Study on Market-Bought Seafood
Not at Risk From TBT in Seafood
Two studies recently published (Cardwell et al. 1999 and Keithly et al. 1999) document that people are not at risk from tributyltin (TBT) exposure from eating market-bought seafood. The studies indicate that the levels of TBT present in seafood purchased at local seafood markets in Europe, Asia, Australia, and North America in 1997 were many times lower than the Tolerable Daily Intake (TDI) levels set to protect human health. Similar results were found in a study conducted in the US in 1989-1990, even though this was only shortly after the US placed restrictions on TBT usage as a ship antifoulant. In addition, exposure to TBT is expected to continue to decline, as monitoring programs worldwide continue to report declining TBT concentrations in bivalves and fish. These declines are attributable to legislation limiting TBT usage passed by several countries in the late 1980s and early 1990s, and the replacement of TBT free-association paints that had uncontrolled release of TBT, to the use of TBT self-polishing copolymer paints with controlled release rates.
The authors purchased seafood, including fish, crab, shrimp, squid, clams, and oysters from retail seafood markets worldwide. The type of seafood purchased in each country is presented in Table 1. Concentrations of TBT were measured in the edible portion of seafood in order to determine exposure levels to evaluate the potential risk to humans from consumption of the seafood.
For most compounds, there is a threshold dose below which no harmful effects (i.e., toxicity) will occur. For TBT, several studies suggest safe doses (TDI) that include a 10- to 100-fold safety factor applied, range from 0.02 to 0.00025 mg/kg/day. A value of 0.0003 mg/kg/day is supported by the U.S. Environmental Protection Agency (USEPA 1997) and was used by Cardwell et al. (1999) and Keithly et al. (1999). That is, an adult can consume 0.0003 mg/kg TBT per day for 70 years without harmful effects (risk) from dietary intake of TBT (including a 100-fold safety factor).
Potential risks to people were evaluated by comparing the estimated doses of TBT from seafood consumption to the safe dose for TBT. The comparisons were quantified using a hazard quotient, which is the ratio of the expected dose from consuming seafood to the safe dose (USEPA 1989). A hazard quotient greater than one indicates that the expected dose is greater than the safe dose. It does not necessarily imply health risks, but rather an increased possibility of health risks, warranting further study. A hazard quotient less than one indicates that health risks from exposure to TBT is unlikely to occur.
All calculated hazard quotients were well below one. Therefore, no health risks to people from consumption of the TBT in seafood were predicted. As can be see from Figure 1, the amount of TBT measured in the seafood is many times lower than the amount needed to achieve a risk quotient of one, and the possibility of health risks.
In summary, the results of the seafood survey in 1997 indicate that while TBT was measured in market-bought seafood, the potential risks to people were negligible. Similar results were also reported in a US 1989-1990 study conducted shortly after the use of TBT was regulated in the US.
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