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ARCHIVED - Draft Follow-up to the 1993 Ecological Risk Assessment of Organotin Substances on Canada's Domestic Substances List

5. Risk Characterization

In the environmental assessments of organotin substances notified 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 are not of concern to the environment.

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 tetrabutyltin and tetraphenyltin to cause environmental harm is also evaluated separately, because of their status as precursors to tributyltin and triphenyltin compounds, respectively.

5.1 Organotins Other than Tributyltins, Triphenyltins and Their Precursors

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.

Table 8: PEC/PNEC Ratios for Various Organotin Substances, Based on the Highest Environmental Concentrations Predicted for New and/or Transitional Organotinsa
 PEC
(µg/L)
CTV
(µg/L)
PNEC
(µg/L)
PEC/PNEC
Monomethyltins2.0178180.1
Dimethyltins2.0756760.03
Monobutyltins2.0161.61.3
Dibutyltins2.0130.1315
Monooctyltins2.0>234>2.34<0.9
Dioctyltins2.04.10.0450

a In this table, CTVs and PNECs are expressed on a hydrolyzed basis, which does not include the anionic moieties, whereas PECs are expressed on a whole-molecule basis, which does include the anionic moieties. PECs expressed on a hydrolyzed basis would be smaller, and therefore the PEC/PNEC ratios would also be smaller.

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. Similar results are obtained if PECs are based on the maximum measured concentrations in Canadian surface waters reported in Table 2.

Table 9: PEC/PNEC Ratios for Various Organotin Substances, Based on the Highest Environmental Concentrations Predicted for New and/or Transitional Organotins Under Industry-wide Stewardship Practicesa
 PEC
(µg/L)
CTV
(µg/L)
PNEC
(µg/L)
PEC/PNEC
Monomethyltins0.008178180.0004
Dimethyltins0.008756760.0001
Monobutyltins0.008161.60.005
Dibutyltins0.008130.130.06
Monooctyltins0.008>234>2.34<0.003
Dioctyltins0.0084.10.040.2

a In this table, CTVs and PNECs are expressed on a hydrolyzed basis, which does not include the anionic moieties, whereas PECs are expressed on a whole-molecule basis, which does include the anionic moieties. PECs expressed on a hydrolyzed basis would be smaller, and therefore the PEC/PNEC ratios would also be smaller.

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 may itself be a significant indication of its potential to cause environmental harm. 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. Evidence that a substance is used in Canada in moderate to large quantities (e.g., greater than 1000 kg/year) in a variety of locations and/or that use quantities are increasing may also be taken as an indicator of significant potential for exposure.

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

Table 10: Evaluation of Indicators of Potential for Tributyltins and Triphenyltins to Cause Environmental Harm
IndicatorEvaluation
TributyltinsTriphenyltins
Persistence (as defined in CEPA 1999 Regulations)Tributyltins are persistent in sediment (t1/2 = 0.9-15 years). However, potential for long-range transport is limited, because they are not persistent in water and do not partition to air.Triphenyltins are persistent in sediment (t1/2 = 3.1 years). However, potential for long-range transport is limited, because they are not persistent in water and do not partition to air.
Bioaccumulation (as defined in CEPA 1999 Regulations)Tributyltins are highly bioaccumulative, with reported BAFs of up to 900 000. Concentrations in tissues of top predators are also high (up to 4 µg/g wet weight). Limited evidence of weak biomagnification has been reported for some marine food chains.Triphenyltins are highly bioaccumulative, with reported BCFs of up to 11 400. Concentrations in the tissues of deep-sea fish are also high (up to 4.2 µg/g wet weight). Although data are very sparse, triphenyltins may have limited potential to biomagnify.
Inherent toxicityTributyltins are toxic at low concentrations. For example, the NOEC for the guppy is reported to be 0.01 µg/L. Furthermore, several specific modes of toxic action are possible, including endocrine disruption.Triphenyltins are toxic at low concentrations. For example, the LOEC (mortality) for rainbow trout yolk sac fry is reported to be 0.209 µg/L.
Natural occurrenceTributyltins are not naturally occurring substances.Triphenyltins are not naturally occurring substances.
Use quantitiesThere are eight tributyltin compounds on the DSL. One transitional tributyltin is reported to be manufactured in moderate quantities (150 000 kg/year) for export at one Canadian facility.There is one triphenyltin compound on the DSL. It is believed that this substance is no longer used in Canadian commerce.
Widespread occurrenceTributyltins have been detected in surface waters and sediments throughout Canada, although levels have decreased since restrictions were placed on use of tributyltins in antifouling paints in Canada in 1989.Triphenyltins have not been detected in Canadian surface waters but have been detected in sediment samples from British Columbia, Ontario and Nova Scotia.

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. Consequently, risk quotients for persistent and bioaccumulative substances are often estimated more conservatively, using, for example, an additional assessment factor of 10 applied to the PNEC.

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. Although an extra assessment factor of 10 was not used to derive these PNECs, all risk quotients in Table 11 are significantly greater than 1.

Table 11: PEC/PNEC Ratios for Tributyltin Substances
 PECCTVPNECPEC/PNEC
Water: modelled PECa0.22 µg/L-0.008 µg/L28
Water: measured PECb0.043 µg/L-0.008 µg/L5.4
Sediment: modelled PECa7.8 mg/kg dry weight1.5 mg/kg dry weight0.015 mg/kg dry weight520
Sediment: measured PECb2.4 mg/kg dry weight1.5 mg/kg dry weight0.015 mg/kg dry weight160

a Highest environmental concentrations predicted for new and/or transitional tributyltins associated with chemical manufacturing, expressed as hydrolyzed tributyltin.
b Highest concentrations measured in Canada (Tables 2 and 3).

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 Tetrabutyltin and Tetraphenyltin

As noted in section 3.1.2, like most other organotins, tetrabutyltin and tetraphenyltin are not believed to be persistent in the environment. Tetrabutyltin is 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, tetrabutyltin 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 tetrabutyltin using a quotient method. A PEC of 1.2 µg/L may be established based on the highest concentration of tetrabutyltin 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 tetrabutyltin and tetraphenyltin have the potential to cause environmental harm. However, it is believed that tetraphenyltin is not currently in use in Canada.

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