Absorption and excretion of organophosphorous insecticide biomarkers of malathion in the rat: Implications for overestimation bias and exposure misclassification from environmental biomonitoring

Malathion is an organophosphorous (OP) insecticide widely used for crop protection. Its degradates, malathiondiacid (MDA), malathion monoacid (MMA), dimethylphosphate (DMP), dimethylthiophosphate (DMTP) and dimethyldithiophosphate (DMDTP) are formed in strawberries and other produce. These same chemical biomarkers are measured in urine in human studies as quantitative measures of exposure. The excretion of malathion and its common biomarkers including MDA, MMA, DMP, DMTP and DMDTP at equal molar doses (73 μmol/kg b.w.) was studied following oral dosing of female Holtzmann rats (240–300 g). Following MDA administration, 36.3 ± 5.4% was recovered as MDA, 0.05 ± 0.02% as DMP, 5.5 ± 0.3% as DMTP, 3.8 ± 2.9% as DMDTP (mole percent), and totally 45.6 ± 7.0% of administered dose in urine after 120 h (over 94% in the first 24 h). Following DMTP administration, 8.3 ± 7.7% was recovered as DMP, 46.6 ± 16.5% as DMTP, and totally 55.0 ± 10.3% of administered dose in urine after 120 h (over 92% in the first 24 h). Similar results were obtained with other malathion biomarkers. Preformed biomarkers of malathion and other OP insecticides when ingested in produce are readily absorbed and excreted. Low level human dietary and non-occupational urine biomonitoring studies will be confounded by preformed pesticide biomarkers used to infer potential human pesticide exposure. This has profound implications for epidemiology studies where subject’s biomarker excretion is used as a surrogate for OP exposures that cannot be related to a particular insecticide residue.