Abstract: For treating room air by ionization in an ionization apparatus having electrical discharge ionization tubes or corona discharge tubes, the values of oxidizable air components; relative humidity; flow velocity or flow volume; ozone level in the supply air; and intensity of oxygen ions of the air are measured. The level of ionization power of the ionization apparatus Is determined by a control device based on the measured values. When a predetermined ozone level is detected, the control device lowers the ionization power, and, when the ozone level continues to increase after the ionization power has been lowered indicating an external ozone source, the control device initiates a decomposition of ozone in the ionization apparatus by changing a time period of the applied periodic alternating voltage, supplied in the form of alternating pulses or packages of alternating pulses of a preset number or a combination of both.