Mobile air cleaners, in particular such with fibrous electret filters, are widely used to improve the indoor air quality. The particle size dependence of the air cleaning efficiency was investigated and concluded that recent testing standards overestimate the cleaning performance with respect to relevant indoor aerosols if considering only particles larger than 0.3 mu m. Furthermore, the filters were artificially aged with cigarette smoke to evaluate their long-term stability. Finally, the results were compared to an aging method proposed by a recent testing standard and a very distinct aging behavior was found.