Screening Assessment for the Challenge

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Peroxide, (1,1,4,4-tetramethyl-1,4-butanediyl)bis[(1,1-dimethylethyl)]

Chemical Abstracts Service Registry Number
78-63-7

Environment Canada
Health Canada

July 2008

(PDF Version - 295 KB)

Table of Contents

• Synopsis
• Introduction
• Substance Identity
• Physical and Chemical Properties
• Manufacture and Import
• Uses
• Releases to the Environment
• Environmental Fate
• Persistence and Bioaccumulation Potential
• Potential to Cause Ecological Harm
• Conclusions
• References
• Appendix 1: Robust Study Summaries

Synopsis

Pursuant to section 74 of the Canadian Environmental Protection Act, 1999 (CEPA 1999), the Ministers of the Environment and of Health have conducted a screening assessment on Peroxide, (1,1,4,4-tetramethyl-1,4-butanediyl)bis[(1,1-dimethylethyl)] (DMHBP), Chemical Abstracts Service Registry Number 78-63-7. This substance was identified as a high priority for screening assessment, and included in the Ministerial Challenge because it had been found to meet the ecological categorization criteria for persistence, bioaccumulation potential and inherent toxicity to non-human organisms and is believed to be in commerce in Canada.

The substance DMHBP was not considered to be a high priority for assessment of potential risks to human health, based upon application of the simple exposure and hazard tools developed by Health Canada for categorization of substances on the Domestic Substances List (i.e., it did not meet the criteria of both being considered to present greatest or intermediate potential for exposure and having been classified by another national or international regulatory agency on the basis of carcinogenicity, genotoxicity, developmental toxicity or reproductive toxicity). Therefore, this assessment focuses on information relevant to the evaluation of ecological risks.

DMHBP is an organic substance that is used in Canada and elsewhere in polymer processing. The substance is not naturally produced in the environment. Between 1000 and 10 000 kg of DMHBP were manufactured in Canada in 2006 while between 10 000 and 100 000 kg of DMHBP were imported into Canada during the same period.

Based on certain assumptions and reported use patterns, most of the substance is transformed during the processing phase. Small proportions may be released to water (0.08%). This substance is not soluble in water and has a tendency to partition to particles because of its hydrophobic nature. For these reasons, DMHBP would likely be found almost entirely in sediments and is not expected to be significantly present in other media.

DMHBP is not expected to meet the persistence criterion as set out in the Persistence and Bioaccumulation Regulations, but it is predicted to have a potential to accumulate in organisms.

Predicted environmental concentrations are a few orders of magnitude lower than the predicted no-effects concentrations for aquatic organisms. This indicates a low probability of risk in the aquatic environment.

This substance will be included in the Domestic Substances List inventory update initiative, to be launched in 2009. In addition and where relevant, research and monitoring will support verification of assumptions used during the screening assessment.

Based on the information available, DMHBP does not meet any of the criteria set out in section 64 of the Canadian Environmental Protection Act, 1999.

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Introduction

The Canadian Environmental Protection Act, 1999 (CEPA 1999) (Canada 1999) requires the Minister of the Environment and the Minister of Health to conduct screening assessments of substances that have met the categorization criteria set out in the Act to determine whether these substances present or may present a risk to the environment or human health. Based on the results of a screening assessment, the Ministers can propose to take no further action with respect to the substance, to add the substance to the Priority Substances List (PSL) for further assessment, or to recommend that the substance be added to the List of Toxic Substances in Schedule 1 of the Act and, where applicable, the implementation of virtual elimination.

Based on the information obtained through the categorization process, the Ministers identified a number of substances as high priorities for action. These include substances that

• met all of the ecological categorization criteria, including persistence (P), bioaccumulation potential (B) and inherent toxicity to aquatic organisms (iT), and were believed to be in commerce in Canada; and/or
• met the categorization criteria for greatest potential for exposure (GPE) or presented an intermediate potential for exposure (IPE), and had been identified as posing a high hazard to human health based on classifications by other national or international agencies for carcinogenicity, genotoxicity, developmental toxicity or reproductive toxicity.

