Ionisers or Ionic Air Purifiers work by creating a stream of negative ions that charge airborne particles so that they become attracted to surfaces, thereby removing them from the air. Ionisers contain electrostatic precipitators or collectors which trap the dust /pollen / allergen particles on oppositely charged metal plates.
Ionisers are often sought over filter based air purifiers as they are often quite or silent, energy efficient, and don't require replacement filters - thereby lowering their operating costs.
Negative ions have been shown to have a beneficial impact on mood, anxiety, improved sleep and a feeling of alertness and vitality, similar to the feeling you get near a waterfall or crashing surf.
When buying an Ioniser look for one with low ozone emissions.