Workshop Report 24

Introduction and stakeholder perspectives of persistence

The aim of this session was to introduce the aims and objectives of the workshop together with some of the key stakeholder perspectives and challenges faced with assessing the persistence of chemicals in the environment. A series of stakeholder presentations introducing the issues relating to persistence assessment was followed by a panel discussion and two syndicate sessions.

Jason Snape (AstraZeneca, UK) provided a brief introduction to the scientific remit of ECETOC and summary of its activities related to environmental persistence. This included Task Force reports describing (i) Refined Approaches for Risk Assessment of PBT/vPvB Chemicals (2011), (ii) The Collation of Existing Marine Biodegradation Data and its Use in Environmental Risk Assessment (2009), (iii) The Risk Assessment of PBT Chemicals (2005), and (iv) The Persistence of Chemicals in the Environment (2003). Jason used the PBT profiler definition of chemical persistence where “persistence is the ability of a chemical substance to remain in an environment in an unchanged form. The longer a chemical persists, the higher the potential for human or environmental exposure to it. The individual environmental media for which a chemicals persistence is usually measured or estimated are air, water, soil, and sediment.”

Jason outlined the biodegradation testing paradigm together with a brief summary of relative strengths and weaknesses of the ready, inherent and higher tiered biodegradability tests. Jason also highlighted the rationale behind why the modified and enhanced biodegradation screening studies had been included within the REACH technical guidance for exposure and persistency assessments. This rationale was to circumvent the high number of false negatives that could be attributed to data from the ready biodegradation tests by addressing the underlying reasons for chemicals failing to degrade in these studies (e.g. testing above the chemical water solubility, low biomass levels excluding competent degraders and testing at concentrations where the chemical is toxic to the inocula).

Jason described the key outputs from the 2007, ECETOC and the Environment Agency (EA) of England and Wales “Biodegradation and Persistence” Workshop at Holmes Chapel in the United Kingdom. Many of the outputs agreed at this workshop had been taken forward by ECETOC and CEFIC. These included workshops, Task Forces and CEFIC/LRI projects to advance the science associated with non-extractable residues, enhanced biodegradation screening studies and the generation of the OECD 314 technical guideline. Jason indicated that the outputs of these activities, and parallel work by other interested parties, would also be shared over the next two days to (i) identify where there is consensus on best practice for persistency assessment at the screening or confirmatory stage, (ii) identify where we still have areas of uncertainty that can be addressed through research, scientific reviews or further dedicated workshops and (iii) prioritise and agree these research needs.

Eric Verbruggen (National Institute for Public Health and the Environment (RIVM), the Netherlands) described the several different regulatory frameworks that exist within Europe to deal with specific groups of chemicals (e.g. industrial chemicals, biocides, pesticides, pharmaceuticals), or which serve a particular purpose (e.g. protection of the marine environment). Several of these regulatory frameworks have their own criteria for dealing with Persistent, Bioaccumulative, and Toxic (PBT) substances or Persistent Organic Pollutants (POP). Eric focused on criteria for persistence of substances and compared the criteria from different regulatory frameworks. It appears that for persistence, differences in criteria are relatively small. Eric highlighted some examples of aspects of persistence and approaches to persistence assessments that are treated differently in the different regulatory frameworks. Some frameworks do not use their own criteria but refer to criteria from REACH or the TGD (Technical Guidance Document) used in the former new and existing substances legislation. Despite these small differences in criteria among the regulatory frameworks, details in the assessment procedure could cause the final assessment of persistence to deviate substantially among the frameworks. Even when the criteria are the same, the way the information from experimental studies is used may vary greatly. For example, the half-life of a substance could refer to degradation only, or it could be a half-life for dissipation. This reflected the way in which bound residues are regarded, with the extreme cases of bound residues being considered as completely disappeared versus completely persistent. It is also important how the results from field studies are considered in the assessment. Eric also highlighted issues associated with temperature for which the criteria are defined (ambient or room temperature), and if a temperature correction should be applied. It was also indicated that the interpretation and use in how to deal with photolysis and hydrolysis is often not well defined. Eric concluded by making the case for further harmonisation of the persistence assessment between different regulatory frameworks. This harmonisation needs to harmonise not only the criteria on which the persistence assessment is based, but also the guidance documents on the interpretation of the data.

