%D 2002 %0 ARTICLE %T Biological monitoring of combined exposure to organophosphates and pyrethroids . %J Toxicol . Lett . %V 134 ( 1-3 ) %P 119-24 %A He F %A Chen S %A Tang X %A Gan W %A Tao B %A Wen B %M pub12191869 %X A total of 132 farmers were selected from two areas ( J and S ) for this study . In area J , the subgroups Ja ( 40 subjects ) and Jb ( 36 subjects ) sprayed on rice crops either the organophosphate ( OP ) insecticide methamidophos or the mixture methamidophos-deltamethrin ( a pyrethroid , Pyr ) . In area S , the subgroups Sa ( 27 subjects ) and Sb ( 29 subjects ) sprayed on cottons either the OP methyl-parathion or the mixture methyl-parathion-deltamethrin . Groups Jc ( 22 subjects ) and Sc ( 20 subjects ) were non-exposed controls . Erythrocyte acetylcholinesterase ( AChE ) was measured by the Ellmans method before spraying , after 2-h exposure and 1 h later . Inhibition of AChE occurred after 2-h exposure to both single OP ( Ja and Sa ) and OP-containing mixtures ( Jb and Sb ) as compared with both pre-exposure and control values ( Jc and Sc ) . A similar recovery was found in subgroups Ja , Jb , Sa and Sb 1 h after spraying , but all remained lower than the pre-exposure level . There was no difference in AChE inhibition and recovery between the single OP subgroups and the OP mixture subgroups . The inhibitory effect on AChE shown in subjects with combined OP-Pyr exposure was induced by the OP component of the insecticide mixture and to a similar extent as that induced by the single OP formulations . In addition to the fact that the OP is more potent than Pyrs , the above results might explain why the clinical manifestations of OP mixture poisonings are similar to those of single OP poisoning .