Portable electrostatic air cleaners (precipitators) were tested in six different classrooms during ordinary lessons. The number concentration of airborne particles in the classrooms was measured using optical particle counters, which separated the particle recordings into six different size fractions in the range 0.3 μm to >25 μm. It was found that the effectiveness of the air cleaners in cleaning the classroom air was decreasing significantly with increasing particle size. For the smallest particles measured, 0.3-0.5 μm, the measured effectiveness was 76 %, whereas for the particle size fraction 10-25 μm it was only 33 %. This reduced effectiveness for the large particles can be explained by the removal process of particle deposition onto room surfaces. The removal rate due to deposition seems to be competitive with the removal rate of the air cleaners as regards large particles. It appears that the deposition effect often needs to be taken into account in effectiveness assessments for particulate air cleaners.