The aerial crop dusting and spraying of fields with the phosphorothioate insecticide parathion in the late 1900s, significantly improved crop yields but resulted in high levels of occupational toxicity in handlers and agricultural workers, as well as cases of intentional self-harm poisoning, culminating in its banning in many western countries by early 2000s. However because of the low solubility and volatility of parathion, most available products were formulated using organic solvents e.g. xylene, to increase the efficacy of the aerosols and dusts. In the present study, the toxicity of parathion was assessed when formulated in an aqueous solvents (ethanol/PBS (1:9)), and delivered to macaques as an aerosol. Doses of 780 μg/kg and 1.56 mg/kg were delivered one day apart, using a modified nebulizer calculated to result in lung deposition of ∼480 μg/kg with a similar or larger amount being swallowed; these doses being similar to the estimated lethal oral dose 286ug/kg - 1.43 mg/kg of formulated parathion in humans. Surprisingly, this dose (a combined amount of ∼14 mg) caused only low AChE inhibition and moderate BChE inhibition with no clinical symptoms, indicating that the use of organic solvents may have previously played a critical role in the severity of parathion toxicity following inhalation exposure. In addition, unlike constitutively toxic OPs, which are highly toxic when inhaled, these results are consistent with the idea that phosphorothioate insecticides appear to be more intoxicating following oral than inhalation exposure. However, this still remains uncertain because the presence of organic solvents in the ingested parathion studies was not always known.
Keywords: Cholinesterase inhibition; Inhalation; Macaque model; Parathion; Solvents.
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