Guidelines For Canadian Drinking Water Quality - Supporting Documents - Terbufos

1987

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Table of Contents

• Guideline
• Identity, Use and Sources in the Environment
• Exposure
• Analytical Methods and Treatment Technology
• Health Effects
• Rationale
• References

Guideline

The interim maximum acceptable concentration (IMAC) for terbufos in drinking water is 0.001 mg/L (1 µg/L).

Identity, Use and Sources in the Environment

Terbufos (C⁹H²¹O²PS³) is an organophosphorus insecticide used to control soil-borne insects, primarily on corn, sugar beets and rutabagas. More than 100 000 kg of terbufos were used in Canada in 1990.^{Footnote 11}

The vapour pressure of terbufos is 34.6 mPa at 25ºC, and its water solubility is 10-15 mg/L at 25ºC. Terbufos is hydrolysed under alkaline conditions in soil or water.^{Footnote 22} Terbufos has some residual action in soil. It was not considered in either the Canadian or U.S. ranking schemes for pesticides deemed to have the potential to leach from soil to water.^{Footnote 33,Footnote 44}

Exposure

In a 1986 study of municipal water in Manitoba, terbufos was not detected in 49 samples (detection limit 0.2 µg/L).^{Footnote 55} Terbufos was not detected in surface water samples from either the Grand or the Thames river basins, which are located in areas heavily used for agriculture (detection limit <0.1 µg/L).^{Footnote 66} If it is assumed that all crops with registered uses were treated with terbufos at the maximum tolerated level, the theoretical maximum dietary intake of terbufos for an adult Canadian would be 0.0008 mg/kg bw per day. No actual residue levels in foods are available, however, as terbufos was not included in total diet residue surveys in either Canada or the United States.

Analytical Methods and Treatment Technology

Terbufos may be monitored in water samples by solvent extraction followed by gas chromatography and flame thermionic or flame photometric detection.^{Footnote 77}Thermionic detectors may offer lower detection limits than flame photometric methods.^{Footnote 88} 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 99}

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 1010} 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 1010}

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 1111} 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 1111}

There were no dose-related tumorigenic effects in an 18-month mouse study^{Footnote 1212} 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:

where:

• 0.0025 mg/kg bw per day is the NOAEL for cholinesterase inhibition in a 28-day dog study^{Footnote 1111}
  
• 50 is the uncertainty factor to compensate for the short duration of the study and other deficiencies of the toxicology data base.
  

An interim maximum acceptable concentration (IMAC) for terbufos is derived from the NDI as follows:

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 IMAC must be measurable by available analytical methods, the PQL is also taken into consideration in its derivation. An IMAC of 0.001 mg/L, the 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 IMAC 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