The air ionic composition in a classroom was determined during the operation of a Neo Tec XJ-2100 ionizer (Germany). The amount of ions in the air was measured before and after the switch-on of the ionizer. It was shown to vary depending on the mode of operation of the device and differ from the recommended normal values. The necessity of checking up the work of air ionizers both in living quarters and at workplaces is discussed with the application of air ion counters making it possible to carry out monitoring of the air ionic composition and estimate its compliance with the sanitary and hygienic norms.