The proposed study was Part Two of a 2-year project to test whether ultraviolet light could serve as a practical and cost-effective method to inactivate aerosolized pathogens in commercial swine facilities. Work performed in Year One showed that the swine pathogens we tested were highly susceptible to inactivation by UV254. The issue that drove the research in Year Two was the fact that the UV inactivation of pathogens is known to be affected by relative humidity and temperature. In general, higher relative humidity decreases the k-value (requires more UV for inactivation) and higher temperature increases the k-value (takes less UV for inactivation) (Tseng and Li, 2005; Walter and Ko, 2007). Understanding the interaction of relative humidity and temperature on the rate of inactivation of swine pathogens under a range of temperature and relative humidity is a critical step moving this technology to the field. Therefore, our objective was to determine the effects of temperature x relative humidity on the rate of inactivation (k-value) of SIV, BVDV, and PRRSV.