Validation of Exposure Models: The Role of Conceptual Verification, Sensitivity Analysis, and Alternative Hypotheses

Available methods for evaluating the veracity and reliability of exposure models extend far beyond simple comparisons of observed and predicted values. To be valid, model tests must, first, be tailored to evaluate a model within its intended range of operation. Appropriate tests must then be developed to ensure that the model performs in accord with its designers' intentions and that significant phenomena have not been neglected or oversimplified. Finally, comparisons between model predictions and realworld data sets can be used to compare the performance of competing models, but such studies usually cannot serve as general proofs of the reliability or validity of an individual model. These points can be elucidated by means of a discussion of some of the ''validation'' studies executed during the development of the exposure analysis modeling system (EXAMS). The EXAMS was designed to evaluate the probable long-term behavior of newly synthesized organic chemicals in aquatic systems.