Quantification of circular economy (CE) is essential for effective implementation, yet also fundamentally challenging, because it is inherently complex, featuring multiple interactions and system-level dynamicity. Two main approaches of systems thinking, commonly used to model complexities in intricate systems, are: system dynamics (SD), providing a top-down, macroscopic view; and agent-based modelling and simulation (ABMS), offering a bottom-up, microscopic perspective. Here we conducted a Preferred Reporting Items for Systematic Reviews and Meta-Analyses for Scoping Reviews (PRISMA-ScR) review, examining 60 studies applying SD or ABMS to CE, across sectors such as bio-based materials, construction and industrial symbiosis. Both methods capture aspects of circularity's feedback loops and time evolution, but they are often used in isolation in the absence of integrated platforms along with concerns over computational costs. This limits their capacity to comprehensively model internal dynamics at multiple scales and provide system-wide decision support. Few studies explore the potential of combining SD and ABMS or attempt to integrate them with static tools, such as life-cycle assessment and multi-criteria decision analysis. Standardised metrics and operational holistic evaluation tools incorporating economic, environmental, technical and social sustainability aspects are missing - especially with the latter. A more unified and comprehensive systems approach to support informed decisions on circularity would improve evidence-based policymaking and empower wider industrial adoption.

Plastic debris in the ocean is widely recognized as a significant environmental issue. As several local strategies are needed to prevent plastic releases into the environment, sub-national estimates are essential, but they are still underexplored. Traditional approaches to determine mismanaged plastic waste (MPW) are based on estimating the proportion of plastics in waste that are not adequately managed, an approach that has limitations on a sub-national scale. Understanding that MPW significantly impacts plastic emission estimations, the present study identifies current limitations and develops an alternative approach to estimate MPW at the sub-national scale. This was based on mass balance, in which the total plastic production of a country is distributed among sub-national areas based on the number of inhabitants and their consumption patterns. Subsequently, regional MPW is estimated by excluding properly managed waste based on local solid waste management data. The method was applied to determine MPW in Brazil, which allowed comparisons with previous research. Results show that between 3.12 and 3.77 million metric tons of plastic leaked into the environment in 2019, consistent with earlier estimates. However, at sub-national scales, differences in estimates are higher than 50 % in 3491 of the 5570 municipalities of Brazil. Although the approach still provides an order of magnitude estimate, a path was established to overcome the main limitations in this research area: accounting for local plastic apparent consumption, uncollected waste, informal recycling practices, and plastic losses unrelated to waste management.

Providing effective solid waste and resources management (SWM) is essential to tackle plastics pollution, reduce carbon emissions and local air pollution from open burning, avoid disease spreading and enable circularity. Official development finance (ODF) is vital in providing relevant infrastructure and capacity development. We developed a novel standardised methodology to analyse OECD ODF data and applied it to 2003-2021, offering insights on the scale, flows and changes in SWM ODF. Despite an 8-fold increase, SWM-focused ODF is still very low, representing just 0.41 % of all ODF; and an order of magnitude less than water and sanitation ODF. Lowincome countries received only 8 % of SWM ODF. Total commitments in 2021 were ca. 1.8 Billion USD significantly short of the >30 Billion USD investment estimated as necessary for substantially reducing plastics pollution. The multi-lateral environmental agreement under negotiation ('Plastics Treaty) needs to set ambitious targets for ODF and wider international co-operation.

Plastic pollution remains a major environmental challenge in the Global South, particularly due to the limited unsderstanding of how informal and formal recycling stakeholders interact along complex value chains. Highdensity polyethylene (HDPE), widely used in consumer packaging, is often overlooked in systemic recycling assessments. In this study, we investigate the systemic complexities of a polymer recycling chain insufficiently investigated, focusing on high-density polyethylene (HDPE) - a core contributor to plastics pollution in the Global South, via mismanaged waste. Rigid blow-moulded HDPE is widely used in fast-moving consumer goods containers; it is also sought-after for the production of 'secondary', recycled HDPE (r-HDPE). Starting from a single location (Brazilian town) and a waste picker cooperative (informal recyclers - IRS), we investigate the collection/sorting of municipal solid waste (MSW) recyclables along the downstream value chain of scrap dealing, reprocessing, and manufacturing. The methodological frameworks of technical networks, material flow analysis (MFA), and complex value optimization for resource recovery were used. Results demonstrate an informal to formal spectrum of operations, with clear transition points. The material quality standards required at manufacturing with r-HDPE are poorly applied at the collection stage, indicating a problematic quality management interfacing. Material rejects (losses), allowable contamination level, and monetised value of recyclate are interlinked at each stage. The most substantial value creation and appropriation is accomplished at the initial collection/sorting waste picker stage, followed by the value creation at the final production of the rHDPE pellet. Reprocessors bear disproportionate material losses, counterbalanced by purchasing pricing. Despite overall rejects of 29 % wt.(ar), a high actual recycling rate for rigid HDPE is achieved: 38 % wt.(ar), indicative of an effective collection for recycling and refined manual sorting by the partially formalized IRS. These novel insights on informal recycling networks can inform effective interventions to expand circularity and prevent plastic pollution.

