Ultraviolet germicidal irradiation (UVGI) uses UVC radiation produced by low pressure Hg vapor lamps to control biological air contaminants. Lamp UV output depends on multiple factors, including accumulated operating time (age) and the thermal effects of ambient air temperature and velocity. Additionally, the life of some lamp types depends on the frequency of on-off cycles. Models of lamp life as a function of cycling rate and lamp output as a function of age and ambient conditions are developed for three common standard output lamp types based on heat transfer theory and manufacturer's lamp performance data. Example results are presented and a parametric study in the form of a 3 k factorial experiment is used to identify significant factors affecting output and their interactions. A typical range of ambient conditions reduces lamp output by more than 30% of rated capacity and the inclusion of aging effects reduces capacity by as much as 70%. For given ambient conditions, performance varies substantially across lamp types due to their differing heat transfer characteristics. It is concluded that short term and long term variation of lamp output is highly significant and that modeling of such effects is necessary for accurate system design and analysis.