Biological monitoring for exposure to methamidophos: a human oral dosing study

An oral dose of the organophosphate insecticide methamidophos was administered to six volunteers at the acceptable daily intake (ADI, 0.004 mg/kg). Urine was collected from the volunteers at timed intervals for 24 h post-exposure. Methamidophos itself was quantified in urine using liquid/liquid extraction and LC-MS-MS analysis (detection limit 7 nmol/L/1 μg/L). Methamidophos exhibited a rapid elimination half-life of 1.1h, (range 0.4-1.5 h). Mean metabolite levels found in 24h total urine collections (normalised for a 70 kg volunteer) were 9.2 nmol/L (range 1.0-19.1). One volunteer was anomalous; excluding this result the range was 6.7-19.1 nmol/L, with a mean of 10.9 nmol/L. Individual urine samples collected during the first 24 h ranged from below the detection limit (ND) to 237 nmol/L. The mean dose recovery excreted as methamidophos in urine was 1.1% (range 0.04-1.71%). Three environmental studies have been reported in the literature with levels ranging from ND to 66 nmol/L. The number of positive results in all three studies was low (<1.5% of total samples analyzed). When compared with our results (ND - 237 nmol/L), the studies suggest general population exposures are within the ADI. However, the very short half-life makes determining intermittent environmental exposures difficult.