Organotins are used as pesticides, biocides, catalysts and stabilisers but have immunotoxic and endocrine disrupting effects. They have also been linked to obesity.
The draft risk assessment was the third to be produced by consultants RPA for the European Commission under the EU "existing substances" rules. Earlier drafts were rejected in 2003 and 2004.
It covers four compounds: tributyl tin (TBT), dibutyl tin, dioctyl tin and triphenyl tin. All are thought to act through the same toxic mechanism and so need to be considered together.
The Scientific Committee on Health and Environmental Risks (SCHER) was asked to review the findings by the European Commission. It found the risk assessment, though complex and often incomplete, was far from reassuring for public health.
SCHER concluded there is a high probability that the public are being exposed to above-tolerable levels of organotins, particularly children and people living near factories producing or using the compounds.
The biggest sources of human exposure are considered to be:
- Food, particularly seafood which can be contaminated by organotins used in antifouling paint.
- Indoor air and dust where PVC flooring and wall coverings contain organotins used as stabilisers.
- Skin absorption via clothing such as printed T-shirts, PVC gloves, sandals and hygiene products such as nappies. Organotins may be present in PVC coatings or in textiles where they may be used as a biocide.
For example, the assessment does not consider exposure to organotins via PVC medical equipment. Exposure from silicone breast implants and rubber pillows also need to be investigated because silicone may contain organotin catalysts.
SCHER also found the assessment underestimates exposure via indoor air because of false assumptions about losses from PVC.
Another criticism concerned the impact of TBT in freshwaters. The compound is used in antifoulants, which have had a devastating effect on the reproduction of marine molluscs. But in freshwater the study concluded that water fleas were the most sensitive organism - despite previous advice that emerging evidence of impacts on freshwater snails needed to be considered.
In fairness to RPA, SCHER accepts that assessing the risks of organotins is difficult. It notes that the large number of compounds and the variety of applications makes it hard to reach general conclusions.
The most sensitive potential health impact of organotins is suppression of the immune system, but they also affect sex hormone metabolism in molluscs and may affect sexual development in rats.
However, Californian researchers recently found TBT also activated genes that promote fat cell formation. Tissue culture experiments and studies on newborn mice showed exposure increased fatty tissues in adults and caused abnormal fat distribution.
SCHER considers these findings significant and notes they occur at a similar level to immunotoxic effects. The committee says the findings confirm the validity of the current tolerable daily intake level of 0.1 micrograms of tin per kilogram bodyweight per day.
The body representing European tin stabiliser manufacturers ETINSA said the risk assessment came to positive conclusions for most applications. More than 95% of tin stabilisers are used in rigid PVC where no problems have been identified, it emphasised, and said it was committed to resolving remaining issues around minor uses.