Air cleaner effectiveness (ε) is the fractional change in concentration of an air contaminant resulting from the addition of an air cleaner to a system. Unlike component single-pass efficiency, it takes into account the aggregate effect of all contaminant removal mechanisms as well as the effects of air cleaner placement in the system. The usefulness of ε in the analysis and application of air cleaners, as well as its shortcomings, is illustrated by the modeling of in-duct ultraviolet germicidal irradiation (UVGI) in a hypothetical two-zone building served by a constant volume system. The impact of design parameters such as the location of UVGI units, particulate filter efficiency, and the nature of contaminant release are investigated, with calculated ? values ranging from 5% to 90% depending on the nature of these parameters.