Environment

A number of different organotin compounds were analyzed in environmental samples in previous surveys and monitoring programs. Levels found in most cases originated from the use of trisubstituted organotin species in biocidal applications. The tributyltin compounds naturally break down to dibutyltin, monobutyltin, and finally, to inorganic tin. However, inorganic tin is biologically methylated in the environment [5, 6]. Consequently, the use of methyltin stabilizers is expected to contribute only marginally to the overall load of methylated tin in the environment. The U.S. EPA estimates that only ppt (parts per trillion) quantities of organotins are contributed as a result of the use of PVC heat stabilizers [7].

The only source of octyltin compounds, when identified in the environment, is their application as PVC stabilizer. In several studies mono- and dioctyltin substances were not found in surface waters or in leachates from landfills, urban raw wastewater, or urban run-offs [8, 9, 10]. However, in a recent Swedish survey on effluents from sewage treatment plants small amounts of dioctyltin substances were occasionally found in the sewage sludge, but no dioctyltin substances were detected in the water phase [11, 12]. After discussions of the results with the Swedish authorities, it was agreed that ORTEPA repeats the analyses following the same protocol. Again, no octyltin substances were found in the water phase [12], however, the concentrations of dioctyltin in all sewage sludge samples were found to be below the detection limit, while monoctyltin was close to the detection limit. ORTEPA is still investigating whether these emissions come from point sources.

Employing standardized model calculations on the distribution of dioctyltin compounds ORTEPA intends to broaden the database on the environmental occurrence. According to various studies performed on rigid PVC pipes, very low concentrations of organotins were found as a result of leaching into water. Under dynamic conditions, leached concentrations rapidly reached a minimum level (parts per trillion) [13, 14, 15, 16, 17, 18].

In summary, there is no environmental risk expected to be associated with the use of organotin compounds as PVC stabilizers.

Degradation
Photochemical degradation of alkyltins released into the aquatic environment probably plays only a minor role in the transformation pathway under environmental conditions, since the light spectrum and intensity is reduced by the water surface and turbidity in the water column. Alkyltin buried in soils (e.g., by landfill disposal) is supposed to be unaffected by sunlight

Biological degradation of methyl-, butyl- and octyltin compounds was observed under aerobic conditions in the aquatic environment. Half-lives were reported to be between some days and several weeks, and decreased with higher temperatures. Organotins are not expected to be persistent for long periods in the aquatic environment [1].

Partial mineralization was found for dioctyltin in standard biodegradation tests [19]. Anaerobic degradation for organotin compounds is generally slow [20].

Organotin compounds used as PVC stabilizers are not persistent in the environment mainly due to microbial activity. All organotin stabilizers eventually degrade into inorganic tin.
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Effects
The short-term toxicity (EC₅₀/24-96 h) of mono- and dimethyltin compounds is in the range of 0.1 to > 9.6 mg/L for the most sensitive species (diatom algae) [21, 22, 23]. The toxicity (EC₅₀/24-48 hours) of mono- and dibutyltin compounds with the water flea, Daphnia magna, is at concentrations of 1 to 10 mg/L [24, 25, 26], and dibutyltin (EC₅₀/48 hours) with oyster larvae at concentrations of 0.1 to 0.2 mg/L [27]. Monooctyltin is not toxic up to the solubility limit in bacterial tests [28, 29]. Reports on the toxicity of dioctyltin compounds cover a large range of concentrations with one value at 0.005 mg/L [30] and others in the range of 1 to 8 mg/L (EC₅₀/24-48 hours) for Daphnia magna [31, 32, 33]. Because of this unexpected discrepancy, ORTEPA is currently conducting a chronic test of dioctyltin with D. magna to extend the database.

Effects on terrestrial organisms are reported only for mixtures of mono- and dimethyltin compounds with LC₅₀/14 days values of 140 to > 1,000 mg/kg in the earthworm Eisenia foetida [34, 35, 36].

For several freshwater and saltwater algae species inorganic tin, which occurs as the end product of degradation processes of alkyltin compounds, is less toxic than all the alkyltins named above [37]. These data show that the ecotoxicity of organotin compounds used for stabilizers and of their degradation products is relatively low since effect concentrations are some orders of magnitude higher than for the tri-substituted organotin compounds used as biocides.
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Risk Assessment
In order to evaluate the environmental risk, the data on measured concentrations in the environment have to be compared with environmental no-effect concentrations, the latter usually extrapolated from laboratory data and termed as predicted no-effect concentrations (PNEC).

For mono- and dimethyltin and mono- and dibutyltin compounds the measured environmental concentrations in water are much lower than the predicted no-effect concentrations.

For octyltin, a risk assessment for the aquatic environment performed in Sweden concludes that octyltin stabilizers could possibly be hazardous to the environment. However, in the analyses of effluents taken from the Swedish wastewater treatment plants, carried out by ORTEPA [12], all dioctyltin concentrations were found to be below the detection limit of 0.01 �g/L. Using a dilution factor of 10 for concentrations in surface waters, the environmental concentration (EC) of dioctyltin is below 0.001 �g/L. Even if the lowest reported EC₅₀ of a standard acute toxicity test with D. magna (i.e., 5 �g/L divided by an assessment factor of 200 as suggested by the [ECETOC] European Center for Ecotoxicology and Toxicology [38]) is used for the calculation, the resulting PNEC is 0.025 �g/L. This means that the ratio EC/PNEC is <1, indicating that no risk is to be expected for the aquatic environment. A Canadian risk assessment carried out by the Department of the Environment came to the same conclusions [39].

No relevant concentrations in terms of ecotoxicity for mono- and dimethyltin, mono- and dibutyltin, and mono- and dioctyltin are expected to be present in sediments and soils or in biota.
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