The Ministers therefore published a notice of intent in the Canada Gazette, Part I, on December 9, 2006 (Canada 2006), that challenged industry and other interested stakeholders to submit, within specified timelines, specific information that may be used to inform risk assessment, and to develop and benchmark best practices for the risk management and product stewardship of these substances identified as high priorities.

The substance peroxide, (1,1,4,4-tetramethyl-1,4-butanediyl)bis[(1,1-dimethylethyl) was identified as a high priority for assessment of ecological risk as it was found to be persistent, bioaccumulative and inherently toxic to aquatic organisms and is believed to be in commerce in Canada.  The Challenge for peroxide, (1,1,4,4-tetramethyl-1,4-butanediyl)bis[(1,1-dimethylethyl) was published in the Canada Gazette on February 3, 2007 (Canada 2007a). A substance profile was released at the same time. The substance profile presented the technical information available prior to December 2005 that formed the basis for categorization of this substance. As a result of the Challenge, submissions of information were received.

Although peroxide, (1,1,4,4-tetramethyl-1,4-butanediyl)bis[(1,1-dimethylethyl) was determined to be a high priority for assessment with respect to the environment, it did not meet the criteria for GPE or IPE and high hazard to human health based on classifications by other national or international agencies for carcinogenicity, genotoxicity, developmental toxicity or reproductive toxicity. Therefore, this assessment focuses principally on information relevant to the evaluation of ecological risks.

Screening assessments under CEPA 1999 focus on information critical to determining whether a substance meets the criteria for defining a chemical as toxic as set out in section 64 of the Act, where

"64. [...] a substance is toxic if it is entering or may enter the environment in a quantity or concentration or under conditions that

Screening assessments examine scientific information and develops conclusions by incorporating a weight of evidence approach and precaution.

This screening assessment includes consideration of information on chemical properties, hazards, uses and exposure, including the additional information submitted under the Challenge. Data relevant to the screening assessment of this substance were identified in original literature, review and assessment documents, stakeholder research reports and from recent literature searches up to May 2008. Key studies were critically evaluated; modelling results may have been used to reach conclusions. When available and relevant, information presented in hazard assessment from other jurisdictions was considered. The screening assessment does not represent an exhaustive or critical review of all available data. Rather, it presents the most critical studies and lines of evidence pertinent to the conclusion.

This screening assessment was prepared by staff in the Existing Substances Programs at Health Canada and Environment Canada and incorporates input from other programs within these departments. Additionally, the draft of this screening assessment was subject to a 60-day public comment period. The critical information and considerations upon which the assessment is based are summarized below.

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Substance Identity

For the purposes of this report, this substance will be referred to as DMHBP, which has been derived from the name 2,5-dimethylhexane-2,5-di-tert-butylperoxide.

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Physical and Chemical Properties

Table 2 contains experimental and modelled physical and chemical properties of DMHBP that are relevant to its environmental fate.

Some of the physical and chemical properties in the above table were generated using quantitative structure-activity relationship (QSAR) models, and there are uncertainties related to the use of these models. For instance, the applicability domain of a model may not cover the entire structure of a given chemical, thus lowering the reliability of predictions. For DMHBP, the comparison of modelled and experimental values for boiling point shows that this property cannot be adequately predicted by the model MPBPWIN. This could also be the case for some of the other physical and chemical properties and associated models.

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Manufacture and Import

Organic peroxide initiators were not manufactured in Canada in 2000. Approximately 300 000 kg of dialkyl peroxides were used in the Canadian polymer resin manufacturing process in 2000 (Cheminfo Services Inc., 2002).

Response to a survey notice pursuant to section 71 of CEPA 1999 indicated that one company manufactured DMHBP in Canada in 2006 at a quantity meeting the 100 kg reporting threshold. Ten companies met the 100 kg reporting threshold and reported importing the substance into Canada at a total quantity for the ten companies between 10 000 and 100 000 kg. One company reported importing the substance at a quantity below 100 kg (Environment Canada 2007a).

It is not known how much DMHBP is imported into Canada in finished articles, for example, as residues in polymeric materials.