Thomas W. Federle (Procter & Gamble, USA) indicated that the ability to accurately determine the potential for an organic chemical to degrade and the rate at which degradation will occur in environmental compartments where it is released and ultimately resides is critical in evaluating its environmental persistence. Moreover, this understanding is essential for accurately estimating environmental exposure when conducting an environmental risk assessment. Historically, ready and inherent biodegradation tests have been the principle regulatory tools utilised for assessing degradability. However, these tests are ineffective for chemicals that are difficult to test due to their physico-chemical properties, are not used as growth substrates by microorganisms or whose degraders are rare in standard test inocula. While some of these limitations are remedied in simulation tests, these tests come with their own unique issues.

Tom surveyed some of the challenges commonly encountered in accurately evaluating the degradation and persistence of organic chemicals. These include challenges that are not only scientific and methodological but also financial and practical. Methodological challenges include dosing difficult to handle substances and having sufficient analytical signal above background to quantitatively measure biodegradation at test concentrations, which are not inhibitory to the microbes or at which mass transfer is not a limitation. Scientific challenges include having an inoculum that is of sufficient size and diversity that rare degraders are present and, in the case of substances that are co-metabolised rather than used as a growth substrate, having a metabolically active microbial community available in the test. The former is complicated by regulatory restrictions on using pre-adapted inocula, which is particularly a problem for chemicals that are new to the world. Tom illustrated the importance of adaptation within the natural environmental and how the biodegradability of a chemical can change over time. This example highlighted that rates of degradation increased over time and the frequency of observing consistent positive biodegradation outcomes also increased through the routine use and discharge of a chemical.

Tom also highlighted that the use of simulation tests come with their own specific challenges, many of which are of a practical nature. These include not only the cost but the difficulty of obtaining high quality and well characterised radiolabelled test materials with the consolidation and contraction that has occurred in the industry during the past few years. Others relate to the complexity of such tests, the difficulty of successfully executing them, uncertainty about the results themselves and even their regulatory acceptance. This latter uncertainty includes whether the results from scientifically sound but non-prescribed tests (e.g. OECD 314) will even be considered by regulatory agencies, potential variability in how individual regulators or agencies will weigh and interpret such tests and how they will consider bound residues in the ultimate assessment. Unfortunately, such uncertainty can translate into reluctance by business managers to proactively fund testing and research that could lead to more definitive understanding on the fate of many chemicals in the environment. Tom concluded with the charge that by identifying the challenges, whether scientific or practical, and the dilemmas that they pose, this workshop can catalyse the development of improved approaches that will ultimately advance our understanding of chemical fate and result in better environmental protection.

Johanna Peltola-Thies (ECHA, Finland) described the regulatory process for PBT assessment before and after the implementation of REACH. The PBT working group, under the interim strategy period before REACH, discussed around 120 existing substances and also several new substances and biocides. The assessment conclusion ‘not persistent’ for existing substances was drawn in equal amounts from biodegradation screening information and other information (e.g. abiotic degradation, information about reactivity). For a large number of the substances, the discussions consisted of considerations on the validity of available data in the light of the physico-chemical (PC) properties and chemical reactivity. In discussion on available experimental degradation studies the relevance of the test conditions for PBT assessment was crucial. Conventions established at that time were incorporated into ECHA’s guidance, but for some paths (photodegradation, anaerobic biodegradation, hydrolysis, bound residues) uncertainties still remained in terms of whether and how to use the respective data in the context of PBT assessment. Johanna also highlighted a number of challenges. These included: identifying the compartment of concern, understanding the relative role and importance of aquatic photodegradation, how to present and introduce the test chemical(s), improving our understanding of the complex fate of substances in tests and in the environment, understanding the importance of anaerobic biodegradation and the use of monitoring data from contaminated sites.

Johanna indicated that a year ago about 150+ registered substances were prioritised for further PBT screening assessment by EU Member State experts as an activity beyond the formal REACH processes. The basis for the prioritisation was mainly quantitative structure-activity relationship (QSAR) estimations. Only a small proportion of submitted registrations contained experimentally derived degradation data. Furthermore, only a small number of degradation simulation testing proposals have been submitted with the registrations. These facts, and also considering the registration data quality findings of ECHA, appear to indicate that understanding on the assessment of persistence among the registrants in general may still be different from the perception of the authorities. It seems that the main part of the ongoing PBT assessment work of the Member State experts will cover similar aspects as the PBT assessment work in the past. However, in addition to the issues of uncertainty mentioned above, search and assessment concepts of potentially persistent constituents or impurities present in unknown, variable composition or biological (UVCB) substances need to be developed or further refined. It has not been possible so far to cover UVCBs in the PBT screening activities in a balanced way.