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Environmental fate & pathways

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The in vitro rate of metabolism was determined for the read across source [1,3(or 1,4)- Phenylenebis(1-methylethylidene)] bis[tert-butyl] peroxide using the in vitro metabolism trout liver S9 assay (OECD Test Guideline 319B).


This organic peroxide metabolized rapidly under biologically active conditions with a metabolic half- life of 67 minutes or 1.13 hours. This data was used to extrapolate in vivo rate constants of metabolism or kMETof 0.184 day-1or 0.475 day-1using hepatic portal blood flow or hepatic portal and arterial blood flow, respectively. The use of these kMET values in conjunction with the Gobas model dropped the calculated BCF from 45,294 kg day-1(assuming no metabolism) to 536 or 209 kg day-1. The impact of metabolism on the read across source [1,3(or 1,4)- Phenylenebis(1-methylethylidene)] bis[tert-butyl] peroxide drops the BCF to less than the action level of 2000 (Reach Criteria) and should not be considered to have significant bioaccumulation potential.


However, following the Echa decision number SEV-D-2114369067-43-01/F, a bioaccumulation in fish study according to the OECD 305 using the aqueous exposure was performed under flow-through conditions on the read across source. The results are based on the time-weighted average as requested in the Echa decision. Based on this, the steady-state BCFss is equal to 1083 L/Kg and the steady-state BCFss normalised to a fish lipid content of 5% is equal to 1635 L/Kg.The lipid noramlised and growth corrected kinetic BCFkLg value is equal to 1820 L/kg. 


The Dutch National Health Institute (RIVM) concluded that BCF derivation should be performed differently and derived a BCFSSL (steady-state BCFSS normalised to a fish lipid content of 5%) of 2480 L/kg and a lipid normalised and growth corrected kinetic BCFkLg of 2201 L/kg for 0.579 µg/L (time-weighted average).
RIVM also noted that from the results presented it is possible that the 1,4 substituted isomer is more bioaccumulative than the 1,3 substituted isomer.


From these results the registrant believes that it is worthwhile confirming this and proposes to perform a similar BCF study with the single 1,3 substituted isomer (CAS 2212-82-9). A contract lab was already approached to perform this study, but before starting this study the registrant will wait for the results of the P-assessment, the OECD 308 study. When it can be shown that the substance is not persistent an additional OECD 305 is not deemed necessary.

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