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Organophosphorus pesticides (OPs) are commonly detected in agricultural products, animal-derived foodstuffs, and environmental samples. Until now, the focus of research has been to evaluate the adverse effect of a single OP. While each OP may be present at concentrations under recognized as "no observed adverse effect level (NOAEL)", the combined effects of multiple OPs present at these low concentrations have not been sufficiently studied. Therefore, we developed an in vitro testing method to evaluate the toxicity of multiple OPs based on the degree of inhibition of cholinesterase (ChE) activity. This method requires only 10 min to complete and no specialized technology. We examined 15 OPs by this method and categorized them into three groups according to the degree of ChE inhibition. A relationship between the OPs' chemical structures and the degree of ChE inhibition emerged with the moiety -P-O-CN- showing the strongest action. The degree of ChE inhibition increased with multiple OPs, and the degree of inhibition seemed to be additive. These results demonstrate that the combined toxicity of multiple OPs present in food or environmental samples is an easily determined and toxicologically relevant measure of overall toxicity of complex OPs mixtures. It is possible to apply this testing method as a monitoring technique in water quality management in order to control OPs. As a result, this method can play the role for the potential risk reduction to the ecosystem and may contribute to the preservation of the environment.

We established a method for quantitative analysis of fenthion (MPP) and its related compounds in water samples, using solid-phase extraction and liquid chromatography/mass spectrometry. With this method, the values of the limit of quantification ranged from 0.2 to 100 ng l-1. Using this method, we examined the fate of MPP in water and the products produced by light irradiation and chlorination. MPP decreased gradually and reached 50% of the initial concentration after 48 hours in water. In particular, MPP-sulfoxide was formed. With light irradiation, MPP decomposed immediately into MPP-sulfoxide, O,O-Dimethyl S-[3-methyl-4-(methylthio)phenyl]phosphorothioate and other compounds. With chlorination, MPP decomposed into MPP-sulfoxide, MPP-sulfone, and their oxons. The concentration of oxons increased in a time-dependent manner. In their effects on organisms, MPP, MPP-sulfoxide and MPP-sulfone showed weak inhibitory activity to cholinesterase, whereas their oxons showed strong activity. It is feared that MPP and its products exist in environmental water and are produced by the disinfection treatment process. Comprehensive evaluation of the toxicity of MPP and its related compounds is important in order to understand the effects of MPP on ecosystems and human health. [ABSTRACT FROM AUTHOR]