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-ionised 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 ionisers produces only neutral to positive reports when tried by patients suffering from respiratory problems and allergies.
Air ionisers are used in air purifiers, though they are not efficient in this respect. Airborn particles are attracted to the electrode in an effect similar to static electricity. Heavier combined particles may precipitate (fall) out of the air.
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 infections) 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 ionizers 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.
Ionisers should not be confused with ozone generators, even though both devices operate in a similar way. Ionisers use electrostatically charged plates 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).
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. Pure 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, 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. 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 ionizers (among other products), 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.