There is a growing awareness in the chemical industry that environmental aspects need to be incorporated more fully into the design and operation of chemical processes. There are several recently developed methodologies to perform environmental assessments of chemical process designs. However, the value of the environmental information is dependent upon the level of uncertainty in the predictions of these impacts. This paper presents an uncertainty analysis for toxicity assessment of chemical process designs. This uncertainty analysis generated representative normalized standard errors for several measured environmental properties (Henry's law constant, octanol-water partition coefficient, hydroxyl radical reaction rate constant, and lethal concentration for human inhalation toxicity) from a subset of the high production volume chemicals. Then, the propagation of these property uncertainties in the calculation of an inhalation toxicity index was conducted. This method of uncertainty analysis has been applied to the environmentally conscious design of a process for volatile organic compound recovery and recycle from a gaseous waste stream.