The transmission of tuberculosis (TB) and other infectious diseases in health-care buildings has been a recognized hazard for decades. Ultraviolet germicidal irradiation (UVGI) of upper room air is used as an engineering control method to prevent the spread of airborne infectious disease. Under full-scale conditions, the efficacy of UVGI for inactivating airborne bacterial spores and active cells was evaluated. A test room fitted with a modern UVGI system was used to conduct bio-aerosol inactivation experiments. UVGI efficacy can be affected by environmental factors such as relative humidity (RH), and air mixing conditions. The impacts of these factors were determined by measuring UVGI efficacy under various environmental conditions. High RH decreased the UVGI effectiveness by 30-45% and the UVGI equivalent air-exchange rate by half. UVGI effectiveness reached its highest with 50% RH. The UVGI efficacy decreased by as much as 80% with incomplete air mixing induced by wintertime ventilation conditions.