Priority Substances List Assessment Report for Non-pesticidal organotin compounds

3.0 Assessment of "Toxic" Under CEPA

3.1 CEPA 11(a) Environment

This environmental assessment considers the six major groups of non-pesticidal organotin compounds that could enter the Canadian environment from the products currently in commerce: mono- and di- methyltin, butyltin, and octyltin species. The quantities of non-pesticidal organotin compounds entering the Canadian environment are not known for any of several possible routes of entry.

Based on limited information on fate, the mono- and di- alkyltin compounds are hydrophilic compounds that do not partition strongly to sediment or tissue. It is also unlikely that they will volatilize to the atmosphere in significant quantities. Most of the available data indicate that mono- and di- alkyltin compounds are not persistent in water with half-lives expected to range from a few days to less than a few months at 20°C. Because the aquatic environment is the most important repository for organotin compounds, this assessment will focus on the most sensitive aquatic biota exposed to organotin compounds.

There is limited information on the toxicity of most non-pesticidal organotin compounds to freshwater and marine biota. In this assessment, effects thresholds for the most sensitive aquatic biota were estimated by dividing the lowest-observed-effect-level in toxicity tests by various factors that account for the limited data available (Table 2). Emphasis was placed on ecologically relevant test results (e.g., mortality rather than Microtox endpoints). The estimated effects thresholds (EET) were then compared to the mean environmental concentrations (EC) observed in Canada for the freshwater and marine environments. If the EET/EC ratio was ≤ l for a given compound, then the potential exists for that compound to cause harmful effects to aquatic biota.

The environmental concentrations listed in Table 2 are assumed to overestimate the true mean concentrations in Canada because most sampling was conducted at sites where maximum concentrations would be expected (e.g., harbours and marinas for the butyltin compounds), and the detection frequencies at these sites were low, ranging from 0 to 29%. Non-detections were not included in the calculations of the mean environmental concentrations.

Comparison of the estimated effects thresholds to environmental concentrations for the mono- and di- methyltin and mono- and di- butyltin compounds indicates that the EET/EC ratios ranged from 5 to >1 350 in the freshwater and marine environments (Table 2). Therefore, these compounds are unlikely to cause harmful effects to freshwater or marine biota in Canada. The mono- and di- octyltin compounds have not been found (to date) in Canada or elsewhere in any environmental medium (Table 2). Although toxicity data are lacking for the octyltin compounds, it is unlikely that they could cause harmful effects to aquatic biota because these biota are not exposed to the octyltin compounds.

No data were identified on the toxicity of non-pesticidal organotin compounds to wildlife. Due to the low toxicity of the non-pesticidal organotin compounds to aquatic invertebrates and fish, adverse effects on aquatic-based wildlife due to decreased availability of prey are considered unlikely.

Therefore, on the basis of the available data, non-pesticidal organotin compounds are not considered to be "toxic" as defined under Paragraph 11(a) of the Canadian Environmental Protection Act.

3.2 CEPA 11(b) Environment on Which Human Life Depends

The non-pesticidal organotin compounds discussed in this assessment are not appreciably volatile and are not expected to contribute to phenomena such as ozone depletion, global warming, or the formation of ground-level ozone. They are not suspected of being associated with other known direct effects on the environment on which human life depends.

Therefore, on the basis of available data, non-pesticidal organotin compounds are not considered to be "toxic" as defined under Paragraph 11(b) of the Canadian Environmental Protection Act.

3.3 CEPA 11(c) Human Life or Health

Population Exposure. Available data on the exposure of the general population to non-pesticidal organotin compounds are extremely limited. Information on concentrations in air has not been identified and data on concentrations in drinking water are limited to only a few samples from Florida. A potential source of non-pesticidal organotin compounds in drinking water, however, is migration of stabilizers from PVC pipe, which is used fairly extensively in distribution systems in Canada.

