Organotins: dialkyltins, tributyltin oxide
(endorsed 1996)
Guideline
Dialkyltins: Data are inadequate to set a guideline value for drinking water.
Tributyltin oxide: Based on health considerations, the concentration in drinking water should not exceed 0.001 mg/L.
General description
The group of compounds known as the organotins comprises a large number of compounds with different properties and applications. Of these the dialkyl and tributyltin compounds have some application in the water industry and are the ones most likely to be found in drinking water supplies.
The dialkyltins are widely used as stabilisers in plastic, and may leach out of PVC water pipes for a short time after installation. In one study, dibutyltin sulfide was detected at a concentration of 0.01 mg/L in water that was in static contact with PVC pipes.
Tributyltins are used as biocides and have occasionally been detected in raw water in Canada, the United States, the United Kingdom and Switzerland, probably because of their use as antifouling agents on boats. The use of tributyl-organotin compounds, particularly tributyltin oxide, in antifouling paints has now been banned in a number of countries because it is extremely toxic to aquatic life. Tributyltin is also used as a biocide in boiler waters. Other organotins are unlikely to be found in water.
Typical values in Australian drinking water
Organotins have not been found in Australian drinking waters. They are included here to provide guidance in the unlikely event of contamination, and because they have been detected occasionally in drinking water supplies overseas.
Treatment of drinking water
No published reports are available on water treatment procedures that can be used to remove organotins from drinking water.
Measurement
The organotins can be analysed using a solvent extraction procedure (Greaves and Unger 1988). They are extracted using a hexane–tropolone mixture and derivatised to form hexylbutyltins. Analysis is by gas chromatography with flame photometric detection. Limits of determination are less than 0.000002 mg/L (2 ng/L).
Health considerations
Few data are available on the absorption and distribution of organotins in the body, but animal studies have reported that some of the compounds are poorly absorbed, and distributed primarily to the liver and kidney.
An extensive review and summary of the human and animal toxicity data for tributyltin compounds is available (IPCS 1990).
The dialkyltins have low general toxicity. A study using rats fed dialkyltin for 3 months reported depressed growth and mild anaemia only at the highest dose used (4mg/kg body weight per day). No toxic effects were observed at lower doses. Other studies with rats and dogs reported similar results. Carcinogenicity bioassays with animals have been inconclusive.
No data are available on the ingestion of tributyltin oxide in humans, although occupational information and dermal exposure are known to cause irritation. A number of long-term animal studies have been undertaken using tributyltin oxide. A 2-year chronic toxicity study using rats concluded that doses of 50 mg/kg body weight per day can induce toxicity to some organs including the thyroid and pituitary glands. A no-effect level of 0.5 mg/kg body weight per day was established from this study (Wester et al. 1990). An immunotoxicity study on the suppression of resistance to nematodes in rats identified a no-effect level of 0.025 mg/kg body weight per day (Vos et al. 1990). The latter immunotoxicity study is considered more sensitive but the significance to humans is questionable. The no-effect levels agree to about an order of magnitude.
Tributyltin oxide was not mutagenic with bacteria and yeast but caused a significant increase in the number of benign tumours of the pituitary gland when fed to rats for 2 years.
Derivation of guideline
The guideline value of 0.001 mg/L for tributyltin oxide in drinking water was determined as follows:
where:
0.025 mg/kg body weight per day is the no-effect level from a 18-month immunotoxicity study using rats (Vos et al. 1990).
70 kg is the average weight of an adult.
0.1 is the proportion of total daily intake attributable to the consumption of water.
2 L/day is the average amount of water consumed by an adult.
100 is the safety factor in using the results of an animal study as a basis for human exposure (10 for interspecies variations and 10 for intraspecies variations).
The World Health Organization guideline value of 0.002 mg/L was based on 20% of total daily intake coming from drinking water. The proportion contributed by drinking water to the total Australian intake is probably less.
References
Greaves J, Unger MA (1988). A selected ion monitoring assay for tributyltin and its degradation products. Biomedical and Environmental Mass Spectrometry, 15:565–569.
IPCS (International Programme on Chemical Safety) (1990). Tributyltin compounds. Environmental Health Criteria, 116. World Health Organization, IPCS.
Vos JG, DeKlerk A, Krajnc EI, VanLoveren H, Rozing J (1990). Immunotoxicity of bis(tri-n-butyltin)oxide in the rat: Effects of thymus dependent immunity and on nonspecific resistance following long-term exposure in young versus aged rats. Toxicology and Applied Pharmacology, 105:144–155.
Wester PW, Krajnc EI, van Leeuwen FXR, Loeber JG, van der Heijden CA, Vaessen HAMG, Helleman PW (1990). Chronic toxicity and carcinogenicity of bis(tri-n-butyltin)oxide (TBTO) in the rat. Food and Chemical Toxicology, 28:179–196.
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