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There are no specific toxicokinetic data for the streams. In accordance with section 1 of REACH Annex XI, testing does not appear to be scientifically necessary since data are available on component substances which are adequate for the purposes of classification and labelling and/or risk assessment of the UVCB substances included in C5 non cyclic category.

Isoprene studies demonstrate a clear species difference between rats and mice with respect to toxicity with the mouse being the more sensitive species. This species difference is thought to be due primarily to differences in the rates of formation and/or deactivation of the biological active epoxide metabolites, with the mouse producing higher levels than the rat (SIDS, 2005; MAK, 46 Lieferung 2009).

In relation to establishing the DNEL for isoprene, the endogenous production and fate of isoprene is considered. Using a physiological toxicokinetic model, the rate of endogenous production of isoprene in a 70 kg man was estimated to be 23.8 umol/h (0.34umol/h/kg); approximately 90% is metabolised, the remaining 10% is exhaled unchanged. The model predicts that endogenous production of isoprene will result in blood concentrations of 9.5 nmol/l in humans. For man, the estimated area under the blood concentration versus time curve (AUC) following exposure by inhalation to 10 ppm isoprene for 8h is 4-fold higher than the 24h AUC resulting from exposure to endogenous isoprene (Csanady and Filser, 2001).

An extensive survey of published values for endogenous isoprene exhaled by the general population has been conducted. The weighted mean and standard deviation of values published since 1990 is 0.064 ± 0.049 ml/m3; older values were excluded due to doubts regarding the specificity of the analytical methods used. A physiological toxicokinetic (PT) model was used to calculate the life time (80 years) exposure to endogenous isoprene; this value, expressed as the area under the venous blood concentration versus time curve (AUC0-80) is 3.6 ± 2.8 mmol h/L. The PT model was then used to calculate the acceptable occupational exposure (8 hours/day, 5 days/week, 48 weeks/year for 40 years) such that the mean AUC for occupational exposure remained within the standard deviation of the endogenous AUC; this gave a value of 2.9 ml/m3 (MAK,46 Lieferung 2009).