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7 <br />tend to be localized near TBT sources. The following discusses the <br />data available for alternative antifoulants and whether sufficient data <br />exists for these compounds to conduct a comparative risk assessment <br />with TBT. <br />Alternative Antifoulants <br />There are a variety of other antifoulants that have been developed or <br />are in the process of development. A copper compound such as <br />cuprous oxide (CuZO), copper thiocyanate (CuSCN), or metallic <br />copper is often used as the principle biocide, but booster biocides are <br />often necessary to protect against copper-resistant fouling organisms <br />(Voulvoulis et al. 1999). These boosters may be either organic or <br />organometallic compounds (Table 1). The following discusses the <br />types of exposure and effects data available for these compounds. <br />Exposure Data. Unlike TBT, concentration data for many of these <br />compounds in the aquatic environment are limited or non-existent <br />because the compounds are not in heavy use as an antifoulant or are <br />still in the developmental stage. Consequently, exposure <br />concentrations for these compounds in the environment would have <br />to be modeled. The triazine compound Irgarol® is one exception. It <br />has been heavily used in some locations, and measured <br />concentrations along the coast of England and in the Mediterranean <br />Sea are approaching levels that are acutel toxic (Voulvoulis et al. <br />1999). For most alternative antifoulants, however, environmental <br />data are not available. <br />Table 1. Ezamples of organic and metal-based booster biocides. <br />Organic Boosters <br />Metal-Based Boosters <br />Dichloro-isothiazolone <br />Copper naphthenate <br />Triazine <br />Copper/zinc pyrithione <br />Diuron <br />Zinc oxide <br />