Malathion Impurity 10

Isomalathion is an inhibitor of AChE. In addition, it is a potent inhibitor of tissue and serum carboxylesterases, which are known to rapidly detoxify malathion. As a result, small concentrations of isomalathion (0.01–0.5%) present as an impurity in malathion products result in an increase in malathion’s acute toxicity. This is not a directly proportional effect; for example, the effect of 0.05% isomalathion is to decrease the acute oral LD₅₀ of malathion 3-fold, but a further 10-fold increase in concentration to 0.5% isomalathion produces only a further halving of the LD₅₀ (Umetsu et al., 1977).

Similar results were obtained in another study comparing the LD₅₀ of purified malathion with that of malathion containing added isomalathion in known quantities (Aldridge et al., 1979). The acute oral LD₅₀ for purified malathion was 10,700 mg/kg BW. Increasing ratios of isomalathion to malathion markedly increased the toxicity of malathion by 12-fold for a 0.02 ratio and by 17-fold for a 0.04 ratio.Based on a recent study, commercially available technical-grade malathion is considered to be moderately toxic by the oral route (LD₅₀ 1778 mg/kg BW). This content of the test batch was 0.44%.