When deciding whether or not to regulate a chemical, regulatory bodies often evaluate the degree to which the public may be exposed by evaluating the chemical's occurrence in food and drinking water. As part of its decision-making process, the United States Environmental Protection Agency (USEPA) evaluated the occurrence of perchlorate in public drinking water by sampling public water systems (PWSs) as part of the first implementation of the Unregulated Contaminant Monitoring Rule (UCMR 1) between 2001 and 2005. The objective of this paper is to evaluate the current representativeness of the UCMR 1 dataset. To achieve this objective, publicly available sources were searched to obtain updated perchlorate data for the majority of large PWSs with perchlorate detections under UCMR 1. Comparison of the updated and UCMR 1 perchlorate datasets shows that the UCMR 1 dataset is no longer representative because the extent and degree of occurrence has decreased since implementation of UCMR 1. Given this finding, it seems appropriate for regulatory bodies engaged in decision-making processes over several years to periodically re-evaluate the conditions that prompted the regulatory effort, thereby ensuring that rules and regulations address actual conditions of concern.

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