Indoor air cleaners can contribute to reducing infection risks by the filtration of virus-carrying droplets. There are various national standards to test indoor air cleaners that determine the clean air delivery rate (CADR), but typically only as a size-integrated value for particles >0.3 μm. Thus, a test method using potassium chloride (KCl) and paraffin as surrogate particles in the size range of viruses and exhaled droplets was developed. We show that air cleaners with fibrous and electrostatic filters are generally capable of reducing the airborne particle concentrations. However, for electret filters, the performance can strongly degrade over time by being loaded with particles. By comparing filters with different efficiencies in the same air cleaner, we demonstrate that the use of high-efficiency filters can be even at the expense of the cleaning efficacy. We developed a mathematical model to estimate the inhaled dose of viruses and show that the combination of natural venting and an air cleaner can lead to a substantial reduction of the infection risk.