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Tributyl Tin Worries

For as long as man has plied the waters of the earth, organisms have sought to come along for the ride. Mollusks such as mussels, barnacles, and a bewildering array of seaweed, algae, worms, and other creatures have claimed the bottom of seafaring vessels as their home. From the age of square riggers to modern-day container laden behemoths, the battle of the bottom has been a relentless one in which organisms seek habitat, and crews and owners have sought to save the performance of their vessels from drag, fouled intakes, and hull degradation.

The prime strategy in today’s paints is to create a surface toxic to marine life and keep it from staking a claim. Recent research by several environmental groups, including Greenpeace, Bluewater Network and Ocean Advocates cite tributyl tin, or TBT, which is commonly found in the anti-fouling paint of commercial ship as the purveyor of widespread marine degradation.

Chemical engineers have been plagued by problems in developing a paint that will keep ship hulls free of marine growth, but which is still cost effective and environmentally friendly. TBT was originally thought to meet these needs. TBT inhibits marine growth on the hull, thus decreasing the vessel’s friction and drag in the water. Reduced friction means less energy consumption that can spell significant economic savings for shipping companies, as well as a reduction in exhaust fumes.

Antifouling uses of tributyl tin began in the 1960s, although it wasn’t until the 1970s that its extreme toxicity to some organisms became evident. Since 1990 there has been a ban on using paint containing TBT on boats under 25 meters (80 feet) in length, and in 1998 the International Maritime Organization adopted a resolution to phase out the substance. France, Australia, and the US banned its use, though the substance is still widely used on other nations’ oceangoing vessels and flag of convenience vessels—ships which claim a country as a hailing port to circumvent shipping regulations or tariffs can complicate the picture further. Trace amounts of TBT can now be found in almost all the world’s harbors.

Problems begin once TBT begins to leach from a ship’s hull. The persistent toxins continue to be introduced into the water for years, and are detrimental in extremely small amounts. Dead dolphins and dead sea otters have been found with TBT concentrations in their livers. Seals, water birds, and shellfish around the world have also been found tainted with the substance. Mollusks and other creatures that filter water are particularly susceptible to TBT. Snails such as the purple snail and other related species have exhibited hormonal disturbance in which the female purple snails exposed to TBT develop male characteristics, sterility, and death. Sterile purple snails are found along the entire Norwegian coast with the exception of Finnmark. Norway is one of the largest producers of TBT-based ship bottom paint. When exposed to TBT certain types of fish , such as the stickleback, exhibit impaired abilities to escape from predators. Mussels and oysters are also affected by the toxin, and exhibit dwarfism and impaired growth in larvae development. In humans, concentrations of TBT can cause headaches, vertigo, eye irritation, psycho-neurological disturbance, vomiting, urine retention and skin burns. It's worth noting that a ban implemented immediately would mean five to seven years more of existing TBT sources finding its way into the world’s oceans.

A recent Greenpeace study at a popular beach in Thessalonkiki harbor in Greece found levels high enough to threaten human health. Near an area where paint is sand blasted from ships and allowed to flow into the sea, levels of TBT were found at 1.8 million times higher than the provisional limit sets by OSPAR, the intergovernmental organization regulating marine pollution in the northeast Atlantic.

Replacements for the toxic paint are focusing on a variety of possibilities that range from developing super-smooth surfaces to which organisms would not be able to attach, to organic biocides with low leaching rates, and even research into nature's own fouling inhibitors. There are a number of organisms in the natural world that are little effected by fouling, such as coral and amphibians.

There was a time that researchers within the field considered it unlikely that TBT would become concentrated within the food chain. More recent research cites the potential for accumulation is greater than originally thought as the world’s burgeoning coastal populations increasingly rely on greater seafood catches. A current resolution calls for a ban on any new application of TBT by January 1, 2003, with a total ban on the use of TBT by January 1, 2008. It remains to be seen whether this will be ratified and internationally enforced.


Mark Matthews is offline  
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