Tuesday, May 17, 2016

5. ORGANOTIN THE NEUROTOXIN


Organotins are a group of related chemicals, of which by far the best known is tributyltin (TBT), as a result of its widespread use in anti-fouling paints on ships and boats. However, several other organotin compounds are in common use, most notably mono- and dibutyltin (MBT, DBT), mono- and dioctyltin (MOT, DOT) and triphenyltins (TPT). Most abundant in consumer products are MBT and DBT, primarily due to use as stabilisers in PVC products including plastisol prints on textile articles. DOT is used in consumer products, including sportswear articles and footwear (Matthews 1996, OSPAR 2011). In some cases, organotin compounds have been used as biocides in certain types of textile products, including sportswear, socks and footwear (Greenpeace 2012c). Although anti-fouling paints have accounted for the majority of TBT used, historically this compound has also been used as a biocide in some consumer products, including certain textiles and carpets, though regulations prohibiting his use now exist in certain countries (Allsopp et al. 2000, 2001, OSPAR 2011).

Organotins have been detected in a wide range of textile articles, predominantly in the forms MBT/DBT and MOT/DOT. For textile articles bearing plastisol prints, various studies have detected MBT (7.2 mg/kg), DBT (0.02-9.7 mg/kg), MOT (0.06 mg/kg), and DOT (1.4 mg/kg) (Laursen et al. 2003, TNO 2003, TNO 2005), and total organotins (0.004-0.474 mg/kg) (Greenpeace 2004). Examples have also been reported of sportswear containing DBT (3.2 mg/kg, BEUC 2012), and outdoor clothing containing MBT (2.3 mg/kg), DBT (5.8 mg/kg), MOT (0.55-13 mg/kg), DOT (5.6 mg/kg), and total organotin (1.129 mg/kg) (Greenpeace 2004, Greenpeace 2012c). The widespread use of tributyl tin (TBT) in anti-fouling paints on ships, combined with the relative persistence of butyltins and their affinity for biological tissues, has led to their widespread occurrence in fish, and marine mammals for many years (Kannan et al. 1996, Iwata et al. 1995, Suominen et al. 2011). Organotin residues have also been detected in human livers, predominantly due to seafood consumption (Takahashi et al. 1999, Nielsen & Strand 2012, GuĂ©rin et al. 2007). While the consumption of seafood probably remains the predominant source of organotin exposure for humans, exposure to consumer products that contain them or to dusts in the home may also be significant. Organotins have been detected in the terrestrial environment, though far fewer studies have been carried out compared to the marine environment (Zuliani et al. 2010). Organotins are known to be toxic to a range of organisms, including mammals, at relatively low levels of exposure. In marine invertebrates, TBT is generally more toxic than DBT, which is in turn more toxic than MBT (Cima et al. 1996). However, this is by no means always the case, as DBT is more toxic than TBT to certain enzyme systems (Bouchard et al. 1999, Al-Ghais et al. 2000). In fish, DBT is frequently a more potent toxin than TBT (O’Halloran et al. 1998), with the immune system the primary target. Organotins have also been found to have immunotoxicity and teratogenic (developmental) properties in mammalian systems (Kergosien & Rice 1998), with DBT again frequently appearing more toxic than TBT (De Santiago & Aguilar-Santelises 1999). DBT is neurotoxic to mammalian brain cells (Eskes et al. 1999, Jenkins et al. 2004). DBT was also found to cause an increased incidence of cell death (apoptosis) in certain tissues of the brains of rats exposed during development (Jenkins et al. 2004). Toxic effects on testes development in mice have also been reported (Kumasaka et al. 2002). Immunotoxicity and impacts on developmental in mammals have also been reported for DOT and MOT, including impacts on the thyroid and T-lymphocyte immune response (Tonk et al. 2011a, 2011b, Faqi et al. 2001).

Until recently, legislative controls on organotin compounds have focused primarily on TBT in anti-fouling paints (IMO 2004).
TBT substances are also ‘priority hazardous substances’ under the EU Water Framework Directive (EU 2001), such that action to prevent releases to water within 20 years will be required throughout Europe. Regulations now exist in certain countries (primarily within the EU) on the presence of organotins in consumer products (EU 2010). Tri-substituted organotin compounds are prohibited in products sold in the EU where the equivalent concentration of tin is above 0.1 % by weight in the article or it parts. In addition, dibutyltin (DBT) compounds are prohibited where the equivalent concentration of tin in an article is above0.1 % by weight (equivalent to 0.20%, 2000 mg/kg, of DBT), with certain exceptions until 2015, including fabrics coated with PVC containing DBT compounds as stabilisers when intended for outdoor applications. Similarly, dioctyltin (DOT) compounds are prohibited in certain articles where the equivalent concentration of tin is above 0.1 % by weight (equivalent to 0.31%, 3100 mg/kg, of DOT), including textile articles intended to come into contact with the skin, gloves, footwear or part of footwear intended to come into contact with the skin, and childcare articles.

Certain industry standards in the textile sector also set limits for certain organotins in textile products. For example, the Global Organic Textile Standard (GOTS) prohibits MBT above 0.1 mg/kg, and certain other organotins including DBT, TBT and DOT above 0.05 mg/kg (GOTS 2011). Similarly, the Oeko-Tex standard for textiles sets a limit of 1.0 mg/kg for TBT and 2.0 mg/kg for DBT and DOT (Oeko-Tex 2012). 

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