Elsewhere, DMHBP has been identified as a US High Production Volume Chemical, with total use reported under the US Inventory Update Rule within the range of 455 to 4545 tonnes per year for 1990, 1994, 1998 and 2002. For 1986, the reported total use was within the range of 227 to 455 tonnes (US EPA 2002). DMHBP has also been identified as an OECD High Production Volume chemical. It is a European Union (EU) Low Production Volume Chemical, indicating that production within the EU has been estimated to be in the order of 10 tonnes per year. The database for Substances in Preparations in Nordic Countries indicates that in 2004, approximately 3 tonnes were in use in Sweden (SPIN database 2000).

Uses

Information on uses of DMHBP in Canada was received in response to the CEPA section 71 Notice for the 2006 calendar year. Uses include use as a polymer additive and crosslinking agent, manufacture of belting, rubber and plastic hose, other rubber products, and manufacture of plastics material and resin.

Published literature indicates that DMHBP is a dialkyl peroxide that may be used in polymer processing as an initiator for crosslinking of polyolefins. The carbon-carbon cross links provide rubber articles with the maximum resistance to heat, oxygen and compression set. It can be used as a polymerization initiator for plastics and in rubber processing for the production of window seals and automotive seals, hoses, and soles of shoes. It may also be used for the curing of some resins for applications ranging from boat hulls and swimming pools to bodywork parts (Arkema 2006). In these uses, the peroxide bonds are broken to produce reactive radicals that initiate polymerization.

Releases to the Environment

DMHBP is not naturally produced in the environment.

Mass flow tool

To estimate potential release of the substance to the environment at different stages of its life cycle, a mass flow tool was used. Empirical data concerning releases of specific substances to the environment are seldom available. Therefore, for each identified type of use of the substance, the proportion and quantity of release to the different environmental media are estimated, as is the proportion of the substance chemically transformed or sent for waste disposal. Assumptions and input parameters used in making these estimates are based on information obtained from a variety of sources including responses to regulatory surveys, Statistics Canada, manufacturers’ websites and technical databases. Of particular relevance are emission factors, which are generally expressed as the fraction of a substance released to the environment, particularly during its manufacture, processing and use associated with industrial processes. Sources of such information include emission scenario documents, often developed under the auspices of the Organisation for Economic Co-operation and Development (OECD), and default assumptions used by different international chemical regulatory agencies. It is noted that the level of uncertainty in the mass of substance and quantity released to the environment generally increases further down the life cycle. Unless specific information on the rate or potential for release of the substance from landfills and incinerators is available, the Mass Flow Tool does not quantitatively account for releases to the environment from disposal.

The tool results indicate that most of the substance (about 96%) is lost by transformation. This occurs mainly during the processing phase at polymer manufacturing facilities, where the peroxide bonds in the substance are broken to form reactive radicals that initiate polymerization. About 4% may end up in waste disposal sites as a result of handling and cleaning processes, manufacture of DMHBP and disposal of off-spec product. A small fraction of solid waste is incinerated, which is expected to result in the transformation of the substance. Based largely on information contained in OECD emission scenario documents for processing and uses associated with this substance, it is estimated that ~0.1% and ~0.1 % of DMHBP may be released to sewers and air, respectively.

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Environmental Fate

Based on its physical and chemical properties (Table 2) and the results of Level III fugacity modelling (Table 4), DMHBP is expected to reside in sediment, air, soil or water, depending on the compartment of release.

According to the mass flow tool results presented in Table 3, expected environmental releases of DMHBP is to air and to sewers during processing. Therefore, the  release scenarios to air and to water shown in Table 4 would be the most relevant for Canada. DMHBP released to air is expected to partly remain in this environmental medium and to partition to sediment. DMHBP released to water is expected to strongly adsorb to suspended solids and sediments, according to its very high log K_oc value of ~5.2 (Table 2) and Level III fugacity modeling results.

