Cochran_1996_Rev.Environ.Contam.Toxicol_146_1

Mevinphos (trade name, Phosdrin), a category 1 organophosphorus insecticide, has been used mainly as a cleanup pesticide for vegetable crops. A risk assessment for occupational and dietary exposure to mevinphos was initiated because of the high acute toxicity of the compound. Repetitive dosing with mevinphos did not cause any discernible histopathological effects in mice or rats, nor was it oncogenic in either species. The principal toxic effects of mevinphos, both short- and long term, were due to inhibition of cholinesterase activity. Consequently, potential adverse effects from short-term exposures were the primary concern. A human no-observed-effect level (0.025 mg/kg) for cholinergic signs was used as the regulatory basis for calculating margins of safety (MOSs) for potential acute dietary and short-term occupational exposures. Estimates of exposure to mixer/loaders, pilots, and flaggers associated with aerial application of mevinphos were based on passive dosimetry. Because no acceptable exposure studies for work tasks associated with ground application of mevinphos were available, surrogate data based on ground application of oxydemeton-methyl were used. Exposure estimates for field workers and harvesters relied on measured dislodgeable foliar residues of mevinphos and transfer factors generated from studies of other active ingredients. MOSs for mean acute occupational exposure of mixer/loader/applicators associated with ground application and of harvesters working in fruit trees were less than the value conventionally recommended to protect people from the toxic effects of mevinphos. MOSs for the 95th percentile of short-term worker exposure for all mixer/loader work categories associated with mevinphos application were also inadequate. Calculated MOSs for potential acute dietary exposure to measured residue levels of mevinphos were adequate for the various population subgroups. However, 25 of the USEPA tolerances for mevinphos on agricultural commodities were not adequate to protect for the toxic effects of mevinphos from theoretical acute dietary exposure to one or more population subgroups if commodities are consumed with residues at the tolerance level. When the mean short-term occupational exposures were combined with potential acute dietary exposure, the MOSs for mixer/loaders engaged in aerial applications, as well as ground applications, were inadequate to protect people from the toxic effects of mevinphos. As mitigation of the estimated excessive occupational exposures did not appear possible, both California and the USEPA were preparing to cancel registration of the product. However, an agreement was worked out between the manufacturer and the two agencies that ended production for domestic use but allowed existing stocks in the channels of trade to continue to be used for a limited period.