Tiered environmental risk assessment of methyltins from heat stabilizers in rigid PVC in Sweden

The methyltins, methyltin 2-ethylhexyl mercaptoacetate and methyltin-2-mercaptoethyloleate sulfide, are used as heat stabilizers in rigid PVC. The degradation products monomethyltintrichloride (MMTTC) and dimethyltindichloride (DMTDC) may leach from the PVC, and enter the environment. A risk assessment was executed for the use of these products in Sweden, with the double aim to (a) assess the potential environmental risks, and (b) explore the possibilities of a tiered risk assessment. An initial assessment was executed with EUSES 1.0. The initial assessment showed no significant environmental risks on a regional scale. On a local scale however, emissions from a sewage treatment plant to the surface water pose a potential risk for the near proximity. In a second tier, this critical exposure route was studied in more detail; an improved exposure scenario was drawn up; and an additional sorption experiment was performed. This refinement resulted in much lower predicted environmental concentrations (≪PNEC). Measured methyltin concentrations in countries where methyltin heat stabilizers are registered, reveal concentrations ranging from < detection limit to intermediate values between concentrations predicted in the initial and refined assessment. The tiered assessment scheme proved valuable. Using this, the environmental risks due to the leaching of MMTTC and DMTDC from rigid PVC in Sweden are of no concern.