Plastics are a grave, growing, and under-recognised danger to human and planetary health. Plastics cause disease and death from infancy to old age and are responsible for health-related economic losses exceeding US$1·5 trillion annually. These impacts fall disproportionately upon low-income and at-risk populations. The principal driver of this crisis is accelerating growth in plastic production-from 2 megatonnes (Mt) in 1950, to 475 Mt in 2022 that is projected to be 1200 Mt by 2060. Plastic pollution has also worsened, and 8000 Mt of plastic waste now pollute the planet. Less than 10% of plastic is recycled. Yet, continued worsening of plastics' harms is not inevitable. Similar to air pollution and lead, plastics' harms can be mitigated cost-effectively by evidence-based, transparently tracked, effectively implemented, and adequately financed laws and policies. To address plastics' harms globally, UN member states unanimously resolved in 2022 to develop a comprehensive, legally binding instrument on plastic pollution, namely the Global Plastics Treaty covering the full lifecycle of plastic. Coincident with the expected finalisation of this treaty, we are launching an independent, indicator-based global monitoring system: the Lancet Countdown on health and plastics. This Countdown will identify, track, and regularly report on a suite of geographically and temporally representative indicators that monitor progress toward reducing plastic exposures and mitigating plastics' harms to human and planetary health.

The need for an effective, health-protective global plastics treaty continues to build. Plastic production is accelerating, waste plastic is accumulating, evidence of the harms of plastics to human and planetary health is growing, and the annual health-related costs of these harms are conservatively estimated at US$1·5 trillion and rising.

The United Nations have agreed to negotiate a legally binding instrument to eliminate plastic pollution which includes provisions to reduce environmental emissions of plastic through improved waste management. However, there is a paucity of scientific evidence to prioritize the actions which will have the greatest impact on plastic pollution mitigation and many of the specific emission and transmission pathways are highly challenging to observe and measure. To this, we apply an expert elicitation study to provide a systematic evidence base on which parts of the solid waste management (SWM) system have the highest potential for plastic emission as well as the effectiveness of specific interventions to reduce them. We found that SWM processes closer to the waste generator, such as the waste storage and collection stage, have the greatest potential for plastic emission, with the most effective mitigations often simple solutions, such as provision of rigid containers or ensuring waste is disposed of in bags. Our results contribute to a much-needed evidence base and demonstrate significant reduction in plastic debris emissions is achievable by deploying tangible and effective local SWM infrastructure and service interventions.

Ongoing negotiations for a ‘Legally Binding Instrument on Plastic Pollution’ recognise the substantial contribution made by the informal recycling sector (IRS - waste pickers) to plastic pollution mitigation as part of just transition. Negotiating parties will require baseline evidence of the sector’s activities to inform the development of local and national actions plans. To this, we carried out a review of IRS prevalence and productivity following the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) method followed by extensive (statistical) data analytics. Waste pickers represent median 0.2% (interquartile range – IQR: 0.1-0.5%) of the urban population worldwide, collecting between 20 kg and 80 kg of engineered materials for recycling each day, of which 30% (mean wt. ar) are plastics. We identify substantial shortcomings in most methodologies used to gather data on the IRS, introducing epistemic uncertainty into some previous estimates of the sector’s activity. We recommend development of a standardised resource-efficient method of sampling and data gathering, suitable for implementation at municipal/local scale. Our work offers verifiable quantitative knowledge on the sector’s activities to date, suitable for use in plastic pollution quantification models and local/national action plans required to baseline and monitor progress towards multilateral targets.

