Electrostatic air cleaners (precipitators) were tested in four classrooms during ordinary lessons for four weeks. Airborne dust in the classrooms was collected on filters using two different kinds of samplers: IOM-sampler and a new kind of ionizing sampler. The amount of collected dust was evaluated by visual inspection (“filter blackening”) and the content of cat allergen was measured using an amplified ELISA assay. It was found that the filter blackening was 64% lower when the air cleaners were active than when they where inactive (p<0.001). This suggests that the air cleaners were effective in reducing the content of airborne particulate matter. The reduction in cat allergen appeared however to be less substantial. The variance of the allergen data was unexpectedly large, presumably due to relatively few allergen-carrying particles in the sampled air. It is believed that larger sampling volumes are needed to get more reliable data of airborne cat allergen in spaces where the allergen is emitted indirectly.