentials used to create air ions are achieved by using capacitors to develop a high voltage (c.20,000 volts), low-current charge at an electrode. Such voltages can also generate ozone (an energetic allotrope of oxygen), and NOx, which, even in relatively low concentrations, may irritate lung tissues, causing chest pain, coughing, throat irritation and aggravated asthma.
However, the Cedars-Sinai Medical Center, Los Angeles has stated its acceptance of anecdotal doctor's reports that the use of ionizers produces only neutral to positive reports when tried by patients suffering from respiratory problems and allergies.