An air ioniser (or negative ion generator) is a device that uses high voltage to ionise (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 ioniser is the ESD ioniser (balanced ion generator) used to neutralise static charge.

Russian scientist and inventor Alexander Chizhevsky produced the so called Chizhevsky Chandelier around 1918. This was the first modern air ioniser.

Air ionisers are used in air purifiers. Airborne particles are attracted to the electrode in an effect similar to static electricity. These ions are de-ionised 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. The SARS Pandemic fuelled the desire for personal ionisers 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 producer ASUS included an integrated air ioniser in the N51Vf laptop.