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This paper addresses the impact of uncertainty evaluation on the analysis of an environmental performance indicator for the process industry. As an industrial case study, the analysis concerns an indicator of the effluent generation of a Brazilian petrochemical industry. The uncertainty evaluation results from the Guide to the Expression of Uncertainty in Measurement (GUM) and its Supplement 1 (GUM-S1) revealed that the current company target to reduce the effluent generation indicator by 5% is unfeasible. This is because the worst-case uncertainty scenario of the indicator has a value of 5.4%. The analysis also demonstrated that the effluent flow rate is the major source of uncertainty in the indicator, and the uncertainty associated with the measurement apparatus (Parshall flume) is the most meaningful factor with respect to this uncertainty. Before establishing any target reduction concerning this indicator, the measurement system of the effluent flow rate should be improved.