Filling the microplastics knowledge gap

Plastic pellets © iStock / DaisyDaisyEnvironmental Concept

Microplastics are spread throughout the environment, yet we still know little about them. Keith Hayward reviews some of the current thinking and concerns, and the need for progress on monitoring and analysis.

There is growing concern about microplastics, driven not least by ever-mounting evidence of just how pervasive they are. Last year, for example, the World Health Organization (WHO) presented a review of findings on microplastics in freshwater and drinking water. It summed up the situation by stating: “Microplastics are ubiquitous in the environment.”

This opens the potential for concern on a number of fronts. One of these is to do with any impacts of the plastic itself. This can be in connection with primary microplastics, which are manufactured plastic products. Or it can be for secondary microplastics, which result from the breakdown of primary plastics, particularly because of their erosion in the environment.

This is potentially of concern because the microplastics can be ingested by organisms. But as far as drinking water and human health are concerned, the WHO report found that “it is not possible to draw any firm conclusions on toxicity related to microplastic exposure through drinking water, particularly for the smallest particles, but no reliable information suggests it is a concern”.

Chemical concerns

Another possible area of concern is with respect to chemicals associated with the microplastics. This includes monomers – unbound plastics precursors – and the many different additives used with plastics. It also includes chemicals that adsorb to microplastics.

Professor Lorena Rios Mendoza, of University of Wisconsin – Superior, has researched this area of adsorbed chemicals for many years, looking in particular at persistent organic pollutants, especially in samples from the North Pacific and the Great Lakes. Work she reported on in 2007, for example, as a co-author, was “the first study showing the concentrations of persistent organic pollutants in post-consumer plastic debris samples”. Chemical analysis of sorbed materials also revealed toxic substances associated with microparticles, Rios reported in a subsequent paper, in which she more specifically highlighted detection of polycyclic aromatic hydrocarbons and polychlorinated biphenyls. The evidence has continued to expand since, and in recent reviews – including a chapter in a book from IWA Publishing – Rios has highlighted substances such as organochlorine pesticides, heavy metals, and pharmaceutical compounds.

As far as drinking water and human health are concerned, WHO assesses the risk in terms of a ‘margin of exposure’ (MOE), in essence combining an indicator of toxicological effect with estimated exposure. Its recent review concluded that “with respect to chemicals, a very conservative exposure scenario and MOE assessment indicates low concern for human health”.


A third area of potential concern is to do with biofilms – microbiological growth – on the plastics, and the attachment of pathogens. In her recent review, Rios notes that the distinct habitat for microorganisms on microplastics is known as the platisphere. She highlighted the association with microplastics means bacteria and viruses can be transported long distances, and that research has included evidence regarding potentially pathogenic Vibrio parahaemolyticusticus.

For drinking water, WHO concluded in its review that “there is currently no evidence to suggest a human health risk from microplastic-associated biofilms in drinking water”. It added: “The potential risk from pathogens found on biofilms associated with microplastic is also far lower than on biofilms associated with water distribution systems.”

Across these areas, therefore, WHO flags relatively limited current concern around drinking water and human health. But it also notes that “there may be widespread risks to aquatic ecosystems within a century if microplastic emission in the environment continues at the current rate”, adding: “It is important to make clear, though, that risks to the aquatic ecosystem do not necessarily equate to human health risks.”

Knowledge gaps

If anything is clear about microplastics, it is that there is a huge amount we do not yet know about them. WHO, for one, presented a wide range of research needs in its report on freshwater and drinking water. The knowledge gap is particularly stark when it comes to the smallest particles, known as nanoplastics. “Routine methods are currently not available to detect nanoplastics in environmental samples,” WHO noted, adding: “A set of standard methods is needed for sampling and analysing microplastics in drinking water and fresh water.”

This goes to the heart of the challenge around microplastics, right up to how to shape any policy action that may be needed. There are a multitude of different aspects. WHO, for example, noted in its report: “Two of the main inputs of microplastics into fresh water are surface run-off and wastewater effluent.” Better understanding of these can only come from methods covering sampling and analysis.

