The total worldwide consumption of organotin compounds has dramatically increased in the last thirty years from about 5,000 tons per year at the beginning of the '60s to over 60,000 tons per year in the midst of '80s. They are mainly used as stabilizers for rigid PVC (mono- and di-organotins) and as biocides (triorganotins). Even if the use as biocides accounts for only 30 % of the total world consumption, it contributes, due to the direct introduction, to the largest portion of organotins in the environment. Furthermore, the total production of organotins in the last thirty years increased by about 10-fold while the production of triorganotins for biocide uses has increased by about 20-fold in the same period. The environmental aspects of non-biocidal organotin compounds has been recently reviewed by Maguire [1]. In the conclusions it is stated that the most important non-pesticidal route of entry of mono- and dimethyltin, butyltin and octyltin to the environment is through leaching of PVC by water. Triorganotin biocides are used in pesticide formulations (mainly triphenyltin (TPhT)) [2] and, above all, in antifouling paints (mainly tributyltin (TBT) but also, increasingly, TPhT) [3]. Triphenyltin acetate (Brestan™) and triphenyltin hydroxide (Duter™) are used for the control of Phytophthora infestants, tricyclohexyltin hydroxide (Plictran™) for the control of Phytophagous; 1-tricyclohexylstannyl-1,2,4-triazole (Peropal™) and triphenylbutatin oxide (Vendex™) are both used as miticides [5]. These products are largely used in agricultural application and contamination could result from run-off water and overspray. Tributyltin-based antifouling paints were introduced at the beginning of the 60's but their widespread use started only in the 70's, replacing copper-based paints due to a superior performance: TBT paints are effective for about 5-7 years while copper paints are effective for no more than two years [4-5]. There are two types of organotin based antifouling paints: (i) conventional or "free association" paints in which the toxicant is loose in the paint and (ii) non conventional or polymeric paints in which the toxicant is chemically bound to a polymeric matrix. Conventional paints are more polluting and have a lower effect duration than polymeric paints, having a higher TBT release rate [6]. TBT is directly released into aquatic environment and its immission can be both continuous (release from the hulls of the boats) or intermittent (release from dockyard activities as paint removal, cleaning, painting, etc.). Environmental persistence and fate of TBT are strictly correlated to the specific characteristics of the aquatic ecosystem such as temperature, salinity, pH, suspended matter, microbial populations, flushing rates, etc. Distribution of TBT among the different environmental compartments is regulated by (i) physical mechanisms (including volatilization, adsorption, etc.), (ii) chemical mechanisms (including photochemical reactions) and (iii) biological mechanisms (including uptake and transformation) [7]. Both TBT and TPhT undergo degradation processes in marine environment, such as microbial and UV degradation, consisting in a progressive dealkylation down to inorganic tin [8]. Sufficient evidence exists of a faster rate for the DBT→MBT degradation in some experimental and environmental conditions [9-11]. As the toxicity of the organotins is maximum for the trisubstituted compounds, the degradation can be considered as a mechanism of detoxification. In fact, elemental Sn and its inorganic compounds are practically non toxic for all living systems: due to their very low solubility in lipids, they are scarcely accumulated by the organisms [12-13]; furthermore, at physiological pH, the element is not reactive and its oxides are practically insoluble [14]. On the contrary, the progressive introduction of organic groups at the Sn atom exerts a profound influence on chemical-physical properties, biological activity, mobility and persistence. This leads to an increasing toxicity of the molecule, reaching a maximum for the trisubstituted compounds [15-16]. For marine organisms the highest toxicity is shown by tributyl, triphenyl and tricyclohexyltin compounds [17]. The inorganic substituents do not significantly affect the toxicity of the compounds, unless they are strongly coordinating groups [18-20].The relative lipophilicity of triorganotin compounds as long as the tendency to bind with complex and simple lipids, make them able to cross biological membranes, producing toxic effects. Many reviews containing toxicological data on organotin compounds have been published [19-23]. The bioaccumulation process depends on the lipophilicity of the substance and on its resistance to metabolism and excretion processes [24]. Studies of kinetics and mechanism of accumulation showed that marine bivalves rapidly and effectively accumulate organotins even when exposed to low concentrations of dissolved material [25]. Bivalves accumulate dissolved TBT from sea water, presumably directly into exposed tissues such as gills, followed by migration to other tissues, or by ingesting tainted food. Very high concentrations can be reached in these organisms, because they are not capable, due to a low activity of the mixed function oxidase system, to metabolize a wide range of xenobiotics, including organotins [26-27]. Laboratory experiments on the accumulation of TBT demonstrated high bioconcentration factors for oysters [28] and for mussels [29-30]. Bioconcentration factors calculated in the field [11, 23, 31-32] resulted to be even higher than those predicted on the basis of octanol-water partition coefficient or calculated from laboratory experiments. The reasons for the discrepancies between laboratory experiments and field data are probably to be found in the difficulty of considering all the important parameters regulating the environmental behaviour in laboratory experiments. The direct introduction in the marine environment and the successive accumulation together with the high toxicity of these compounds towards "non-target" organisms, such as oysters and mussels, can cause environmental and economic damages as observed in the past in the Arcachon Bay (France) [33]. France was the first country to restrict the use of the TBT based antifouling paint by a legislation that regulated the use of these paints on boats with hulls less than 25 m long [34]. Similar regulations were enacted by many other countries such as UK, USA, Canada, etc. Recently, recommendations to extend the restriction to all organotin paints not only for boats, but also for industrial water cooling systems, mariculture structures, etc. were taken into account. Many references on organotin legislative information can be found in literature [6, 34-37]. © 1995 Elsevier B.V. All rights reserved.

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