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PBT assessment

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PBT assessment: overall result

PBT status:
the substance is not PBT / vPvB
Justification:

Persistence Assessment Abiotic Degradation Photodegradation of 2,4-di-tert-butylphenol was assessed using the calculation method contained in AOPWin v 1.92 and revealed a half life of 2.614 h (overall OH Rate Constant = 49.1082 E-12 cm3/molecule-sec). A structurally similar compound, 2,6-di-tert-butyl-m-cresol (CAS No. 128-37-0; PBT Summary Sheet No. 121) was investigated and observed to oxidise in water to several transformation products with a moderate to rapid rate in the dark and faster with sunlight, as discussed in the publication by the PBT Working Group (TC NES Subgroup on Identification of PBT and vPvB substances, ECB Summary Fact Sheet, PBT Working Group- PBT List No. 13, 29 March 2007). Photodegradation was also demonstrated by Freitag et al. (1982) in another structurally similar compound, 2,6-di-tert-butylphenol (CAS No. 128-39-2; PBT Summary Sheet No. 13). The test substance was dispersed on silica gel at a concentration of approximately 30 ng/g silica gel and a microphotoreactor was used to irradiate the sample at 290 nm for 17 h. Volatile organic compounds and CO2 were collected. Photomineralisation was measured to be 29.5% of the total CO2 and “organic fragments” were found to be 0.1%. The results of this test indicate that photo-degradation may be possible however it is not expected to be a major route of degradation, a conclusion supported by the PBT Working Group (TC NES Subgroup on Identification of PBT and vPvB substances, ECB Summary Fact Sheet, PBT Working Group- PBT List No. 13, 29 March 2007). Biotic degradation Two studies have been conducted which assess biodegradability in water (activated sludge respiration inhibition test). 1. Inherent biodegradability, Modified MITI Test (II) (OECD 302C) 2. BOD-Test for Insoluble Substances In the first test, the inherent biodegradability of 2,4-DTBP was evaluated. The method used followed that described in the OECD Guidelines for Testing of Chemicals / Section 2: Degradation and Accumulation, Test No. 302C "Inherent Biodegradability, Modified MITI Test (II). A mixed population of activated sewage sludge microorganisms was used as inoculum and aniline as reference substance. Initial concentration of test substance was 30 mg/l and reference substance 100 mg/l. 2,4-DTBP attained negligible degradation within 28 days calculated from oxygen uptake (% of COD and ThOD) and therefore, cannot be considered as inherently biodegradable under the strict terms and conditions of the OECD Guidelines. TOC analysis of 2,4-DTBP was not possible due to the insoluble nature of the test material. A modified MITI test was carried out due to this fact. A COD value of 2.85 mg O2/mg was obtained for the test material. In the second test, 2,4-DTBP was evaluated for its biodegradability according to ISO Draft (BOD Test for Insoluble Substances). Test substance, 6.3, 7.6 and 6.8 mg, 2,4-DTBP/flask and reference substance, diethylene glycol (13.8, 13.8 and 13.7 mg/flask) were weighed into flasks and samples were taken weekly over 28 days to measure the oxygen content. The test material attained negligible degradation (2%) within 28 days calculated from oxygen uptake (% of ThOD) and therefore, cannot be considered as biodegradable. In conclusion, whilst 2,4-DTBP may oxidize in aquatic solutions and may undergo photodegradation, based upon the above data 2,4-di-tert-butylphenol should be considered to meet the Persistent, P/vP criteria. This conclusion was supported by the PBT Working Group when assessing 2,4-di-tert-butylphenol (TC NES Subgroup on Identification of PBT and vPvB substances, ECB Summary Fact Sheet, PBT Working Group- PBT List No. 12, 29 March 2007). Bioaccumulation Assessment Based upon the results of a flow through fresh water test (MITI 1992) conducted over 8 weeks with Cyprinus carpio at two nominal concentration levels, 2 µg/l (135-360 L/kg) and 20 µg/l (128-436 L/kg), it can be concluded that 2,4-di-tert-butylphenol has a moderate bioaccumulation potential. Using the BCFWIN (v3.0) model with a measured Log Kow value of 4.8, the BCF fish is predicted to be 409 L/kg wwt for 2,4-DTBP. This predicted BCF fish is consistent with the experimental values obtained in the MITI study and therefore the worst case experimental value of 436 L/kg in fish has been used in the risk assessment. The TGD calculations have been shown to be very conservative, estimating a BCF for fish of 2400 L/kg wwt. On the basis of the above data, it can be concluded that 2,4-di-tert-butylphenol has a moderate bioaccumulation potential, a conclusion supported by the PBT Working Group when assessing 2,4-di-tert-butylphenol (TC NES Subgroup on Identification of PBT and vPvB substances, ECB Summary Fact Sheet, PBT Working Group- PBT List No. 12, 29 March 2007). Metabolism in higher animals from studies conducted using the structurally similar compound 2,6-di-tert-butylphenol, also indicates that 2,6-di-tert-butylphenol is unlikely to bioaccumulate in the food chain. Following systemic exposure to 2,6-DTBP in the circulation, a