Rios sums up the need in her recent review: “To understand the potential, fate and accumulation of NP (nanoplastics) in the environment, as well as its sorption and accumulation of toxic compounds, one needs to know the effects of its composition (kind of polymer), size, density, shape, surface charge, and dynamic fragmentation… Without consensus in a standardisation of analytical methods for collection, identification and quantification of micro(nanoplastics) in the environment, their concentrations, spatial and temporal changes, and risks will be unknown.”

The concern referred to by WHO that there may be widespread risks to aquatic ecosystems within a century was a conclusion of an important review released last year by Science Advice for Policy by European Academies (SAPEA). This is part of the European Commission’s Scientific Advice Mechanism, which provides “independent, interdisciplinary, and evidence-based scientific advice on policy issues to the European Commission”.

SAPEA’s 2019 report ‘A scientific perspective on microplastics in nature and society’ highlights a whole range of knowledge gaps around what it refers to as NMPs – nano- and microplastics.

On freshwaters, it comments that: “The long-term ecological impacts of NMPs in freshwaters remain unknown.” It also states: “In freshwaters, we do not know to what extent peak events such as flooding influence NMP transport and to what extent this transport is dynamic in time,” adding: “Sampling and analysis methods of nanoplastics are not yet established and, therefore, information on their occurrence in freshwaters is currently unavailable.”

More specifically on wastewaters, it says: “Sewer systems transport microplastics into WWTPs (wastewater treatment plants), which are highly efficient barriers preventing microplastics from entering aquatic ecosystems. They are designed to remove particulate matter. The latest studies demonstrate that WWTPs retain 87–99% of the microplastics load.” In some respects this sounds very positive, although, in fact, it means the load enters the wastewater sludge stream.

The report adds: “Due to the lack of a feasible technology, nanoplastics have not yet been detected in wastewater and thus information about their sources, occurrence and fate is unavailable.” It says that, in wastewaters, “nanoplastics are an unknown. While we think they are generated due to larger plastics ageing, we cannot be sure, because the mechanism is unknown and we cannot measure them.”

Advice to the European Commission

SAPEA’s advice fed through to support a Scientific Opinion issued in April 2019 by the Group of Chief Scientific Advisors, which provides independent advice to the European Commission.

The Opinion stated: “Relatively few studies record microplastics in nature at or below the 10-50 micron size range because they are below the detection limit of the most often used analysis equipment. Some experimental studies have shown increasing concentrations of microplastics with decreasing size, suggesting that actual concentrations in the environment could be higher than those reported to date. Furthermore, toxicity and the relative ease with which microplastics cross biological barriers are expected to increase with decreasing size. This raises further concerns about smaller microplastics, and, in particular, nanoplastics.”

It added: “Growing scientific evidence on the hazards of the uncontrolled, irreversible, and long-term ecological risks due to microplastics do exist for some coastal waters and sediments. Scientists predict that, if emissions to the environment continue at the current rate or increase, ecological risks could be widespread within a century. Since most laboratory studies to date have been conducted for conditions that do not reflect real-world exposure, a better understanding is needed of the effects of different concentrations, compositions, sizes, and shapes of microplastic on ecosystems and humans before robust conclusions can be drawn about real risks.”

It added that, although current evidence suggests there is not widespread risk to humans or the environment, “there are significant grounds for concern and for precautionary measures to be taken”. It said that high-quality risk assessment approaches are essential to prioritise measures and to determine when and where to apply them.

The Opinion therefore set out recommendations under several headings. There is a need to broaden policy cover to prevent and reduce microplastic pollution. As part of this, it advised that the European Commission “should take steps to enable the scientific community to fill knowledge gaps regarding the presence, concentration, and behaviour of nanoplastic pollution in different situations.

It also recommended promoting global cooperation, high-quality scientific exchange and policy coherence, including on international scientific standards and methodologies. The Opinion advised: “Initiate the development of consensual international definitions and standards for the measurement and monitoring of microplastic pollution and its impact on ecosystems and human health, enabling: i) a globally-coherent picture of the nature and threats of microplastic pollution and, ii) clear, unambiguous technical prescriptions and criteria for regulatory measures, when these are needed.”

These recommendations are driving research, development of quality assurance tools, and harmonisation efforts. In particular, the European Commission’s Joint Research Centre has a programme of activity focused on such topics (see box).

