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It should be noted that the test substance is not considered as posing a hazard to the aquatic environment. Indigo is a natural dye which is gained from Indigofera tinctoria and Indigofera suffruticosa and is known to be not readily biodegradable. The structurally identical synthetic Indigo as well is a solid under all environmental conditions and is insoluble in water (< 0.05 mg/L) with an n-octanol/water partition coefficient of log Kow = 2.7. It has a low volatility based on a vapour pressure of < 3.5E-5 Pa at 100°C and sublimes at ca. 170°C before melting; thus compartmentalising into air compartment is not expected.

Based on the physico-chemical data of Indigo, it is expected to be found predominantly in soil with an estimated half-life of 75 days (PBT-profiler) – to a low extent in water and not in sediment. It is very efficiently adsorbed in wastewater treatment plants (88% to 97%) as shown in adsorption studies. However, with a log Koc of 2.85 it has a comparably low affinity for soil/sediment.

The measured bioconcentration factor for aquatic bioaccumulation (log BCF) for Indigo is < 4.5 and does not exceed the bioconcentration criteria. Furthermore, there is no indication of bioaccumulation in mammals from kinetic or toxicity studies. Therefore, Indigo is not regarded as bio-accumulative.Indigo was nottoxic in mammalian studies therefore in the event of exposure to environmental organisms, effects due to secondary poisoning can be excluded.