Persistence and Bioaccumulation Potential

Persistence

As mentioned above, the only direct releases of DMHBP to the environment could be to air and to surface water through sewers (Table 3). Once in air, the fate analysis presented in Table 4 indicates that this substance would partition mainly to air (53%) and sediments (33%), and to a lower extent to soil (14%). If released to water, DMHBP would partition mainly into sediments (98%; Table 4). Different of lines of evidence are presented below to assess the potential for persistence of DMHBP in these environmental media.

While peroxides are generally considered to be reactive because of the nature of the peroxide bond, there are differences in the level of reactivity among different categories of organoperoxides, and even among different substances within a category.

Dialkyl peroxides are among the most stable of all the commercially available organoperoxides, with a shelf half-life of at least one year at their recommended storage temperature of < 38°C (ATOFINA 2001). However, storing conditions does not reflect the transformation pathways that can exist in the natural environment, such as hydrolysis, photolysis and biodegradation.

In the atmosphere, a substance may transform as the result of several processes such as photolysis, atmospheric oxidation and ozone reaction. Regarding photolysis, there are no data available on the absorption spectrum of DMHBP. However, di-tert butyl peroxide (CAS RN 110-05-4), another dialkyl peroxide, has been found to absorb light up to 340 nm and to photolyze to form tert-butoxy radicals (HSDB 1983 – ). The rate of this process is not known. Because this substance is structurally similar to DMHBP, the latter may also be subject to photolysis when exposed to light. The only data available to assess the potential of DMHBP for atmospheric oxidation and ozone reaction were modelled using AOPWIN 2000. However, the results obtained with this model are considered to be of lower reliability since no chemicals of structural comparability to DMHBP are contained in its training set. This substance is not expected to be persistent in this medium, based on the line of evidence available for an analogue (potential for photolysis).

In water, a substance may undergo hydrolysis, photolysis and/or biodegradation. Regarding hydrolysis, DMHBP does not contain functional groups expected to react with water. As mentioned above, DMHBP may be subject to photolysis, based on experimental data available for another dialkyl peroxide (CAS RN 110-05-4). There are experimental data available to assess the biodegradation of DMHBP and other dialkyl peroxides. These data suggest that DMHBP is not persistent in water.

First, DMHBP was degraded with removal rates of nearly 100% in the semi-continuous activated sludge (SCAS) test in which the substance was exposed to sewage sludge microorganisms for eight weeks (OPPSD 2008). Since such a test includes loss from solution by adsorption to solids, a proportion of DMHBP may have indeed followed this path due to its high hydrophobicity. In a ready-biodegradation test (modified MITI test – OECD 301C), DMHBP showed only 4% biodegradation over 28 days as measured by gas chromatography analysis (NITE 2002). This result confirms that this substance can be resistant to hydrolysis and ultimate degradation under certain test conditions.

Studies addressing biodegradation were available for other organic peroxides. Even though these results are given a lower weight, they are still used as additional lines of evidence to assess the potential for persistence of DMHBP. In a ready biodegradability closed bottle test (OECD Guideline 301D), a dialkyl peroxide, dicumyl peroxide (CAS RN 80-43-3), showed no biodegradation during the acclimation period of 15 days. After this period, 18% and 60% biodegradation had occurred at days 28 and 57, respectively. (OPPSD 2008).

In a risk assessment of tertiary butyl hydroperoxide (CAS RN 75-91-2), a hydroperoxide, the Netherlands Chemical Substances Bureau reported that this substance was not appreciably degraded in abiotic degradation tests. In these tests, half-lives for primary degradation ranged from 170 to 6900 days in 10-day tests in ultra-pure water and from 36 to 45 days in 10-day tests with sterilized sludge (Chemical Substances Bureau 2004). The substance was not readily biodegradable in the modified Sturm test or the closed bottle test, both of which measure ultimate degradation, but the substance was biodegraded in 1-hour activated sludge tests, with primary degradation half-lives of 18–24 minutes (Chemical Substances Bureau 2004). These results show that this hydroperoxide does not undergo hydrolysis and that it has a strong tendency to sorb to organic matter. The results also show that this peroxide can undergo primary degradation within minutes. However, it is resistant to ultimate degradation. It should be noted that in hydroperoxides, the peroxide bond is at the end of the molecule, where it is more accessible to attack than in dialkyl peroxides, where the peroxide bond is closer to the centre of the molecule.

