Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Terbufos

Terbufos may be monitored in water samples by solvent extraction followed by gas chromatography and flame thermionic or flame photometric detection.Footnote 7Thermionic detectors may offer lower detection limits than flame photometric methods.Footnote 8 The practical quantitation limit (PQL) (based on the abilities of laboratories to measure terbufos within reasonable limits of precision and accuracy) is 1 µg/L; detection limits range from 0.1 to 0.5 µg/L.

No information has been found on the effectiveness of current treatment technologies in removing terbufos from drinking water.

Health Effects

Terbufos is readily absorbed by the oral, inhalation and dermal routes. Eighty-three percent of a single oral dose of technical 14C-labelled terbufos (0.8 mg/kg bw) was eliminated in the urine of rats 168 hours after dosing; 3.5% of the dose was recovered in the faeces. Terbufos does not accumulate in body tissues.Footnote 9

Terbufos is highly toxic by both the oral and dermal routes; oral LD50 values are 9.0 mg/kg bw for female rats and between 1.5 and 4.5 mg/kg bw for male rats.Footnote 10 The principal toxic effect of terbufos is acetyl-cholinesterase inhibition. High doses also cause congestion of the liver, kidneys and lungs. Symptoms of acute toxicity include muscle tremors, salivation, diuresis, hyperpnoea and tachycardia.Footnote 10

A no-observed-adverse-effect level (NOAEL) of 0.0025 mg/kg bw for cholinesterase inhibition was observed in a 28-day dog study.Footnote 11 Chronic oral administration of doses up to 0.3 mg/kg bw in rats in a two-year feeding study reduced body weights, increased mortality, increased several organ weights and increased the incidence of eye diseases. The NOAELs were 0.04 mg/kg bw for cholinesterase depression and 0.2 mg/kg bw for changes in organ/body weight ratio.Footnote 11

There were no dose-related tumorigenic effects in an 18-month mouse studyFootnote 12 or in the two-year rat study.Footnote 11 Terbufos was not mutagenic in the Ames test in four strains of bacteria or in a similar test on Escherichia coli with or without metabolic activation. No results of other short-term tests for mutagenic or genotoxic effects are available. No reproductive, foetotoxic or teratogenic effects other than reduced body weights in both adults and weanlings were observed in a two-generation rat study. Data base deficiencies include lack of mutagenicity and subchronic/chronic data on a non-rodent species.Footnote 11

Rationale

A negligible daily intake (NDI) for terbufos was established by the Food Directorate of Health Canada as follows:

NDI = 0.0025 mg/kg bw per day ÷ 50 = 0.000 05 mg/kg bw per day

where:

  • 0.0025 mg/kg bw per day is the NOAEL for cholinesterase inhibition in a 28-day dog studyFootnote 11
  • 50 is the uncertainty factor to compensate for the short duration of the study and other deficiencies of the toxicology data base.

The maximum acceptable concentration (MAC) for terbufos is derived from the NDI as follows:

MAC = 0.000 05 mg/kg bw per day × 70 kg × 0.20 ÷ 1.5 L/d ≈ 0.0005 mg/L

where:

  • 0.000 05 mg/kg bw per day is the NDI established by the Food Directorate
  • 70 kg is the average body weight of an adult
  • 0.20 is the proportion of daily intake of terbufos allocated to drinking water
  • 1.5 L/d is the average daily consumption of drinking water for an adult.

Because the MAC must be measurable by available analytical methods, the PQL is also taken into consideration in its derivation. A MAC of 0.001 mg/L, the practical quantitation limit, is therefore established for terbufos in drinking water.

The theoretical maximum dietary intake of terbufos for an adult Canadian is 0.0008 mg/kg bw per day (see "Exposure"), which exceeds the NDI established by Health Canada, and intake of drinking water at the MAC will further exceed the NDI. It should be noted, however, that terbufos residues have not been detected in food. Every effort should be made to keep terbufos concentrations in drinking water as low as possible.

References

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