The PERI (Nilsson, 1999) (Pesticide Environmental Risk Indicator) aims at evaluating pesticide use by farmers over a range of years as part of a certification process. It was developed as a part of the International Organization for Standardization (ISO)-14001 certification process for use by farmers to record and evaluate possible environmental risks over time. PERI combines variables from groundwater, surface water, and air compartments in one equation to obtain an environmental risk score (ERS). The potential emission to groundwater and air is represented by a score between 1 and 5, using data on soil degradation, mobility and Henry’s constant. The final score is determined by combining the score for emissions with a score for toxicity (in different compartment).