Other types of studies were available to assess biodegradation which suggest that DMHBP is not persistent. In an in vitrometabolism study using a trout liver S9 enzyme fraction reported by OPPSD (2008), DMHBP was metabolised rapidly under conditions of incubation and it also degraded rapidly in controls in which the S9 enzyme fraction was denatured. In an identical study conducted with a closely related dialkyl peroxide, (1,1,4,4-tetramethyl-2-butyne-1,4-diyl)bis[(1,1-dimethylethyl)peroxide], CAS number 1068-27-5, an expected breakdown product, tertiary butanol, and another unidentified degradation product were detected in the controls (OPPSD 2008). The reported half-life in the controls was 1.89 hours. The results of these studies indicate that DMHBP and other dialkyl peroxides may undergo both biotic and abiotic degradation reactions quickly in the environment, and therefore would not be considered persistent.

Toxicity tests conducted with (1,1,4,4-tetramethyl-2-butyne-1,4-diyl)bis[(1,1-dimethylethyl)peroxide] (DMBP), CAS number 1068-27-5, also suggest that dialkyl peroxides may not be persistent in water. In two tests, the measured concentration of DMBP in water decreased from 3.76 mg/L to < 0.081 mg/L after 72 hours, and from 5.31 mg/L to 0.375 mg/L after 48 hours (Study Submission 2006a and 2006b). Considering the breakdown of DMBP in a metabolism study cited above, this disappearance may have been the result of degradation of the substance. Also, DMBP may have volatilized from the solution or may have sorbed to test organisms or to the walls of test containers. However, sorption was probably not important enough to account for the drop seen in the measured concentrations. These laboratory tests indicate that the dialkyl peroxide DMBP is unstable in aqueous solution and dissipates within days. A similar behaviour could be expected for DMHBP.

The potential for persistence of DMHBP in sediment is of particular concern since this substance would partition mainly to this environmental compartment should it be released to surface water (Table 4). Information submitted to Environment Canada states that the reactivity of organic peroxides in the presence of metals such as iron and manganese should prevent their accumulation in soils and sediments (Challenge Submission 2008). These metals are indeed abundant in these matrices. Otherwise, it is generally accepted that the half-life of a substance in sediment is longer than that in water (factor of 1:4, as proposed by Boethling et al. 1995). Considering that the metabolism studies conducted with DMHBP and the close analogue DMBP, as well as the toxicity studies conducted with the latter, indicate that its half-life in aqueous solutions is probably of the order of hours, its half-life in sediments should be in the order of days or weeks. Similarly, according to Boethling et al.’s factor, the half-life of DMHBP in soil would be expected to be in the same order as the half-life in water.

Although experimental data on the degradation of DMHBP and analogue substances are available, QSARs were also applied using degradation models. Modelling indicates that DMHBP would be persistent in air, water, soil and sediment. However, the modeled values are considered to be of lower reliability as no chemicals of structural comparability to DMHBP are contained in their training sets. Indeed, these fragment-based models do not consider the peroxide bond, which can be reactive in some substances. Given that experimental data are available and given that the modeled values are of lower reliability, the latter are given a very low weight in the assessment of the environmental persistence of DMHBP.

Different lines of evidence were presented above to assess the persistence of DMHBP, should it be released to the environment. Based on these lines of evidence, it is concluded that DMHBP does not meet the persistence criteria for air (half-life ≥ 2 days), water and soil (half-life ≥ 182 days) or sediments (half-life ≥ 365 days) as set out in the Persistence and Bioaccumulation Regulations (Canada 2000).

Bioaccumulation

Experimental steady-state bioconcentration factor (BCF) values for fish from the NITE database were 2250 and 3690 L/kg, (Table 5a) as reported by OPPSD (2008). In this study, fish were exposed under flow-through conditions for 8 weeks. Test water analyses were done twice a week and fish analyses were done every two weeks. Fish were not fed on the days of fish sampling.

