This paper presents an index that will be used to assess the impact of continuous chemical processes on the environment. The index is calculated using a global mass balance combined with a scoring system for the impact of each pollutant. The scoring system is generated using a tree structure and is based on selected parameters which are indicative of environmental impact The scores are then combined with discharge data based on quantities rather than the more traditional use of environmental concentrations. The index considers both the long-term and short-term environmental impacts of processes. For ease of use, the index has been implemented as a computer-based process engineering tool. It will be used for selecting the best environmental option for new processes and also in the redesign of existing plants in order to meet increasingly stringent discharge limits. A simple case study is presented to illustrate the operation of this new index in a process retrofit context.