Follow-up to ecological risk assessment of organotin substances on domestic substances list: chapter 5

5. Risk Characterization

• 5.1 Monoalkyltins and Dialkyltins
• 5.2 Tributyltin and Triphenyltin Compounds
• 5.3 Tetrabutyltins and Tetraphenyltin

In the environmental assessments of organotin substances notified as new and/or transitional substances between August 1994 and March 2000, Environment Canada used conservative estimates of rates of release to water and assessment factors to calculate estimated concentrations of concern. It was concluded that concentrations of these substances in water and sediments would be high enough to potentially harm organisms. Therefore, the notified substances are suspected to be toxic under Paragraph 64(a) of CEPA 1999 (Environment Canada, 2006).

Environment Canada (2006) noted that with effective stewardship practices in effect at facilities using organotin stabilizers, releases of these substances would be minimized to levels that would not be expected to harm aquatic organisms.

Because of the unique concerns relating to persistent and bioaccumulative substances, the potential for tributyltin and triphenyltin compounds to cause environmental harm has been evaluated separately. The potential for tetrabutyltins and tetraphenyltin to cause environmental harm is also evaluated separately, because of their status as precursors to tributyltin and triphenyltin compounds, respectively. 5.1 Monoalkyltins and Dialkyltins

Table 8 presents ratios of PEC to Predicted No-Effect Concentration (PNEC) calculated using the toxicity data for pelagic organisms presented in section 4 of this report and the PEC based on the highest concentration predicted in the assessments of new and/or transitional organotins (section 3.2.1). Use was assumed to be as PVC stabilizers. The quantities of organotins used as catalysts and in glass coatings are believed to be much less than the quantity used as PVC stabilizers, so PECs and PEC/PNEC ratios resulting from these uses would be substantially lower than the values presented in Table 8. In Table 8 and subsequent tables, the toxicity values are termed Critical Toxicity Values, or CTVs. In general, PNECs are calculated by dividing the CTVs by assessment factors (100 for lethal acute endpoints and 10 for algal EC₅₀ and chronic endpoints). Mono- and dibutyltins and dioctyltins have PEC/PNEC ratios greater than 1 using this approach which indicates a potential to cause harm.

5.1 Monoalkyltins and Dialkyltins

Table 8 presents ratios of PEC to Predicted No-Effect Concentration (PNEC) calculated using the toxicity data for pelagic organisms presented in section 4 of this report and the PEC based on the highest concentration predicted in the assessments of new and/or transitional organotins (section 3.2.1). Use was assumed to be as PVC stabilizers. The quantities of organotins used as catalysts and in glass coatings are believed to be much less than the quantity used as PVC stabilizers, so PECs and PEC/PNEC ratios resulting from these uses would be substantially lower than the values presented in Table 8. In Table 8 and subsequent tables, the toxicity values are termed Critical Toxicity Values, or CTVs. In general, PNECs are calculated by dividing the CTVs by assessment factors (100 for lethal acute endpoints and 10 for algal EC₅₀ and chronic endpoints). Mono- and dibutyltins and dioctyltins have PEC/PNEC ratios greater than 1 using this approach which indicates a potential to cause harm.

With industry-wide stewardship practices related to use as PVC stabilizers in place, the PEC is lower (section 3.2.1), and therefore the PEC/PNEC ratios are reduced to below 1, as shown in Table 9.

5.2 Tributyltin and Triphenyltin Compounds

Tributyltin and triphenyltin compounds meet both persistence and bioaccumulation criteria specified in the Persistence and Bioaccumulation Regulations of CEPA 1999 (Government of Canada, 2000).

There are special concerns with such highly persistent and bioaccumulative substances. Although current science is unable to accurately predict the long-term ecological effects of these substances, they are generally acknowledged to have the potential to cause serious and possibly irreversible impacts. Assessments of such substances must therefore be performed using a preventative, proactive approach, to ensure that such harm does not occur.

