WWF: Chemicals and Food: Organotin compounds - incl. butyltins (TBT) and octyltins
Description
Background Organotin compounds (OTCs) are man-made chemical compounds, based on a hydrocarbon structure combined with tin. The most well known organotin is tributyltin (TBT), used widely in marine antifoulant paints to prevent the growth of organisms such as barnacles on the hull of ships. Other organotins include the mono- and dibutyltins (MBT, DBT), octyltins (monooctyltin, MOT and dioctyltin, DOT) and mono-, di- and tri- phenyltins (MPT, DPT, TPT). Organotins have many applications, which include use as stabilisers in PVC, catalysts in chemical reactions, glass coatings, agricultural pesticides, biocides in marine antifoulant paints and wood treatments and preservatives (Batt, 2006) and in anti-odour/anti-fungal treatments for textiles and textile polymers (Greenpeace, 2003b, Peters, 2006). The five major commercial applications (biocides, PVC stabilizers, catalysts, agrochemicals and glass coatings) account for approximately 20, 000 tons of tin consumption per year (Batt, 2006). TBT is now an omnipresent global contaminant. Extensive use in antifouling paints on watercraft led to the widespread distribution of TBT (and its breakdown products DBT and MBT) in the global marine and freshwater environment, in water, sediment and biota (wildlife species) (de Brito et al., 2002, Ciesielski et al., 2004, Connelly, et al., 2001, Elgethun et al., 2000, Kannan et al., 1996, 1999a, Lee et al., 2006, 2005, Strand & Jacobsen, 2005, Sudaryanto et al., 2004, Ueno et al., 2004). TBT has even been found in marine sediments in Antarctica (Negri, et al., 2004). The persistence of some organotin compounds means that they remain in sediments, particularly anoxic (without oxygen) sediments, for long periods of time e.g. the half-life of TBT in deep sediment has been estimated to be approximately 87+/-17 years (Viglino et al., 2004). “Hot spots” of particularly high levels of TBT in water, sediment and biota are normally associated with commercial ports, harbours, shipyards, shipping lanes, marinas and the like (Alzieu, 2000, Shim et al., 2002, Smith et al., 2006), although the situation is slowly improving following legislation banning the use of TBT in antifoulant paints (see below). Dredging activities can, however, resuspend previously buried residues of TBT back in to the water column, creating localised pollution in such areas (Canellas-Bolta et al., 2005). The damaging environmental effects of organotins released from antifoulant paints first came to light in the late 1970’s and early-mid 1980’s, with two regional case histories in particular demonstrating the extent of the impacts – the collapse of oyster fisheries in Arcachon Bay, France and the phenomenon of “imposex” (the development of male sexual characteristics) in female marine snails from UK coastal waters, which led to widespread population declines (Santillo, et al., 2001). At the end of the 1970’s widespread reproductive failures decimated populations of the commercially important Pacific oyster (Crassostrea gigas) in Arcachon Bay in France, but it took until the early 1980’s until a causal agent was identified (Santillo et al., 2001). High levels of TBT in the waters of the bay were found to have impaired reproduction (by inhibiting embryogenesis and larval development) and causing shell deformities in the osyters (Alzieu et al., 1986, Alzieu, 2000, Ruiz et al., 1996). Similar impacts were seen in C.gigas from UK coasts (Waldock et al., 1987, Waldock & Thain, 1983, Thain & Waldock, 1986). Restrictions (see below) on the use of TBT as a biocide TBT has since reduced ambient water concentrations, allowing oyster populations to recover (Alzieu, 2000). Around the same time, following earlier observations of the appearance of a penis in female dogwhelks (Nucella lapillus) from Plymouth Sound, UK (which increased in prevalence with proximity to the harbour) (Blaber, 1970), research revealed imposex in N.lapillus occurring widely throughout the coastal waters of the southern UK (Bryan et al., 1986). This phenomenon was linked with exposure to high levels of TBT in coastal waters and is due to the androgenic properties of TBT i.e. TBT acts like a male hormone. Symptoms of TBT exposure in these invertebrates includes the development of male sexual characteristics e.g. a penis and vas deferens (imposex) by females (Bailey & Davies, 1988a, b,) leading to widespread population declines (Vos et al., 2000). Again, recovery in some populations has been observed following the ban on the use of TBT based paints on vessels (Birchenough et al., 2002, Miller et al., 1999). Imposex has been observed in many other species of molluscs (e.g. ten Hallers-Tjabbes et al., 2003, Santos et al., 2004) and it is reported that over 150 species of prosobranch gastropods (snails) worldwide have now been affected by TBT (and TPT) (Matthiessen et al., 1999). TBT also affects higher animals where currently concerns are particularly focussed on its effects on the immune system. It has been detected in fish, birds, otters, seals, sharks, porpoises and whales (Guruge et al., 1996, Iwata, et al., 1995, Law et al., 1998, Lee et al., 2005, Kannan et al., 1996, 1999a, Tanabe et al., 1998). Following the finding that TBT was severely impacting shellfish populations, bans on the use of organotin antifoulants were introduced in the EU, US and Japan in the 1980s for small vessels (<25m), and have been followed by a convention adopted by the International Maritime Organisation (IMO) prohibiting the application of organotin antifoulants on all vessels and offshore installations from January 2003 and requiring they are removed (or covered by a barrier to prevent their leaching) by January 2008 (ENDS Report, 2001, IMO, 2001). The convention will enter into force 12 months after 25 States representing 25% of the world's merchant shipping tonnage have ratified it (IMO, 2001). Currently 16 states representing 17,27% of the world's merchant shipping tonnage have ratified the convention and WWF is urging other countries to do so. In addition to the IMO convention, the EU has taken its own action on TBT. The European regulation 782/2003 entered in force in 2003, banning TBT in new ship coatings (EU, 2003). Under this law, from 2008 ships carrying an active TBT based paint will no longer be allowed to enter European harbours. Organotins have also been found in air (Boraiko et al., 2004), household dust (Fromme et al., 2005, Greenpeace, 2003a), soil (Marcic et al., 2006), sewage and sewage sludge (Chau et al., 1992, Fent, 1996) and landfill leachate (Mersiowsky et al., 2001). Compared to the marine species, information on the presence of organotins in terrestrial species and humans is much more limited. MBT, DBT and TBT have however been found in the liver of humans in Denmark and Japan (Nielsen & Strand, 2002, Takahashi et al., 1999), in the blood of humans in the US (Kannan et al., 1999b) and in dogs and monkeys (Takahashi et al., 1999).
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