Ionizing air purifiers are devices that can produce ozone indirectly, but can still emit a large amount of ozone, especially compared to other standard appliances. Ozone, a lung irritant, is produced both indirectly by ion generators and some other electronic air filters and directly by ozone generators. Despite some vendors' claims, there is no difference between ozone in outdoor smog and ozone produced by these devices. Under certain conditions of use, ion generators and other air purifiers that generate ozone can produce levels of this lung irritant significantly above levels considered harmful to human health.
The Food and Drug Administration (FDA) has set a limit of 0.05 parts per million ozone for medical devices. Although ozone can be used to reduce odors and pollutants in unoccupied spaces, the levels needed to achieve this are above those generally believed to be safe for humans. Ionizers are often found in the same unit as an air purifier, but they can also be purchased as separate units. Ionizers work by causing particles to adhere to nearby surfaces or to each other and deposit out of the air, but they can generate unwanted ozone.
It is important not to confuse ionizers with ozone air purifiers, which use ozone to clean the air and create or emit hazardous ozone. Indoors, ozone is produced by air ionizers (if you have one in your home). Ionizers commonly use corona discharge to emit negative ions, but the gas emitted by the corona discharge can be toxic to humans and the environment.