Evidence that a substance is persistent and bioaccumulative when taken together with potential for environmental release or formation and potential for toxicity to organisms provides a significant indication that it may be entering the environment under conditions that may have harmful long-term ecological effects. Persistent substances remain in the environment for long periods of time, increasing the probability and the duration of exposure. Persistent substances that are subject to long-range transport are of particular concern because they can result in low-level, regional contamination. Releases of extremely small amounts of persistent and bioaccumulative substances may lead to relatively high concentrations in organisms over wide areas. Very bioaccumulative and persistent substances may also biomagnify through the food chain, resulting in especially high internal exposures for top predators. Because they are widespread, several different persistent and bioaccumulative substances may be present simultaneously in the tissues of organisms, increasing the likelihood and potential severity of harm.

Other factors can increase concerns regarding the potential for persistent and bioaccumulative substances to cause environmental harm. For example, there is a particular concern for substances that have the potential to harm organisms at relatively low concentrations and/or that have specific modes of toxic action (in addition to narcosis). Evidence that a substance does not occur in the environment naturally may also indicate an elevated potential to cause harm, since organisms will not have had very long to develop specific strategies for mitigating the effects of exposures. Evidence from monitoring studies indicating that a substance is widespread in the environment and/or that concentrations have been increasing over time is an indicator of elevated exposure potential.

Each of the lines of evidence mentioned above is reviewed for tributyltin and triphenyltin compounds in Table 10.

Although risk quotients may also be used to indicate potential to cause environmental harm for persistent and bioaccumulative substances, risks are likely to be underestimated using this approach. For example, if steady state has not been achieved in the environment and concentrations are continually increasing, measured PECs will be too low. In addition, PNECs may be too high because of the long time needed to achieve steady state and the lack of exposure through food consumption in typical short-term laboratory toxicity tests.

Nevertheless, risk quotients for tributyltins were calculated for comparison purposes. PEC/PNEC ratios for tributyltin compounds, based on both modelled and measured PECs in water and sediment, are shown in Table 11. The Canadian Water Quality Guideline for the Protection of Freshwater Aquatic Life of 0.008 µg/L (CCME, 1999) was used as the PNEC for tributyltins in water. The most sensitive benthic organism reported was the mayfly, Hexagenia spp., with a 21-day IC₅₀ (growth) of 1.5 mg tributyltin/kg dry weight. Dividing this toxicity value by an assessment factor of 100 (10 to extrapolate from an acute to a chronic no-effect level, and 10 to account for extrapolation from laboratory to field conditions and for inter- and intraspecies variability) gives a chronic PNEC of 0.015 mg/kg dry weight for tributyltins in sediment. All risk quotients in Table 11 are significantly greater than 1.

Based on the various lines of evidence presented above, it is concluded that both tributyltin and triphenyltin compounds have the potential to cause environmental harm. However, it is believed that triphenyltins are not currently in use in Canada.

5.3 Tetrabutyltins and Tetraphenyltin

As noted in section 3.1.2, like most other organotins, tetrabutyltins and tetraphenyltin are not believed to be persistent in the environment. Tetrabutyltins are expected to degrade by removal of one of the alkyl groups attached to the tin atom, producing tributyltins. Tetraphenyltin is expected to degrade to triphenyltin compounds in a similar manner. As precursors to persistent and bioaccumulative compounds that have the potential to cause environmental harm, tetrabutyltins and tetraphenyltin are themselves considered to have the potential to cause harm.

Data are also available to evaluate risks to pelagic organisms from direct exposure to tetrabutyltins using a quotient method. A PEC of 1.2 µg/L may be established based on the highest concentration of tetrabutyltins predicted in the assessments of new and/or transitional organotins associated with use as a chemical intermediate (Environment Canada, 2006). A PNEC of 0.45 µg/L may be derived by dividing the acute CTV of 45 µg tetrabutyltin/L (see Table 7) by an assessment factor of 100. Based on these data, the PEC/PNEC ratio is 2.7, suggesting some potential to cause harm to pelagic organisms.

Considering the above lines of evidence, it is concluded that both tetrabutyltins and tetraphenyltin have the potential to cause environmental harm. However, it is believed that tetraphenyltin is not currently in use in Canada.