Air ioniser
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. Conversely, 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
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.
The use of negative ions continues to be a less accepted mainstream therapy in Eastern Europe and the Far East than in Western Europe or the United States, although problems with nosocomial infections (hospital acquired "super-bugs") have led the National Health Service (NHS) in the UK to do extensive research into the effect of negative ions on this area of hygiene.
Recent SARS outbreaks have fueled the desire for personal ionisers in the far east, including Japan (where many products have been specialized to contain negative ion generators, including toothbrushes, refrigerators and washing machines). There are no specific standards for these devices.

# Benefits of negative ions
Cedars-Sinai has a page last reviewed 03-15-2006 which discusses negatively ionized air as an alternative treatment for certain conditions.  The Geophysical Institute of the University of Alaska Fairbanks published an article September 26 1981 which discusses the beneficial role of negative ions and the positive ion fields created by CRT (cathode ray tube) computer monitors.

# Air purifier criticisms
Criticisms of ionizers 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.
- Generation of ozone as a beneficial byproduct.
- Creation of an ionic wind, caused by the repulsion of similarly charged ions near the electrodes.
- Even the best ionizers will produce a small amount of ozone, which is highly toxic (see below). This can be mitigated to some degree by special devices designed to remove ozone.
- Ironically, the ozone generating ionizers can make asthma and other lung conditions worse.  This causes precisely the opposite of the desired effect when purchasing an air purifier.

# Ions vs ozone
Ionisers should not be confused with ozone generators, even though both devices operate in a similar way. Ionisers use an electrostatically charged plate to produce positively or negatively charged gas ions that particulate matter sticks to (in an effect similar to static electricity). Ozone generators are optimised to attract an extra oxygen ion to an O2 molecule, using either a corona discharge tube or UV light.
Even the best ionisers will produce a small amount of ozone, and ozone generators will produce gaseous ions of molecules other than ozone (unless fed by pure oxygen, not air).
Ozone is claimed by some alternative medicine proponents Template:Weasel-inline to be relatively harmless to humans, but this is a demonstrable fallacy. Ozone is a highly toxic and extremely reactive gas.
A higher daily average than 0.1 ppm (0.2 mg/m3) is not recommended and can damage the lungs and olfactory bulb cells directly.
At high concentrations, ozone can also be toxic to air-borne bacteria, and may destroy or kill these sometimes infectious organisms.  However, the needed concentrations are toxic enough to man and animal that the FDA explicitly demands ozone therapy not be used as medical treatment,
and has taken action against businesses that fail to comply with this regulation.

# Consumer Reports court case
Consumer Reports, a non-profit U.S.-based product-testing magazine, reported in October 2003 that air ionizers do not perform to high enough standards compared to conventional HEPA air filters.  In response to this report, The Sharper Image, a manufacturer of air ionizers (among other things), sued Consumer's Union (the publishers of Consumer Reports) for product defamation.  The Sharper Image's Ionic Breeze unit did meet all EPA guidelines, including less than 50 ppb ozone production. Consumer Reports gave the Ionic Breeze and other popular units a "fail" because they have a low Clean Air Delivery Rate (CADR).  CADR measures the amount of filtered air circulated during a short period of time, and was originally designed to rate media-based air cleaners.  The Sharper Image claimed that this test was a poor way to rate the Ionic Breeze, since it does not take into account other features, such as 24-hour a day continuous cleaning, ease of maintenance, and silent operation.  The U.S. District Court for the Northern District of California subsequently struck The Sharper Image's complaint and dismissed the case, reasoning that The Sharper Image had failed to demonstrate that it could prove any of the statements made by Consumer Reports were false.  The Court's final ruling in May 2005 ordered The Sharper Image to pay $525,000 USD for Consumer Union's legal expenses.