What evidence exists on the environmental occurrence and toxic effects of the tire additive 6PPD: a systematic map protocol

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BACKGROUND: The tire additive N-(1,3-dimethylbutyl)-Nʹ-phenyl-p-phenylenediamine (6PPD) is widely produced in large volumes as a rubber antidegradant. This chemical can be released into the environment throughout the lifecycle of rubber products. Recently, 6PPD has become the subject of regulatory interest in some jurisdictions due to its widespread environmental occurrence and the acute toxicity of its transformation product N-(1,3-dimethylbutyl)-Nʹ-phenyl-p-phenylenediamine-quinone (6PPDQ) to some salmonids. As research advances for these emerging contaminants, it is critical to understand whether 6PPD and 6PPDQ concentrations in the environment are high enough to pose a risk to living organisms. Here, we present a protocol to conduct two linked systematic evidence maps related to (1) the occurrence of 6PPD and 6PPDQ in the environment and (2) the effects of 6PPD and 6PPDQ on living organisms. Our objective is to collate information on quantification methods, occurrence data, studied species, and toxicity endpoints. This work will contribute to synthesizing a rapidly expanding body of literature and providing insight into knowledge gaps to direct future work. METHODS: The systematic maps will be developed in accordance with the Collaboration for Environmental Evidence Guidelines and Standards for Evidence Synthesis in Environmental Management. A unified search strategy using chemical names, acronyms, identifiers, and trade names for 6PPD and 6PPDQ will be used for both maps. Searches will be conducted in seven databases, and grey literature will be sourced from key websites and Advisory Board input. Search results will be managed in a reference management software and screened at the title/abstract and full-text levels against predefined eligibility criteria based on the Population-Outcome and Population-Exposure-Comparison-Outcome framework for Map 1 and Map 2, respectively. A decision tree designed a priori will guide concurrent screening to determine article eligibility for either or both maps. For all eligible articles, bibliographic and study-specific data will be coded and entered into a searchable database. Both article screening and data coding will be completed by two independent reviewers. Two systematic evidence maps will summarize the evidence base using narrative synthesis, figures, and tables.

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