rapid metabolism via glucuronisation and sulphatation may be predicted from the chemical structure with excretion mainly via the kidney. This theory can be supported by a study conducted in a structurally similar compound, Tebufelone (Sietsema W.K. et al. 1993). When the pharmacokinetics was considered, oral and IV administration in the dose range up to 2 mg/kg PO gave a half life of 2.8 h (IV route) and 3.7h (PO route). Although non-linear kinetics were present for Tebufelone, this was attributed to saturation of metabolic elimination at the higher dose of 10 mg/kg PO. This information for a chemically similar compound would imply a short half life for 2,4-DTBP at the very low level of potential human exposure with no likelihood for bioaccumulation. The results above shows no indication of a bioaccumulation potential in aquatic organisms (BCF 2000) or for higher animals via the food chain. Thus, neither the B nor the vB criterion is fulfilled. This conclusion was supported by the PBT Working Group when assessing 2,4-di-tert-butylphenol (TC NES Subgroup on Identification of PBT and vPvB substances, ECB Summary Fact Sheet, PBT Working Group- PBT List No. 12, 29 March 2007). Toxicity Assessment Two studies were conducted in fish using 2,4-DTBP; a rainbow trout 96 h study and a Golden ide 48 h study. The LC50 from the rainbow trout was found to be greater than the highest value tested and is therefore not considered appropriate for setting the PNEC. The second study conducted using Golden ide is also not considered appropriate for setting the PNEC as the study was performed over 48 h and not 96 h, and the test species is not a standard species as defined in the OECD Guideline 203. It was therefore considered more appropriate to read across to the studies conducted using the structurally similar compound 2,6-DTBP. Four studies were conducted in fish. Two acute studies (rainbow trout and zebra fish) and two prolonged toxicity tests (14 days; rainbow trout and fathead minnows). From the 14 day studies conducted in fathead minnows and rainbow trout, it was also possible to determine 96 h LC50 values thereby satisfying the requirements of an acute toxicity test. Due to a poor dose response relationship the results from the 14 day prolonged toxicity test with rainbow trout were considered to have lower reliability than the fathead minnow results. The two rainbow trout 96 h LC50 results were greater than the highest value tested. The 96 h LC50 fathead minnow result therefore represents the lowest and most accurate result. The NOAEL (14 days) was 0.30 mg/L. There were no long-term fish studies. One acute toxicity invertebrate study was performed using Daphnia magna. The lowest endpoint was found to be EC50 (48 h) 0.50 mg/L (Lebertz H. (2001)). The No Observed Effect Concentration through 48 hours was 0.20 mg a.i./L. A chronic Daphnia magna study was conducted on 2,4-di-tert-butylphenol. The 21 day NOEC was 0.1 mg/l. Algae were shown to be the most sensitive aquatic species (ErC50 = 0.37 mg/l; NOErC = 0.075 mg/l). Based upon the above mentioned aquatic toxicity studies 2,4-di-tert-butylphenol should not be classified as Toxic. 2,4-di-tert-butylphenol has not been classified as carcinogenic, mutagenic or toxic for reproduction. A combined 1-Generation Reproduction Toxicity Study / Repeated Dose Toxicity Study was conducted in rats. This study was carried out to investigate the oral toxicity of 2,4-di-tert-butyl-phenol (DTBP) to prenatal animals prior and through mating, gestation and lactation. F1 progeny was maintained on the experimental diet for 13 weeks after weaning. Recovery was investigated in a number of F1 animals throughout an additional subsequent 4 week treatment-free period. Three groups of parental generation rats each comprising 15 males and 15 females were fed diets containing 2,4 -di-tert-butyl-phenol (DTBP) at dose levels of 50, 150 or 300 mg/kg/day for 4 weeks before mating and throughout mating, gestation and lactation. Groups of 20 F1 generation progeny of each sex were maintained on the experimental diets for 13 weeks after weaning. Similar groups of animals fed untreated powdered diet for similar periods acted as control group. Five male and 5 female F1 generation progeny from each of the groups were maintained untreated after the 13 week feeding phase, to investigate the nature of any treatment-related changes observed. One male animal of the parental generation at dose level 300 mg/kg/d was killed and necropsied on day 39, following a generalised deterioration in condition. A cause of clinical deterioration could not be deduced from the findings at necropsy. A pregnant animal at dose level 50 mg/kg/d died after a prolonged gestation period, and at necropsy was noted to have a fetus presented for birth in the breech position. All other P generation animals survived the scheduled treatment period, and showed no overt signs of toxicity attributable to the ingestion of DTBP. Parental generation animals treated with DTBP at 150 mg/kg/d showed retarded body weight gain during the pre-mating period, and those treated at 300 mg/kg showed body weight losses during the first 2 weeks of the study. Animals treated at 50 mg/kg/d showed a growth