Value of ED50 testing in assessing hazards of acute poisoning by carbamates and organophosphates.

It is shown from the kinetics of inhibition of cholinesterase by -methylcarbamates and organophosphates that the LD dose is likely to be a much greater multiple of the dose causing signs of poisoning in 50% of the animals (the ED) for the carbamates than for the organophosphates. The expected difference was demonstrated by a comparison of the LDs and EDs, intravenous and intramuscular, of five carbamates (2-propoxyphenyl -methylcarbamate, 3-propylphenyl -methylcarbamate, 6-chloro-3,4-xylyl -methylcarbamate, 3,4,5-trimethylphenyl -methylcarbamate, and 3-methyl-5-propylphenyl -methylcarbamate) and two organophosphorus compounds (diethyl 4-nitrophenyl phosphate and dimethyl 4-nitrophenyl phosphate). The slightest evoked tremor was chosen as the most reliable sign of poisoning from which to estimate the ED values. Carbamates gave much greater LD/ED ratios than organophosphorus compounds. It is likely that occupational exposure to carbamates will produce incapacitating symptoms at doses well below lethal levels.