Dr Birgit Sokull-Kluttgen, Deputy Head of the Consumer Products Safety Unit at the JRC, comments: “Microplastic particles are now being found throughout the environment, and even smaller plastic particles, nanoplastics, have been observed. There is concern that accumulation of these particles could have an impact on the environment and on human health through the contamination of land, water and air. Until now, the extent and impacts of said accumulations are not well understood. Therefore, as the science service of the European Commission, the Joint Research Centre works towards a better knowledge base needed for potential future policy actions.”

Science and sound policy

Action on any issue is ultimately a political decision, but development of sound policy depends on a solid understanding of the relevant issues and concerns. In the case of microplastics, and of nanoplastics in particular, this solid understanding depends on science. The need for progress and a better understanding puts monitoring and analysis at the heart of the policy debate. •

Striving for a better knowledge base in the area of micro(nano)plastics

Text supplied by the European Commission’s Joint Research Centre

As the European Commission’s science and knowledge service, the Joint Research Centre (JRC) provides other Commission Services and European Agencies with independent scientific advice and support to EU policies. In the context of the European Green Deal and the European Plastics Strategy, the JRC, among others, executes work on micro- and nanoplastics, which spans a wide variety of actions, including in-house experimental studies, as well as interacting with external bodies such as CEN and ISO.

The issue of micro(nano)plastics is not new for the scientific community, but there are still many knowledge gaps about their formation, composition, size distribution, fate and potential effects. Analytical methods to detect and quantify them do exist, but are limited. In the case of particles in the size range above a few micrometres, some methods already exist, but the quality, reproducibility and comparability of the data obtained is limited by a lack of harmonisation and validation of the applied methods. For particles of smaller size, below a few micrometres and down to the nano size range, there are no established methods available for routine detection and quantification.

Regarding microplastics, the JRC is working towards providing the scientific community and legislators with improved, fit-for-purpose analytical methods and quality assurance tools to enable a better understanding of the extent/impact of microplastic pollution. Actions include:

• Working towards harmonisation of methods for microplastic analysis (e.g., drinking water, food), including inter-laboratory studies such as the ongoing 2020 JRC study on methods to quantify microplastics in water

• Developing quality control tools, such as reference materials

• Carrying out a feasibility study: production/sourcing of representative microplastic materials for analytical/toxicological method development

• Testing and optimisation of instrumental methods for identification and quantification of microplastics

• Testing and optimisation of protocols to extract microplastics from key complex matrixes

• Support to (and participation in) other inter-laboratory comparisons to assist validation/harmonisation efforts.

For the evaluation of the smaller, sub-micrometre particles, activities are primarily experimental studies with the following aims:

• Developing instrumental methods for nanoplastic particulates based on experience from nanomaterial characterisation

• Developing protocols to recover nanoplastics from common complex matrixes.

Emphasis is being placed on sample types that are most relevant to the near future needs of European legislators, such as assessing microplastics in drinking water. Furthermore, wherever possible, studies are concentrating on methods that could most easily and economically be implemented in EU control/monitoring laboratories.

The JRC has also launched an EU monitoring campaign and onsite study on microplastics in wastewater, and supports EU policies in the area of marine litter and pollution.

In order to find solutions, the JRC brings together experts from academia, regulators and industry, and publishes the results of these meetings. In addition, the JRC provides external scientists with open access to its scientific laboratories and research facilities.

Why risk assessment and the setting of limits for micro-plastics in the aquatic environment is problematic

Dr Maria Fürhacker, of BOKU – University of Natural Resources and Life Sciences in Austria, and Secretary of the IWA Specialist Group on Assessment and Control of Hazardous Substances in Water, recently published a paper (in German) entitled ‘Why a risk assessment and the setting of limits for micro-plastics in the aquatic environment is problematic’, raising queries on some of the challenges around microplastics.

In the paper, Fürhacker highlights that a particular problem in the discussion of microplastics is their definition. She says an upper limit of 5mm is accepted by EU Member States and many international organisations. However, the situation is very different for the definition of a lower limit, e.g., this includes 1nm for the European Chemicals Agency (ECHA) or 100nm or 1µm for the European Food Safety Authority (EFSA).

