TTC-based risk assessment of tetrachlorobutadienes and pentachlorobutadienes – The in vitro genotoxic contaminants in ground and drinking water

Tetrachlorinated butadienes (TetraCBDs), pentachlorinated butadienes (PentaCBDs) and hexachlorobutadiene (HexaCBD) were detected in groundwater wells of drinking water supplier near Basel up to maximally 157 ng/L (sum value), 15 ng/L (sum value), and <50 ng/L, in 2006. HexaCBD is toxicologically well characterized and the WHO has derived a TDI of 0.2 μg/kg body weight (bw)/day and a drinking-water guideline value of 600 ng/L. However, due to insufficient toxicity data, neither health-based guideline values nor maximum contaminant levels have been established for the TetraCBDs and PentaCBDs yet. Since TetraCBDs and PentaCBDs show structural alerts for potential genotoxicity, the genotoxicity of TetraCBDs and PentaCBDs was examined in vitro. All the TetraCBDs and PentaCBDs as well as HexaCBD were clastogenic in the chromosome aberration test. In addition, 1,1,3,4-TetraCBD and 1,2,3,4-TetraCBD were positive in the Ames test while the other polychlorinated butadienes including HexaCBD were negative. Using the threshold of toxicological concern (TTC) concept, the target value for TetraCBDs and PentaCBDs in drinking water was set at 75 ng/L (sum value of both substance groups). For the main component 1,1,4,4-TetraCBD, which is negative in the Ames, it is recommended to perform a third in vitro genotoxicity test, the HPRT test, before it can be decided whether to refer to the guideline value of HexaCBD at 600 ng/L or still to use the conservative TTC-based target value of 75 ng/L.