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We uncover the systemic complexities of a polymer recycling chain insufficiently investigated, focusing on high-density polyethylene (HDPE) – a core contributor to plastics pollution in the Global South, via mismanaged waste. Rigid blow-moulded HDPE is widely used in retail packaging and fast-moving consumer goods containers; it is also sought-after for the production of ‘secondary’, recycled HDPE (r-HDPE). Starting from a single location (Brazilian town) and a waste picker cooperative (informal recyclers – IRS), we investigate the collection/sorting of municipal solid waste (MSW) recyclables along the downstream value chain of scrap dealing, reprocessing, and manufacturing. The methodological frameworks of technical networks, material flow analysis (MFA), and complex value optimization for resource recovery were used. Results demonstrate an informal to formal spectrum of operations, with clear transition points. The material quality standards required at manufacturing with r-HDPE are poorly applied at the collection stage, indicating a problematic quality management interfacing. Material rejects (losses), allowable contamination level, and monetised value of recyclate are interlinked at each stage. The most substantial value creation and appropriation is accomplished at the initial collection/sorting waste picker stage, followed by the value creation at the final production of the r-HDPE pellet. Reprocessors bear disproportionate material losses, counterbalanced by purchasing pricing. Despite overall rejects of 29% wt.(ar), a high actual recycling rate for rigid HDPE is achieved: 38% wt.(ar), indicative of an effective collection for recycling and refined manual sorting by the partially formalised IRS. These novel insights on informal recycling networks can inform effective interventions to expand circularity and prevent plastic pollution.

Negotiations for a global treaty on plastic pollution(1) will shape future policies on plastics production, use and waste management. Its parties will benefit from a high-resolution baseline of waste flows and plastic emission sources to enable identification of pollution hotspots and their causes(2). Nationally aggregated waste management data can be distributed to smaller scales to identify generalized points of plastic accumulation and source phenomena(3-11). However, it is challenging to use this type of spatial allocation to assess the conditions under which emissions take place(12,13). Here we develop a global macroplastic pollution emissions inventory by combining conceptual modelling of emission mechanisms with measurable activity data. We define emissions as materials that have moved from the managed or mismanaged system (controlled or contained state) to the unmanaged system (uncontrolled or uncontained state-the environment). Using machine learning and probabilistic material flow analysis, we identify emission hotspots across 50,702 municipalities worldwide from five land-based plastic waste emission sources. We estimate global plastic waste emissions at 52.1[48.3-56.3]million metric tonnes (Mt) per year, with approximately 57%wt. and 43%wt. open burned and unburned debris, respectively. Littering is the largest emission source in the Global North, whereas uncollected waste is the dominant emissions source across the Global South. We suggest that our findings can help inform treaty negotiations and develop national and sub-national waste management action plans and source inventories.

Increasing aspirations to develop a circular economy for waste plastics will result in an expansion of the global plastics reprocessing sector over the coming decades. Here we focus on two critical challenges within the value chain that as a result of such increased circularity may exacerbate existing issues for occupational and public health (1): Legacy contamination in secondary plastics, addressing the risk of materials and substances being inherited from the previous use and carried through into new products when the material enters its subsequent use phase; and challenge (2): Extrusion of secondary plastics in reprocessing, an end process of conventional mechanical recycling of plastics, involving heating secondary plastics under pressure until they melt and can be formed into new products. Via a systematic review (PRISMA guidelines, adapted), we considered over 4,000 sources of information, refined and consolidated into 20 relevant sources, which were critically assessed. We also derive prevalent risk scenarios of hazard-pathway-receptor combinations, subsequently being ranked. Our critical analysis highlights that despite stringent regulation, industrial diligence and enforcement, occasionally small amounts of potentially hazardous substances are able to pass through these safeguards and re-enter in the new product cycle. Although many are present at concentrations unlikely to pose a serious and imminent threat, their existence may be an indication of a wider or possibly increasing challenge of pollution dispersion, as the plastics reprocessing sector proliferates. But, in the Global South context, such controls may not be in place. Several studies showed emission control by passive ventilation, through open doors and windows followed by dilution and dispersion in the atmosphere, resulting in increased occupational exposure. It is recommended that further investigations are undertaken to establish the scale and magnitude of such phenomena, especially given the limited evidence base, with results informing improved future risk management protocols of a circular economy for plastics.

Systems to safely store, handle, treat and dispose of medical (healthcare) waste are well developed in the 21st century. Yet across many parts of the Global South (low- and middle-income countries) such systems, resources and know-how are lacking; to the extent that medical waste could be posing a serious threat to the health, safety and lives of millions of healthcare workers and waste handlers who regularly interact with this material. We present here a novel scope and dimension to investigating the risks and hazards to people who come into contact with medical waste, focusing on activity types and established medical practice. Based on a systematic review of the evidence (PRISMA approach, adapted), we critically analyzed and comparatively summarized data, and identified prevalent combinations of hazards, exposure and risk with a global scope. Subsequently, we assigned indicative comparative risk scores for such combinations. Our critical analysis unveils extensive mismanagement of medical waste globally, including the co-disposal with municipal solid waste (MSW), burning in open pits, and dumping even on public streets. Alarmingly, a small but non-negligible trade in reused medical equipment is proliferated by a cohort of waste reclamation specialists (sub-group of waste pickers): they collect hypodermic needles, and other single use medial items for resale to substance abusers and back into the healthcare system. We also highlight the dilemma faced by medical waste handlers in many parts of the world where a difficult choice is made between creating hazardous emissions from burning waste in the open or discarding it on land (e.g. in dumpsites) from where it risks accidentally infecting people with pathogens.