The steady-state BCF values from the NITE database were used to derive an in vivo-based metabolic rate constant (k_M) according to the method of Arnot et al. (2008). In this method, km is derived according to the following equation:

where:

The method of Arnot et al. (2008) provides for the estimation of confidence factors (CF) for the k_M to account for error associated with the in vivo data (i.e., measurement variability, parameter estimation uncertainty and model error and uncertainty with the predicted log K_ow). A CF of ±3.9 was calculated for the available BCF data.

Because metabolic potential can be related to body weight and temperature (e.g., Hu and Layton 2001, Nichols et al. 2007), the k_M was further normalized to 15^oC and then corrected for the body weight of the middle trophic level fish in the Arnot-Gobas model (184 g). The middle trophic level fish was used to represent overall model output as suggested by the model developer (Arnot pers. comm.) and is most representative of fish weight and size likely to be consumed by an avian or terrestrial piscivore. After normalization routines, the k_M ranges from 0.01 to 0.16 with a median value of 0.04.

Two in vitro metabolism studies using trout liver S9 enzyme fractions and trout whole hepatocytes were reported by OPPSD (2008). Whole body fish metabolism rate constants, kmet, from these studies were derived by OPPSD using methods of Cowan-Ellsberry et al. (2008) for the S9 and Han et al. (2007) for the hepatocyte. The S9 kmet for arterial and portal blood flow (most realistic) was reported as 0.09 (Table 2 OPPSD, 2008). The kmet value derived using whole trout hepatocytes was 0.20 (rounded) as reported in Attachment 1: Bioaccumulation Assessment of Luperox 101 Using an In Vitro Trout Hepatocyte Assay (Nabb study) submitted by the OPPSD. A value of 0.36 was reported in Table 2 of the OPPSD submission, but this value could not be substantiated in Attachment 1.

Unlike the procedure of Arnot et al. (2008), estimates for k_met based on in vitro assays do not provide for the calculation of confidence factors. Cowan-Ellsberry et al. (2008) suggests that for acceptance of in vitro methods, understanding of uncertainty of these methods and testing on more types of chemicals should be performed to evaluate the various assumptions used in their approach. Han et al. (2007) also indicate that uncertainty of model parameters should be understood for the hepatocyte method. As no bounds of uncertainty could be directly estimated for the in vitro data, a one order of magnitude error (CF = ±10) was assumed for potential variability and uncertainty in the parameters used to derive the k_met. The S9 and hepatocyte k_metvalues were also normalized to the weight of the middle trophic level fish in the Arnot and Gobas model as was done for the in vivo data.

The in vivo and in vitro metabolic rate constants were used to adjust the predicted BCF and BAF values from the Arnot and Gobas model’s default of zero metabolism. The results are presented along with other QSAR estimates in Table 5b.

There is good agreement between the metabolic rate constants derived using in vivo methods and S9 in vitro methods and as a result, the predicted BCFs, BAFs and half-lives are also in general agreement. Metabolic rate constant values and BCF and BAF values generated using the hepatocyte method are generally an order of magnitude higher (at median values). BCF values ranged from 68 to 14493 with an average of ~4000 regardless of which method was used for metabolic correction. BAF values ranged from 105 to 735384 with an average BAF of ~156900 regardless of metabolic correction used. Half-lives ranged from less than 1 day to 118 days. The geometric mean steady-state BCF reported in the NITE database is 2881 which is only a factor of 1.1 higher than the closest metabolism corrected BCF of 2570 corresponding to a metabolic rate constant of 0.04. Greatest confidence is associated with the BAF predicted using this metabolic rate correction. The BAF corresponding to the BCF of 2570 is 45709 which allows for a substantial margin of error if the criterion of BAF ≥ 5000 is considered.

The modeled values in table 5c however are considered less reliable as no metabolism considerations are taken into account by these models (directly) and no chemicals of structural comparability are contained in their training sets.

