Biphasic Dissipation Kinetics for Dislodgeable Foliar Residues in Estimating Postapplication Occupational Exposures to Endosulfan

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Agricultural workers are exposed to pesticide residues via dermal contact with foliage upon entry of treated fields. Restricted Entry Intervals (REIs) are established based on both toxicity and exposure. Key factors for estimation of potential worker exposures are dislodgeable foliar residues (DFRs) and the manner in which DFRs dissipate over time. DFR dissipation curves vary in form and shape, and are often biphasic, reflecting different rate processes, chemical-physical influences, and partitioning. Biphasic dissipation behavior for endosulfan [6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepin 3-oxide] previously reported for tomatoes and peppers has been confirmed here for endosulfan on melon, grape, and peach foliage. Use of biphasic kinetics results in more robust r2 values for the regression curves that describe foliar dissipation of endosulfan compared to use of simple first-order kinetics. For endosulfan, the use of biphasic kinetics to describe the overall dissipation accurately predicts daily DFR values. In contrast, first order kinetics may overestimate DFRs and, potentially, postapplication worker exposures during the critical period when entry of treated fields is most likely to occur.