3, 4-dimethylphenyl N-methylcarbamate (MPMC) is in use to control agricultural pests in the paddy field. For field application, dusting of MPMC is made with a “pipe-duster” equipped with a 30-40 meter long polyethylene tube with many tiny holes, through which the dust is jetted. Since workers must carry the tube with their own hands during the field work, they might seriously be exposed to the dust in high concentrations.
This study was to see the inhalation toxicity of MPMC to rabbits. MPMC is a carbamate compound known as an anticholine-sterase agent. The assay of the cholinesterase (ChE) activity by inhibition through carbamates such as MPMC has not been established yet. In this study, however, Michel's method (the electrometric determination) was employed to determine the plasma ChE activity, but modified without changing the basic procedures as follows : 1) In the case of rabbits, a sample of 0.2 ml of plasma was necessary, because of low ChE activity of the rabbit plasma. 2) As for the duration of reaction with acethylcholine, it was essential to make the duration as short much as possible, because the ChE activity inhibited by carbamates is quickly recovered by ChE in the substrate. But in the duration less than 15 minutes, ΔpH values obtained were extremely low. In this study, determinations were made at 15, 30 and 60 minutes from the start for every sample.
Results were as follows :
1) With inhalation by rabbits at the concentration of 20 mg/m3 for 30 minutes, plasma ChE activity seemed to show no significant decrease, while at 38.6mg/m3 for 30 minutes the activity resulted in 26% inhibition.
2) Exposures at 20 mg/m3 for 30 minutes and successively at 0.6 mg/m3 for 60 minutes caused 21% inhibition of the activity. At 38.6 mg/m3 for 30 minutes and successively at 8.4 mg/m3 for 60 minutes, 40% inhibition was observed.
3) After the exposure, spontaneous recovery of the inhibited ChE activity occurred within at least 2 hours in all the rabbits.
4) Highest concentration of dust was noted at the tip of the tube out of three points determined along the tube of “pipe-duster” at different distances from the duster.
