Air ioniser

This photo shows the sterilisation effects of negative air ionization on a chamber aerosolised with Salmonella enteritidis. The left sample is untreated; the right, treated. Photo taken in a lab operated by the United States Department of Agriculture.

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. Cations are positive ions missing one or more electrons, resulting in a net positive charge. Most commercial air purifiers are designed to generate negative ions. Another type of air ionizer is the ESD ionizer (balanced ion generator) used to neutralise static charge. In 2002, Cecil Alfred 'Coppy' Laws was credited with being the inventor of the domestic air ionizer in an obituary in The Independent newspaper.

Ionic air purifiers

Air ionizer and purifier with its dust collection plates removed

Air ionizers are used in air purifiers. Airborne particles are attracted to the electrode in an effect similar to static electricity. These ions are de-ionized by seeking earthed conductors, such as walls and ceilings. To increase the efficiency of this process, some commercial products provide such surfaces within the device. The frequency of nosocomial infections in British hospitals prompted the National Health Service (NHS) to research the effectiveness of anions for air purification, finding that repeated airborne acinetobacter infections in a ward were eliminated by the installation of a negative air ionizer—the infection rate fell to zero. The SARS Pandemic fuelled the desire for personal ionizers in the Far East, including Japan (where many products have been specialised to contain negative ion generators, including toothbrushes, refrigerators, air conditioners, air cleaners and washing machines). There are no specific standards for these devices. Notebook computer producer ASUS included an integrated air ionizer in the N51Vf laptop.

Ions versus ozone

Ionizers should not be confused with ozone generators, even though both devices operate in a similar way. Ionizers use electrostatically charged plates to produce positively or negatively charged gas ions (for instance N₂⁻ or O₂⁻) 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 ionizers will produce a small amount of ozone.

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 humans and animals that the FDA in the United States explicitly demands ozone therapy not be used as medical treatment, and has taken action against businesses that fail to comply with this regulation. 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.

Consumer Reports court case

Consumer Reports, a non-profit U.S.-based product-testing magazine, reported in October 2003 that air ionisers do not perform to high enough standards compared to conventional HEPA filters. The exception was a combination unit that used a fan to move air while ionizing it. In response to this report, The Sharper Image, a manufacturer of air ionisers (among other products), sued Consumer's Union (the publishers of Consumer Reports) for product defamation. 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 down 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 US $525,000 for Consumer Union's legal expenses.

Electrostatic neutralizer in electronics

Air ionizers are sometimes used in electronic work environments to eliminate the buildup of potentially damaging static charges on insulators or conductors. This is sometimes necessary when ground straps cannot be used.

See also

• Dehumidifier
• Electrostatic precipitator
• Humidifier
• Nebuliser
• Negative air ionization therapy
• Seasonal affective disorder

External links

• New Scientist – Air ionizers wipe out hospital infections
• Do "corona discharge" devices alleviate asthma?
• NIOSH Immediate Danger to Life or Health (Ozone) Documentation
• Ionic Air Purifiers and Smog
• The Negative-Ion Myth (archive)
• Air ion behavior in ventilated rooms Indoor and Built Environment, 17 (2). pp. 173–182., Fletcher, L.A., Noakes, C.J., Sleigh, P.A., Beggs, C.B. and Shepherd, S.J. (2008)