Fürhacker notes that the methods for sampling and analysis and assignment to particle sizes, and the investigation of the relevant effects, are not yet standardised. Currently, she sees that the quantitative determination of microplastics is mainly carried out using microscopic, spectroscopic or thermo-analytical methods. She states that spectroscopic methods can determine the number of particles, the particle size and the material, but that reliable data are only observed up to sizes of 20µm (using FTIR – Fourier-transform infrared spectroscopy) or 1µm (Raman spectroscopy).

The potential toxic effects of exposure to microplastics are diverse, and Fürhacker states that exposure data are not readily comparable with impact data, making risk assessment and setting of limits difficult, and, indeed, questionable.

Fürhacker also highlights that, although various experts who have carried out preliminary risk assessments negate a specific risk from microplastics (mostly >300µm) for the size examined in the aquatic environment, the ECHA and UN Environment Programme choose a precautionary approach for the use of microplastics (definition 1nm-5mm) and place microplastics on the substances of very high concern (SVHC) list of Annex XV of the EU’s REACH chemicals regulation. She explains that this is because microplastics are persistent, easily taken up and, thereby, entering the food chain with the potential for negative effects on human health, and because it is impossible to remove microplastics from the environment.

Source: ‘Why a risk assessment and the setting of limits for micro-plastics in the aquatic environment is problematic’.


Microplastics and the Great Lakes

Earlier this year, a research group was awarded a grant of $113,000 by the Freshwater Collaborative of Wisconsin to study microplastics in the St Louis River Estuary and western Lake Superior. Professor Lorena Rios Mendoza, of University of Wisconsin – Superior, was among those to receive the grant, along with researchers at UW-Eau Claire, UW-Madison and the Lake Superior National Estuarine Research Reserve.

The St Louis River is the largest US tributary of Lake Superior, and its estuary forms part of an Area of Concern – an area of the Great Lakes identified as having experienced environmental degradation. “I am studying toxic compounds adsorbed onto microplastic surfaces,” says Rios. She says the concentrations detected from samples collected in the St Louis River Estuary have corresponded to the Area of Concern.

“Microplastics are gaining attention from researchers and the community due to the enormous quantities reported in the oceans. In freshwater environments, such as the Great Lakes, they have become an intensive research topic,” Rios comments. She sees that quantitative analysis of microplastics, size distribution, and composition (type of plastic polymer) present huge challenges for developing harmonised methods to collect, analyse, and report results.

“In my opinion, the main challenge, or the main limitation, for the analysis of microplastics or nanoplastics is the capacity of the analytical instrumentation,” she says. “The distribution of microplastics in the Great Lakes is poorly characterised and mostly unknown, mainly because of the difficulty and analysis of micro-size-particles in an understandable way. However, there are some advances in studying their sources, quantification, and impacts, including improvements in the sampling and extraction analysis method for microplastics from surface waters, sediments, and biota.”

More information:

Microplastics in freshwater environments: A review of quantification assessment. Rios and Balcer, 2019.

Microplastics in water and wastewater – 2nd Edition

IWA Publishing

A recent book from IWA Publishing covers in detail the topic of microplastics in water and wastewater. An introductory chapter looks at the human water cycle and points where microplastics could interact with water. Subsequent chapters examine evidence for the presence of microplastics in freshwater, including rivers and lakes, and for hazardous chemicals associated with microplastics in such systems.

Other chapters discuss the presence of microplastics in wastewater, their sources, their transfer through a wastewater treatment plant, their concentration in effluents throughout the world, and their distribution and effects on the surrounding environment. Sampling methods are covered, including the sample treatment and analysis techniques used so far for identifying microplastics in wastewater.

The book also discusses the presence of microplastics in sewage sludge and in soils irrigated with wastewater or fertilised with sludge, as well as the possible effects on plants and human health. A concluding chapter discusses the need for plastics strategies.

A study guide has been added to the second edition. Each section covers expected learning outcomes, key concepts, a study plan, additional literature, and a set of self-assessment exercises and activities for each chapter of the book. Indicative answers to the self-assessment exercises and activities are provided.

Microplastics in water and wastewater – 2nd Edition

Editor(s): Hrissi Karapanagioti and Ioannis K Kalavrouziotis

ISBN: 9781789061680