484 Archives of Environmental Health ORGANOPHOSPHORUS COMPOUNDS are chemi-

cals that contain both carbon and phosphorus atoms.1 They are derivatives of phosphoric (H3PO4), phosphorus or phosphonic (H3PO3), and phosphinic (H3PO2) acids (Fig. 1). The biological action of organophosphorus compounds is related to their phosphorylating abilities. This is dependent on the electrophilicity (positive char-acter) of the phosphorus atom, which is determined by its substituent groups. Steric factors of substituents also play a major role in determining the biological activity of these chemicals. Lipid solubility is important because it enhances the ability of these compounds to cross bio-logical membranes and the blood-brain barrier, leading to increased biological activity. Organophosphorus compounds are an economically important class of chemical compounds with numerous uses, such as in pesticides, industrial fluids, flame retardants, therapeu-tics, and nerve gas agents. Most modern synthetic organophosphorus com-pounds are tailor-made to inhibit acetylcholinesterase (AChE), an enzyme essential for life in humans and other animal species. Tetraethylpyrophosphate was the 1st or-ganophosphate synthesized as an AChE inhibitor in 1854.2 Later, dimethyl and diethyl phosphorofluoridates were synthesized.2 During World War II, organophos-phorus compounds were developed primarily as agri-cultural insecticides, and later as chemical warfare agents. The majority of organophosphorus insecticides are organophosphorothioates; nerve agents are organo-phosphonates or organophosphonothioates; industrial chemicals are typically organophosphates3 (Table 1). Biologically, organophosphorus compounds are neu