In strictly still air, the propagation of ions is caused by their attraction by the surfaces of the room. Large quantities of ions are measured up to 3-4 metres from an ioniser. In moving air, just as in a real room where the air is carried along by temperature differences between surfaces and by convection due to ventilation and the presence of occupants, the propagation of ions is even more encouraged. Ion propagation can be significantly improved if the ions are emitted close to an air flow.