rate similar to that of the controls, as did all DTBP-treated pregnant females during gestation and lactation. The food consumption of parental generation animals treated at 150 or 300 mg/kg/d was reduced during the pre-mating period and water consumption was markedly increased. In conjunction with the reduced weight gain, these data are suggestive of a reduction in the palatability of the diets. The food and water consumption of animals treated at 50 mg/kg/d were similar to those of the controls. Reproductive capability of the parental generation animals was unimpaired by the ingestion of DTBP at dose levels of up to 300 mg/kg/d. However, the data suggested that ingestion of 300 mg DTBP/kg/d elicited a reduction in the mean number of progeny born and reduced the growth rate of the F1 progeny during lactation. The former effect was not apparent at dose levels of 50 and 150 mg/kg/d. Although reduced growth rate was also apparent in the progeny of animals treated at 150 mg/kg/d, it was associated with a higher average litter number. Post-mortem examination of the parental generation animals and the surplus F1 progeny did not reveal any gross lesions attributable to the ingestion of DTBP at any of the dose levels employed. There were no deaths in any of the experimental groups during the F1 generation feeding phase. Daily observations of these animals did not reveal any overt signs of toxicity attributable to the ingestion of DTBP. From the onset of treatment of the F1 generation animals selected for further study, there was a dose-dependent reduction in the food consumption and body weight gain at all dose levels of DTBP employed. The effect in the females was less marked than in the males. At dose levels of 50 and 150 mg/kg/d the severity of the effect on food consumption was of a similar magnitude to that of the effect on body weight, and probably indicated reduced diet palatability. However, at the highest dose level employed (300 mg/kg/d) the retardation of body weight gain was disproportionally high in relation to the effect on food consumption. Therefore, a primary toxic effect on growth rate may have been produced by the ingestion of DTBP at 300 mg/kg/d. The water consumption of DTBP-treated groups of animals was generally lower than that of the controls during the 13 week F1 generation feeding phase. No ocular defects were observed in the F1 generation animals treated with DTBP at 300 mg/kg/d which could be related to the ingestion of the test article. A low incidence of defects was observed in the control and dose level 300 mg/kg/d animals. Ocular lesions observed included intravitreal haemorrhage, lenticular opacity and fibrous corneal adhesions. The nature of the lesions suggested that they were traumatic lesions resulting from the retro-orbital sinus puncture procedure for blood sampling. At the start of the F1 generation treatment period, animals treated with 300 mg/kg DTBP showed reduced haemoglobin, packed cell volume, mean cell volume and white blood cell counts. These effects were considered to reflect the degree of growth retardation observed during the lactation period. Investigation of the blood chemistry and urine constituents in animals treated at 300 mg/kg/d did not reveal any consistent evidence of an effect of DTBP ingestion. The relative liver weight of male and female animals at 300 mg/kg/d was significantly higher than that of the controls. A similar but less marked effect was also apparent in female animals treated at 150 mg/kg/d. After 4 weeks without DTBP ingestion, relative liver weights had returned to normal values in animals treated at 300 mg/kg/d. These observations are indicative of a physiological (adaptive) response to the ingestion of DTBP. The relative weight of the kidney of males treated at 150 and 300 mg/kg/d, and the relative spleen weight of males treated at 300 mg/kg/d were high in relation to those of the controls. The biological significance of these observations is questionable in the absence of microscopic changes in the architecture of these organs. Post-mortem examination of the parental generation animals treated with DTBP at dose levels of up to 300 mg/kg/d did not reveal any treatment related gross lesions. Post-mortem and microscopic examination of the tissues and organs of F1 generation animals treated with DTBP at 300 mg/kg/d for 13 weeks did not reveal any gross or microscopic lesions which could be attributed to the ingestion of DTBP. Dietary administration of DTBP at a dose level of 300 mg/kg daily for 13 weeks to the F1 generation rats probably elicited a primary toxic effect on growth rate, and at dose level of 150 mg/kg/d, elicited growth retardation which was secondary to reduced diet palatability. With the exception of adaptive, hepatic changes and palatability effects, the "no adverse effect" level established in this study was 150 mg DTBP/kg/d. Based upon the above results there is no indication of a toxicity potential. Thus the T criterion is not fulfilled. This conclusion was supported by the PBT Working Group when assessing 2,4-di-tert-butylphenol (TC NES Subgroup on Identification of PBT and vPvB substances, ECB Summary Fact Sheet, PBT Working Group- PBT List No. 12, 29 March 2007).