This part of ISO 29042 specifies a test bench method for the measurement of the separation efficiency by mass of an air cleaning system with an unducted outlet, operating under defined conditions. The method is applicable to those systems that clean the air of aerosols (smoke, dust, fume, mist), vapour or gas. Measurement of the separation efficiency by mass of an air cleaning system for an intended use can serve for the a) evaluation of the performance of the air cleaning system, b) evaluation of the improvement of the air cleaning system, c) comparison of air cleaning systems, d) ranking of air cleaning systems according to their separation efficiency by mass, and e) determination of the state of the art of air cleaning systems of the same intended use with respect to their separation efficiency by mass.