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As TBHP (substance manufactured and supplied) contains ca. 70% water all results have been corrected to 100% TBHP.

The absorption, distribution and blood kinetics of [14C]TBHP has been extensively investigated in the rat following single sub-cutaneous or single i.v. administration (TNO Study Director, 2003; 2004). Comparable toxicokinetics were observed after i.v. or s.c. injection of [14C]TBHP with rapid / high levels of uptake and distribution by both routes. After absorption, radioactivity was progressively eliminated with a terminal half life around 20 hr. Both Tmax and AUC were proportional to dose, with the volume of distribution (roughly 60% of total body weight) indicating distribution to total body water. Terminal half life appeared greater in males than females with a corresponding lower AUC for females compared with males. Around 60-70% of the dose was excreted in urine over 48 hr and up to 90% over 168 hr irrespective of route of administration. Small amounts of radioactivity were present in exhaled air (7 -17% of dose) with minimal excretion in faeces (up to 2% of dose over 168 hr). [14C]carbon dioxide accounted for one third to one half of the radioactivity present in exhaled air with the remainder was present as uncharacterised volatiles. Radioactivity (expressed as Parent Compound Equivalents; PCE) was distributed throughout the body irrespective of the route of administration with high levels present in kidney, liver and plasma. Tissue residues decreased 30-100 fold within 168 hr from the maximum level at Tmax (dependent on tissue type and dose). tert-Butanol along with 8 other metabolite peaks (but no unchanged TBHP) were present in urine, with the main urinary metabolites tentatively identified as tert-butanol, 2-methyl-1,2-propanediol and 2-hydroxyisobutyric acid (main metabolite). tert-Butanol only (accounting for >90% of radioactivity present) was found in exhaled air.

Oral absorption of 100% was apparent based on the comparable kinetic parameters for total radioactivity after oral gavage, i.v. and s.c exposures together with the high urinary excretion of TBHP-related radioactivity. However, the bioavailability of TBHP (i.e. presence of parent substance in the systemic circulation) is very low or absent due to its rapid conversion to 2-methylpropan-2-ol as demonstrated by the absence of TBHP in the blood at 15 minutes after i.v. injection. This absence of systemic bioavailability is confirmed by the available toxicological data which indicate only local toxicity and no systemic effects.

No information is available on uptake after inhalation exposure. However the extensive absorption recorded after oral exposure together with good water solubility and high vapour pressure support 100% absorption by the lung.

With regard to dermal contact, skin damage was observed in in vitro uptake studies following exposure to 60% TBHP but not with 10% or 1% aqueous TBHP (TNO Study Director, 2004). Total absorption of TBHP after dermal application of 1 -10% TBHP under non-occluded conditions was 3.5% of the dose after 8 hours. These conditions are considered representative of occupational use and subsequent exposure to non-corrosive mixtures of TBHP, and an uptake of 3.5% TBHP will be used for risk characterisation.