Since the environmental awareness increases and regulations become more restrictive, chemical industries are forced to adopt measures for minimizing the environmental impact and to include these techniques in the process design. This work proposes a methodology where environment and human health considerations are coupled with the process design of new and existing plants. With this aim, a new assessment tool for the environmental evaluation of chemical processes is presented. It includes the development of a new environmental indicator (Material Balance Environmental Index, MBEI) based on the toxicities of the chemicals involved and the materials flows between the process and environment. Moreover, a total index is computed following three levels of aggregation using the geometric mean of the ratios of several environmental impact categories. The environmental evaluation tool is tested using two case studies (formaldehyde and styrene production), where data are obtained from rigorous process simulation validated with industrial data.