According to the Persistence and Bioaccumulation Regulations (Canada 2000) measures of BAF are the preferred metric for assessing bioaccumulation potential of substances. This is because BCF does not adequately account for the bioaccumulation potential of substances via the diet, which predominates for substances with logK_ow > ~4.0 (Arnot and Gobas 2003). No empirical BAF were available for DMHBP consequently BAF was modelled. Kinetic mass-balance modelling was considered to provide the most reliable prediction method for determining the bioaccumulation potential of DMHBP because it allows for metabolism correction and DMHBP is within the logK_ow domain of the model.

Metabolism corrected BCF and BAF values range from 68 to 14493 and from 105 to 735384, respectively,  depending on the rate of metabolism.  Environment Canada has analyzed these values and determined that the most reliable metabolism rate is reached when the metabolism corrected predicted BCF is in close agreement with the empirical BCF.  Using this metabolic rate to correct the predicted BAF results in a BAF 393 405. Therefore, based on the available empirical and kinetic-based modelled values corrected for metabolism and considering evidence from both in vivo and in vitro techniques for metabolic potential, DMHBP meets the bioaccumulation criterion (BAF ≥  5000) as set out in the Persistence and Bioaccumulation Regulations (Canada 2000).

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Potential to Cause Ecological Harm

A quantitative evaluation based on exposure and ecological effects was conducted for this substance as part of the weight of evidence evaluation of its potential to cause harm.

First, a predicted environmental concentration (PEC) was determined based on an analysis of exposure pathways. A predicted no-effect concentration (PNEC) was derived by selecting a critical toxicity value (CTV) from the available toxicity data and dividing this value by an assessment factor.

Ecological Exposure Assessment

No empirical data have been found regarding levels of DMHBP in the environment. It was estimated that 0.08% of the quantity used at a polymer manufacturing facility may be released in liquid effluents. A conservative predicted environmental concentration (PEC) was calculated using the following equation (Environment Canada 2007c):

Where:

Based on this equation, the PEC in receiving waters is 0.0002 mg/L.

Ecological Effects Assessment

An acute toxicity study was conducted with the ricefish Oryzias latipe exposed to DMHBP. The study measured a 96-hour LC₅₀ of 4.5 mg/L (NITE Database). Since the estimated water solubility of DMHBP is < 1 mg/L (Table 2), so the substance might not be soluble enough in water to cause acute effects to fish (Study Submission 2008).

There is also acute aquatic toxicity information available for another closely related dialkyl peroxide, peroxide, (1,1,4,4-tetramethyl-2-butyne-1,4-diyl)bis[(1,1-dimethylethyl) (DMBP), CAS No. 1068-27-5. Two tests were conducted with this substance, one with the green algae P. subcapitata and the other with the water flea D. magna (Study Submission 2006a and 2006b). Even though significant losses of DMBP from solution occurred during these tests, endpoints could be calculated based on the geometric mean of measured concentrations. The results were >0.21 mg/L and >1.41 mg/L for the alga and water flea, respectively, as no adverse effects were observed at the highest exposure concentration in both tests. As for DMHBP, DMBP has an estimated low water solubility (<1 mg/L) and hence may not be soluble enough in water to cause acute effects. A solvent was used in the laboratory tests reported.

A range of aquatic toxicity predictions (0.042 to 2.76 mg/L) were obtained from various QSAR models. However, the modeled values are of low reliability as no chemicals of structural comparability to DMHBP are contained in their training sets.

Based on the experimental toxicity data available for DMHBP and for the close analogue DMBP, and based on its low solubility, DMBP is probably not expected to be highly hazardous to aquatic organisms (i.e., acute LC/EC₅₀ > 1.0 mg/L).

In order to help characterize the ecological risk of DMHBP, a predicted no-effects concentration (PNEC) was derived. To do this, a Critical Toxicity Value (CTV) of 4.5 mg/L was chosen based on the test conducted with ricefish exposed to DMHBP. This CTV was then divided by an assessment factor of 100 to account for interspecies and intraspecies variability in sensitivity, to estimate a long-term no-effects concentration from a short-term LC₅₀, and to account for uncertainty in laboratory-to-field extrapolation. It is noted that the chronic toxicity levels of this substance may be significantly lower than acute toxicity levels due to bioaccumulation. A PNEC of 0.045 mg/L was obtained.