Identifying sources is crucial for proposing effective actions to combat marine litter pollution. Here, we used an innovative approach to identify hotspots of mismanaged plastic waste (MPW) within Brazil and subsequent leakage to the ocean, based on population density, socio-economic conditions, municipal solid waste management and environmental parameters. We estimated plastic waste generation and MPW for each of the 5570 Brazilian municipalities, which totaled 3.44 million metric tons per year. Then, we estimated the probability of litter mobilization and transport (P) and the relative risk of leakage to the ocean (MPW × P). The Guanabara Bay and La Plata River comprised the main oceanic entry hotspots of litter produced in Brazil. The use of national databases allowed us to increase spatial and temporal granularity, offering a detailed baseline for the application of prevention and mitigation actions. However, overcoming data limitations is still a challenge in Brazil as in other Global South countries.

Mismanaged municipal solid waste (MSW), the major source of plastics pollution and a key contributor to climate forcing, in Global South cities poses public health and environmental problems. This study analyses the first consistent and quality assured dataset available for cities distributed worldwide, featuring a comprehensive set of solid waste management performance indicators (Wasteaware Cities Benchmark Indicators - WABI). Machine learning (multivariate random forest) and univariate non-linear regression are applied, identifying best-fit converging models for a broad range of explanatory socioeconomic variables. These proxies describe in a variety of ways generic levels of progress, such as Gross Domestic Product - Purchasing Power per capita, Social Progress Index (SPI) and Corruption Perceptions Index. Specifically, the research tests and quantitatively confirms a long-standing, yet unverified, hypothesis: that variability in cities' performance on MSW can be accounted for by socioeconomic development indices. The results provide a baseline for measuring progress as cities report MSW performance for the sustainable development goal SDG11.6.1 indicator: median rates of controlled recovery and disposal are approximately at 45 % for cities in low-income countries, 75 % in lower-middle, and 100 % for both upper-middle and high-income. Casting light on aspects beyond the SDG metric, on the quality of MSW-related services, show that improvements in service quality often lag improvements in service coverage. Overall, the findings suggest that progress in collection coverage, and controlled recovery and disposal has already taken place in low- and middle-income cities. However, if cities aspire to perform better on MSW management than would have been anticipated by the average socioeconomic development in their country, they should identify ways to overcome systemic underlying failures associated with that socioeconomic level. Most alarmingly, 'business as usual' development would substantially increase their waste generation per capita unless new policies are found to promote decoupling.

Over the coming decades, a large additional mass of plastic waste will become available for recycling, as efforts increase to reduce plastic pollution and facilitate a circular economy. New infrastructure will need to be developed, yet the processes and systems chosen should not result in adverse effects on human health and the environment. Here, we present a rapid review and critical semi-quantitative assessment of the potential risks posed by eight approaches to recovering value during the resource recovery phase from post-consumer plastic packaging waste collected and separated with the purported intention of recycling. The focus is on the Global South, where there are more chances that high risk processes could be run below standards of safe operation. Results indicate that under non-idealised operational conditions, mechanical reprocessing is the least impactful on the environment and therefore most appropriate for implementation in developing countries. Processes known as 'chemical recycling' are hard to assess due to lack of real-world process data. Given their lack of maturity and potential for risk to human health and the environment (handling of potentially hazardous substances under pressure and heat), it is unlikely they will make a useful addition to the circular economy in the Global South in the near future. Inevitably, increasing circular economy activity will require expansion towards targeting flexible, multi-material and multilayer products, for which mechanical recycling has well-established limitations. Our comparative risk overview indicates major barriers to changing resource recovery mode from the already dominant mechanical recycling mode towards other nascent or energetic recovery approaches.

Recycling by the informal sector provides a rapid, inexpensive solution to plastic pollution, whilst supporting the livelihoods via their inclusion and empowerment. This solution will have the greatest benefit to the environment if supporting interventions are targeted at types of plastic pollution that are the most damaging from an ecological and wider risk perspective. Interventions should target three aspects of the pollution: reducing barriers to collection, improving the revenue from the materials and wider informal recycler remuneration, and increasing the quality of the materials. Done well, these interventions will increase the collection rate, reduce pollution from plastics, and help millions of people escape poverty. They present a scalable international solution to a global challenge; and are likely the only viable solution to the widespread lack of solid waste services and infrastructure across low-and middle-income countries.