Follow-up to ecological risk assessment of organotin substances on domestic substances list: chapter 1


1. Introduction

Between August 1994 and March 2000, the Minister of the Environment received notifications for nine organotin substances as “new” and/or “transitional” substances pursuant to subsection 26(2) of the 1988 Canadian Environmental Protection Act (CEPA) - now subsection 81(1) of the Canadian Environmental Protection Act, 1999 (CEPA 1999, the statute that has repealed and replaced CEPA). These substances were proposed for importation or manufacture in Canada as stabilizers for polyvinyl chloride (PVC) products, as intermediates used in the manufacture of organotin stabilizers and as material preservatives for building material formulations.

These new and transitional substances were assessed, and it was concluded that the nine substances are entering or may enter the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity. Therefore, these substances are suspected of meeting the criterion set out in paragraph 64(a) of CEPA 1999.

Environment Canada has produced a consolidated assessment report that presents the basis for the environmental concerns with these nine substances (Environment Canada, 2006). The report, entitled Risk assessment of new and transitional organotin substances, is available upon request from the Director, Ecological Assessment Division, Environment Canada, at nsn-infoline@ec.gc.ca.

The purpose of the present document is to determine if the conclusions reached for the nine notified individual organotins would also apply to other organotin substances on Canada's Domestic Substances List (DSL).

Organotin substances are generally represented by the formula RxSnL4-x (where x = 1-4). For these chemicals, "R" is a typical organic group, such as methyl, octyl, butyl or phenyl, which is covalently bonded to the tin atom by a carbon-tin bond. The other anionic moieties on tin, designated "L," may be halogens (chloride, fluoride, etc.) or other sulphur- or oxygen-based organic moieties, such as -SR', -OR', -OC(O)R', -S-, -O-, etc. Organotins are designated mono-, di-, tri- or tetrasubstituted depending upon the number of carbon-tin bonds in the molecule.

There are 109 organotin substances on the DSL, 104 of which belong to nine subcategories of organotins, as outlined in Table 1. Six additional subcategories, tri- and tetramethyltins, tri- and tetraoctyltins and mono- and diphenyltins, contain no substances on the DSL. These subcategories are not considered in this ecological assessment. Tetrabutyltins are not on the DSL, but a tetrabutyltin was one of the nine organotins notified as new and/or transitional substances under CEPA 1999, and tetrabutyltins are addressed in this report. The remaining five organotin substances on the DSL, three discrete organotin substancesFootnote 1 and two UVCBsFootnote 2 (Unknown or Variable Composition Complex Reaction Products or Biological Materials), do not belong to these subcategories, and these five substances were not addressed. Annex 1 presents the organotin substances on the DSL.

Table 1: Organotin Subcategories -- Number of Organotin Substances on the DSL
  Methyltin Butyltin Octyltin Phenyltin
Mono- 13 18 2 0
Di- 15 38 8 0
Tri- 0 8 0 1
Tetra- 0 0 0 1

"Non-pesticidal organotin compounds" were assessed after addition onto the first Priority Substances List (PSL1) and were concluded not to be toxic to the environment (Government of Canada, 1993). At that time, there was insufficient information available to conclude on risk to human health. Since that time, Health Canada has completed a PSL1 follow-up report on organotins and concluded that non-pesticidal organotins do not present a danger to human life or health (Government of Canada, 2003).

The PSL1 assessment (Government of Canada, 1993) focused on mono- and dimethyltins, mono- and dibutyltins and mono- and dioctyltins and based exposure on monitoring data. Most environmental sampling was done in harbours, marinas and shipping lanes, where contamination by butyltin compounds from antifouling paints was expected. It was determined that mean concentrations of non-pesticidal organotins found in these waters were below the estimated effects thresholds for these substances.

There are renewed environmental concerns with organotins for several reasons. The quantity of organotin compounds used as PVC stabilizers has increased substantially in recent years. The PSL1 assessment (Government of Canada, 1993) estimated that 1300 tonnes were imported into Canada in 1984. In the assessment of the nine notified substances, Environment Canada (2006) estimated that up to 400 tonnes of tin stabilizers could be used annually at a single facility. Rinsing of shipping containers, storage tanks and associated transfer hoses was identified by Environment Canada (2006) as a potentially significant source of release of organotin substances to the environment. Further, some recent toxicity studies indicate that some organotin substances are more toxic to sensitive aquatic organisms than had previously been recognized. Finally, additional concerns have emerged regarding the potential for persistent and bioaccumulative substances to cause harm in the environment. There are limitations in using traditional short-term laboratory toxicity tests for such substances and uncertainties in applying risk quotient-based assessment methods to such substances.

This document focuses principally on the freshwater environment, because the organotin substances considered in this report have mainly non-pesticidal uses and their releases are expected to be primarily to fresh water. However, information from marine or estuarine environments is sometimes used -- for example, in the discussion of the persistence and bioaccumulation of organotin substances.

This assessment uses a group approach, reaching a conclusion for each organotin subcategory that contains substances on the DSL and for tetrabutyltins.

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