Characterization of Ecological Risk

The approach taken in this ecological screening assessment was to examine various supporting information and develop conclusions based on a weight-of-evidence approach and using the precautionary principle as required under section 76.1 of CEPA 1999. Particular consideration was given to risk quotient analysis, persistence, bioaccumulation, toxicity, sources and fate in the environment.

A mass flow tool was used to estimate the releases of DMHBP to the environment at different stages of its life cycle. The results indicate that DMHBP is mainly lost by transformation during its use in industrial operations. A low proportion is expected to end up in waste disposal sites, while an even lower proportion (0.08%) could end up in sewers or be released to air (0.06%). Based on that, DMHBP could reach water bodies through effluents from sewage treatment plants, and it could reach air through atmospheric releases from industrial facilities. Once released to aquatic ecosystems, DMHBP will partition mainly into sediment, while a minor proportion will stay in the water column. Once released to air, DMHBP will partly stay in this compartment in addition to partitioning to sediment and soil. Based on experimental evidence available for DMHBP as well as for other organic peroxides, this substance has been determined not to be persistent in the environment. However, it has been determined to be bioaccumulative, based on estimated Bioaccumulation Factors (BAFs). Because it is not expected to persist in water and sediments, DMHBP should not bioaccumulate substantially in organisms if it is released in aquatic ecosystems. In addition, DMHBP is probably not highly hazardous to aquatic organisms. 

A risk quotient analysis (PEC/PNEC), integrating conservative estimated potential exposure with conservative levels for potential adverse toxic effects, was performed for the aquatic environment in Canada. A PEC of 0.0002 mg/L was estimated. A PNEC of 0.045 mg/L was calculated, as described above. The resulting risk quotient is (PEC/PNEC) = 0.0002/0.045 = 0.004. This value indicates that pelagic organisms would not likely be at risk should DMHBP be released in aquatic ecosystems.

If DMHBP is released into a water body, it will partitions to sediments, where sediment-dwelling organisms would be exposed to the substance. Because no environmental monitoring data or toxicity data specific to sediment-dwelling organisms are available, the equilibrium partitioning approach could be used to calculate a sediment PEC and PNEC based on the aquatic compartment values presented above. The risk quotient (PEC/PNEC) for the sediment compartment would therefore be as the same as that for the aquatic compartment, 0.004. Again, this indicates that benthic organisms would not likely be at risk should DMHBP be released in aquatic ecosystems.

Uncertainties in Evaluation of Ecological Risk

There remains uncertainty about the persistence of DMHBP in air, water, soil and sediments under environmental conditions. While some biodegradation tests and a metabolism study indicate that DMHBP disappeared quickly from the test medium, some other tests conducted with this substance as well as with other types of organoperoxides suggest that these substances do not hydrolyze and that they are reluctant to ultimate biodegradation.  Those studies, in which DMHBP declined over time, do not all report the presence of degradation products, so it is unclear if the observed disappearance was due to degradation of the substance or to sorption.

There is also some uncertainty about the potential bioconcentration of DMHPB as only a single bioconcentration study was available from NITE. There is also uncertainty associated with the estimation of metabolism of DMHPB in fish as demonstrated by the range of k_M and kmet. The uncertainty bounds were, however, used to determine the most reliable rate of metabolism for correction of BAF predictions for conclusion of bioaccumulation potential. 

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Conclusion

Based on the information presented in this screening assessment, it is concluded that DMHBP is not entering the environment in a quantity or concentration or under conditions that have or may have an immediate or long-term harmful effect on the environment or its biological diversity or that constitute or may constitute a danger to the environment on which life depends. Similarly, it is concluded that DMHBP meets the criterion for bioaccumulation but not for persistence as set out in the Persistence and Bioaccumulation Regulations (Canada 2000).

Therefore it is concluded that DMHBP does not meet the definition of toxic as set out in paragraph 64(a) of theCanadian Environmental Protection Act, 1999.

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