Ozone, a lung irritant, is produced both indirectly and directly by ion generators and other electronic air filters. Indirect ozone production is a cause for concern, but the direct introduction of a lung irritant into indoor air is even more worrying. 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. Ozone can be used to reduce odors and pollutants in unoccupied spaces, such as removing smoke odors from homes involved in fires, but the levels needed to achieve this are above those generally believed to be safe for humans. Ionizers are commonly used to emit negative ions. However, the gas emitted by the corona discharge can be toxic to humans and the environment.
Some ozone air purifiers are manufactured with an ion generator, sometimes called an ionizer, in the same unit. You can also purchase ionizers as separate units. Ionizers remove particles from the air by causing them to adhere to nearby surfaces or to each other and deposit out of the air, but they can generate unwanted ozone. Most people should not buy an air purifier with an ionizing function.