In this paper the authors introduce a new criterion - SONOX - which can be used to analyze the thermopower plants (TPPs) operation impact on the environment trough noxious gas emissions (sulphur and nitrogen oxides) in the atmosphere. Based on this criterion and applying the equivalence and the compensation principles, developed by the authors in some of their previous papers, we analyze some main Romanian TPPs and some recommendations are given in order to join the European Union norms regarding the respective emissions limits. Published by Elsevier Ltd.