With the general increase in the worldwide incidence of tuberculosis there is increasing interest in the use of upper room ultraviolet germicidal irradiation (UVGI) systems to disinfect air. A number of researchers have demonstrated experimentally the ability of such systems to inactivate airborne microorganisms. However, relatively little theoretical work has been done to explain the results observed and few models exist to describe the performance of upper room UVGI systems. This paper presents a new model, which can be used both to design such systems and to evaluate their germicidal effectiveness. A theoretical study is undertaken, which indicates that although upper room UVGI systems work well at lower ventilation rates, they are of limited benefit in highly ventilated applications. The paper also demonstrates and quantifies the relationship between inter-zonal air velocity and room ventilation rate. In particular, the paper shows that under steady-state conditions the number of passes made by bioaerosol particles through an upper room UV field is independent of the ventilation rate.