Follow-up to ecological risk assessment of organotin substances on domestic substances list: chapter 4

4. Effects

The toxicity of organotins to aquatic organisms is generally understood to result from exposure to the R_xSn^(4-x)+ moiety and to be largely independent of the anionic moiety (Maguire, 1992). There are a few exceptions to this generality, as highlighted in Table 6. For acute toxicity to red killifish, Oryzias latipes, dibutyltin maleate is less toxic than dibutyltin oxide, diacetate and dilaurate. In the bioassays with the diatoms Skeletonema costatum and Thalassiosira pseudonana, there is a decrease in toxicity in the order dibutyltin diacetate, dichloride and difluoride.

Toxicity data for selected organotin substances were presented in Government of Canada (1993) and Maguire (1992). A search of the ECOTOX database was conducted in January 2003 for aquatic toxicity data for the organotin substances on Canada's DSL. Table 7 presents the most sensitive aquatic toxicity results for a number of subcategories of organotins.

Toxicity data presented in Table 7 are not necessarily the same as those used in the assessment of the nine new and/or transitional organotins. Environment Canada (2006) considered toxicity data for substances that are most closely related to the nine specific substances that were assessed, whereas the data presented in Table 7 represent the most toxic results within subcategories of organotins.

For monomethyltins, the most sensitive freshwater organism is the green alga, Scenedesmus obliquus, with a 96-h EC₅₀ (growth) of 178 µg/L (from trichloromethylstannane) (Huang et al., 1996).

For dimethyltins, the most sensitive freshwater organism is Scenedesmus obliquus, with a 96-h EC₅₀ (growth) of 756 µg/L (from dichlorodimethylstannane) (Huang et al., 1993).

For monobutyltins, the most sensitive freshwater organism is the water flea, Daphnia magna, with a chronic Maximum Acceptable Toxicant Concentration (MATC) of 16 µg/L (from butyltin tris(2-ethylhexylmercaptoacetate)) (Organotin Environmental Programme sponsored study, cited in Environment Canada, 2006).

For dibutyltins, the most sensitive freshwater organism is Daphnia magna, with a 48-h EC₅₀ of 13 µg/L (from dibutyltin bis(2-ethylhexylmercaptoacetate)) (IUCLID, 2002a). A 21-day No-Observed-Effect Concentration (NOEC) of 8 µg/L (from dibutyltin dichloride) was reported for Daphnia magna (Analytical Bio-Chemistry Laboratories, 1990).

It should be noted that tributyltin can be present as an impurity in commercial and probably laboratory dibutyltin formulations and can contribute significantly to the toxicity of these formulations. For example, Lytle et al. (2003) noted that a tributyltin impurity as low as 0.1% may have a significant influence on the perceived toxicity of dibutyltins. The authors expressed the opinion that it may not be possible to reduce levels of impurities in dibutyltins much below 0.1%.

For tributyltins, the most sensitive freshwater organism is the guppy, Poecilia reticulata, with a 90-day NOEC of 0.01 µg/L (from hexabutyldistannoxane) (Becker, 1992; data presented in ECOTOX). The 110-day Lowest-Observed-Effect Concentration (LOEC) for rainbow trout, Oncorhynchus mykiss, yolk sac fry was 0.173 µg/L (from tributyltin chloride), based on increased mortality and decreased resistance to Aeromonas (de Vries et al., 1991). Induction of "imposex" (the imposition of male sexual characteristics on females) has been reported for dogwhelks, Nucella lapillus, a marine gastropod, at tributyltin concentrations of 0.001 µg/L, whereas sterilization of females is initiated at 0.007-0.012 µg/L (Bryan et al., 1988; Gibbs et al., 1988). Shimasaki et al. (2003) reported that tributyltin oxide induced sex reversal of genetic female Japanese flounders, Paralichthys olivaceus, into phenotypic males when they were exposed to the substance in food at concentrations of 0.1 µg/g (25.7% sex reversal) and 1.0 µg/g (31.1% sex reversal). In sediments, the 21-day IC₅₀, for the mayfly, Hexagenia spp., based on growth, was 1.5 µg/g dry weight (from tributyltin chloride) (Day et al., 1998). This is consistent with the observation of Meador (2000) that chronic effects on benthos may occur at tributyltin concentrations in the range 0.1-1 µg/g dry weight of sediment. According to Meador (2000), tributyltin toxicity may result from several specific modes of action, including endocrine disruption, as well as alterations in energy production, the P-450 enzyme system and heme metabolism.

For tetrabutyltin, the most sensitive freshwater organism is the fathead minnow, Pimephales promelas, with a 96-hour LC₅₀ of 45 µg/L (Geiger et al., 1990; data presented in ECOTOX).

For monooctyltins, a 48-hour LC₅₀ for Daphnia magna of >234 µg/L (from monooctyltin trichloride) was the only study result identified (Schering AG, 1998).

For dioctyltins, the most sensitive freshwater organism is Daphnia magna, with a 48-hour LC₅₀ of 4.1 µg/L (from dioctyltin dichloride) (Steinhauser et al., 1985).

For triphenyltins, the most sensitive freshwater organisms were rainbow trout yolk sac fry, with a 110-day LOEC of 0.209 µg/L (from triphenyltin chloride, based on increased mortality, liver glycogen depletion and decreased resistance to Aeromonas) (de Vries et al., 1991). The Canadian Water Quality Guideline for the Protection of Freshwater Aquatic Life is 0.022 µg/L (CCME, 1999).

For tetraphenyltin, a 48-hour LC₅₀ for the Japanese medaka, Oryzias latipes, of 398 µg/L was the only study result identified (Nagase et al., 1991).