Lorenzo Ricolfi , Catharina Vendl, Jennifer Bräunig, Matthew D. Taylor, Daniel Hesselson, Gregory Neely, Malgorzata Lagisz, Shinichi Nakagawa

Background
Per- and polyfluoroalkyl substances (PFAS) are a class of widely used anthropogenic chemicals. Concerns regarding their persistence and potential adverse effects have led to multiple secondary research publications. Here, we aim to assess the resulting evidence base in the systematic secondary literature by examining research gaps, evaluating the quality of reviews, and exploring interdisciplinary connections.
Methods
This study employed a systematic evidence-mapping approach to assess the secondary literature on the biological, environmental, and medical aspects of exposure to 35 fluorinated compounds. The inclusion criteria encompassed systematic reviews published in peer-reviewed journals, pre-prints, and theses. Comprehensive searches across electronic databases and grey literature identified relevant reviews. Data extraction and synthesis involved mapping literature content and narrative descriptions. We employed a modified version of the AMSTAR2 checklist to evaluate the methodological rigour of the reviews. Furthermore, we conducted bibliometric data analysis to uncover patterns and trends in the academic literature. A research protocol for this study was previously pre-registered (osf.io/2tpn8) and published (Vendl et al., Environment International 158 (2022) 106973). The database is accessible for free on the online, interactive, and user-friendly systematic evidence map at https://hi-this-is-lorenzo.shinyapps.io/PFAS_SEM_Shiny_App/.
Results
Our map includes a total of 175 systematic reviews that met the eligibility criteria. Over the years, there has been a steady increase in the annual number of publications, with a notable surge in 2021. The majority of reviews focused on human exposure, whereas environmental and animal-related reviews were fewer and often lacked a rigorous systematic approach to literature search and screening. Review outcomes were predominantly associated with human health, particularly focusing on children's reproductive and developmental health. Animal reviews primarily focused on studies conducted in controlled laboratory settings, and wildlife reviews were characterised by a taxonomic bias. Recent reviews increasingly incorporated quantitative synthesis methodologies. The methodological strengths of the reviews included detailed descriptions of study selection processes and disclosure of potential conflicts of interest. However, weaknesses were observed in the critical lack of detail in reporting methods. A bibliometric analysis showed a bias in research toward certain geographical areas. Some regions were highly productive in terms of research, mainly with authors from within the country, but they had limited and clustered international collaborations.
Conclusions
In this overview of the available systematic secondary literature, we map literature content, assess reviews’ methodological quality, highlight data gaps, and draw research network clusters, aiming to guide future research initiatives, facilitate literature reviews, and enhance opportunities for cross-country collaboration. Furthermore, we discuss how this systematic evidence map and its publicly available database benefit scientists, regulatory agencies, and other stakeholders by providing access to current systematic secondary literature on PFAS exposure.