Available data on concentrations in food are limited to small surveys of individual foodstuffs designed principally for development of analytical methods (Forsyth and Cléroux, 1991; Forsyth et al., 1992). Information relevant to estimation of population exposure is restricted to concentrations of monobutyltin and dibutyltin in a limited number of marine species (Forsyth and Cléroux, 1991), monobutyltin in fruit drinks (Forsyth, 1992), and mono-octyltin and dioctyltin in edible oils and fruit drinks (Forsyth, 1992).

In addition, available models and data on sources and physical/chemical properties are inadequate to allow prediction of which medium, e.g., food, air, or drinking water, is potentially the most important source of exposure of the general population to the non-pesticidal organotin compounds.

Available data are considered insufficient, therefore, to qualitatively or quantitatively estimate exposure of the general population in Canada to any of the non-pesticidal organotin compounds.

Effects. The initial step in evaluating whether any of the non-pesticidal organotin compounds considered here are "toxic" as defined under Paragraph 11(c) of CEPA is an assessment of the weight of evidence for carcinogenicity (an effect for which it is generally believed that there is no threshold), based on the scheme developed by the Bureau of Chemical Hazards in the derivation of the "Guidelines for Canadian Drinking Water Quality" (Health and Welfare Canada, 1989). Only one carcinogenicity bioassay has been reported for any of the compounds (i.e., dibutyltin diacetate) considered for this assessment. In this study, there was no evidence of carcinogenicity in male rats and mice and no convincing evidence in female mice exposed to dibutyltin diacetate. Results in female rats were inadequate for the assessment (United States National Cancer Institute, 1978). The results have been mixed from a limited number of investigations concerning the genotoxicity of dibutyltin compounds. Owing to the limitations of the carcinogenesis bioassay (in particular, the accidental loss of tissue samples for high dose females), dibutyltin compounds have been classified in Group V ("inadequate data for evaluation") of the classification scheme for carcinogenicity developed for use in the derivation of the "Guidelines for Canadian Drinking Water Quality" (Health and Welfare Canada, 1989).

Additional data on the carcinogenicity of any of the non-pesticidal organotin compounds considered in this assessment are restricted to inadequate studies on mixtures of monomethyltin and dimethyltin compounds (Mosinger, 1979).

Therefore, all non-pesticidal organotin compounds considered in this assessment have been classified in Group V ("inadequate data for evaluation") of the classification scheme for carcinogenicity developed for use in the derivation of the "Guidelines for Canadian Drinking Water Quality" (Health and Welfare Canada, 1989).

For compounds classified in Group V of this scheme, tolerable daily intakes (TDIs) are developed based on division of relevant effect levels by appropriate uncertainty factors. Based on a preliminary review of available information, it appears that adequate data on subchronic repeated dose toxicity in experimental animals to serve as a basis for development of a TDI are only available for monomethyltin, dimethyltin, and dibutyltin compounds. Though subchronic studies on dioctyltin compounds are available, they appear, on the basis of preliminary review, to be limited in terms of number of dose levels, duration of exposure, group sizes and possibly, the range of endpoints examined.

To assess whether compounds classified in Group V are "toxic" under CEPA, tolerable daily intakes are compared to estimates of population exposure. However, available data are considered insufficient to qualitatively or quantitatively estimate exposure of the general population in Canada to any of the non-pesticidal organotin compounds considered here. Therefore, it is not possible to evaluate whether current concentrations of any of these compounds present in the environment constitute a danger in Canada to human life or health.

Therefore, on the basis of available data, it is not possible to assess whether any of the non-pesticidal organotin compounds considered here are "toxic", as defined under Paragraph 11(c) of the Canadian Environmental Protection Act.

3.4 Conclusion

On the basis of available data, non-pesticidal organotin compounds are not considered to be "toxic" as defined under Paragraphs 11(a) and 11(b) of the Canadian Environmental Protection Act. It has been concluded that available data are insufficient to assess whether non-pesticidal organotin compounds are "toxic", as defined under Paragraph 11(c) of the Canadian Environmental Protection Act