Are nanoplastics hazardous? The way forward to overcome the uncertainties of risk assessment

Setting the scene: What are nanoplastics?

Referring to a ‘Nanopinion’ entitled “Nanoplastics – it’s a name game” (https://euon.echa.europa.eu/nanopinion/-/blogs/-nanoplastics-game), microplastics and nanoplastics are becoming increasingly familiar terms when discussing plastics pollution. Nanoplastics are solid particles of synthetic or heavily modified natural polymers with sizes between 1 nm and 1000 nm, although some authors have suggested an upper limit of 100 nm, as for engineered nanomaterials. Indeed, the upper limit is arbitrary and is more important to set limits for regulation of primary nanoplastics production. In contrast, microplastic particles are between 1000 nm and 1000 µm in size (see Figure 1). At present, nanoplastics have not been as strictly defined as engineered nanomaterials have been, as these latter are already covered by different regulatory frameworks. The majority of nanoplastics in the environment is actually of secondary origin. This means that they are formed via a fragmentation process from larger plastic items that have been released unintentionally into the environment. It is currently a matter of debate whether and how nanoplastics particles are formed in the environment. To date, their detection and identification from environmental media is highly challenging, and hence the formation process has been demonstrated only under laboratory conditions, but nanoplastics were already detected in the soil and in the ocean currents of the North Atlantic gyre.


 
Figure 1. Size classification of plastics particles in relation to biological structures (adopted after https://nanopartikel.info/en/basics/cross-cutting/nanoplastic-in-the-environment/, with permission).

 

Primary nanoplastics versus “plastics soup”

The huge concern of the potential effects of nanoplastics on organisms in the environment has initiated various research projects in the field. While composed of the same materials, nanoplastic particles are much smaller and therefore assumed to have a higher interaction with organisms than microplastic, and hence pose a higher hazard. Most of the studies published to date have involved primary polystyrene nanoparticles (i.e., nano-polystyrene) intentionally produced in certain sizes, to determine their effects on different species from different habitats, and with different feeding behaviours. However, the nanoplastics present in the environment differ in many aspects from these primary, laboratory-produced, particles. Indeed, secondary nanoplastics are mixtures of different polymers that show a broad size distribution, and come in various shapes and colours. The same degree of variation applies to secondary microplastic particles. We could even refer to this mixture of different micro- nanoplastics as a ‘plastics soup’. In addition, secondary nanoplastics in the environment are under constant transformation due to conditions such as wind and waves action and/or solar radiation. Therefore, it is difficult to provide a real estimation of the hazard posed by these nanoplastics in the environment, as the well-known properties of the currently tested primary nanoplastics are not particularly comparable to the nanoplastics present in the environmental context.

Figure 2. Characteristics of laboratory nano-polystyrene samples compared to those of the ‘plastics soup’ from the environment (© Andreas Mattern/ UFZ).

 

Uncertainties in environmental risk assessment

There are many unknowns when we think of nanoplastics risks, both in terms of the exposure and the actual level of hazard. The sampling and characterisation of environmental nanoplastics also remains challenging, and hence there is little information available on the quantities of nanoplastics in different regions of the world, and on their compositions. Furthermore, the detailed characterisation of nanoplastics that are formed from plastics debris under natural conditions is difficult, particularly due to the high background of organic particles that these samples can contain. The size and polymer type of the nanoplastics will also influence their environmental fate. For example, if their density is greater than that of water, the nanoplastics will sink in the water bodies and accumulate in the sediment; however, if their density is lower than that of water, the nanoplastics will float, and thus be exposed to more UV light, which will facilitate their fragmentation.

For the assessment of environmental hazards, current research deals with questions such as whether different fractions of plastics particles need different treatments, i.e., whether nanoplastics might pose a bigger hazard than microplastics. Also, certain particle shapes (e.g., fibres) or colours (some animals may prefer to select microplastics over food ) can be decisive in terms of the potential hazards. Another complex question is the leaching of the plastics-associated chemicals, which might have an important role in the toxic potential of plastics particles. It is thus crucial to be able to describe the properties of plastics particles in as detailed as possible a way, to allow identification of specific properties that need to be taken into consideration in regulatory contexts in the future. This may allow to better link laboratory studies working with primary nanoplastic particles to potential effects of secondary nanoplastic particles within the nanoplastics soup found in the environment.

Quality criteria as a crucial step to ensure the relevance of ecotoxicity data to be used for risk assessment

However, what are the relevant criteria, parameters and/or properties that need to be defined and reported as part of ecotoxicity studies? We have worked on a list of criteria that were initially developed for ecotoxicity studies on engineered nanomaterials. We adapted these criteria to be suitable for nano-polystyrene nanoparticles as these are currently the most studied nanoplastics.

Some of the criteria initially suggested for engineered nanomaterials were not specifically suitable for nano-polystyrene, as nano-polystyrene is mainly in the form of suspensions (e.g., specific surface area, porosity), although these criteria can be relevant for other types of primary nanoplastics that are in a powder form prior to their dispersion in a test medium. On the other hand, some criteria do not apply to either of these forms of primary nanoplastics, such as the release of potentially toxic ions. For nanoplastics, the leaching of plastics-associated chemicals and the residual monomers are important criteria. Magnetic properties become relevant only when magnetic particles are included in plastics as additives, and the formation of reactive oxygen radicals is mainly important for secondary nanoplastics. Some of the new specific considerations for nanoplastics that have been suggested in terms of their polymer-specific properties include the polymer chemical composition, and its source, production and field collection, impurities and chemical additives, density, hydrophobicity and colour, and also their chemical leaching. We consider the quality evaluation approach using modified criteria to be valid for primary and secondary nanoplastics. In the future, it might also be implemented for primary and secondary microplastics.

Figure 3. Summary of the relevant criteria as applied to nanoplastics. Most criteria are also relevant for engineered nanomaterials. The properties that specifically apply to primary and secondary nanoplastics are indicated in italics (modified from Jemec Kokalj et al., 2021; © Andreas Mattern/ UFZ).

 

In summary, nanoplastics are complex environmental pollutants that come in multiple shapes, sizes, colours and polymer compositions. There are currently two main uncertainties for their environmental risk assessment: the challenging sampling situation in the environment, and the subsequent risk quantification; and that the majority of current ecotoxicity studies investigate laboratory-produced primary nanoplastics with well-defined properties. These latter studies do, however, enable the development of guidelines for further research on secondary nanoplastics. One of the outcomes of existing research with primary nanoplastics is that they can be used as a model to define the criteria to evaluate which physico-chemical properties of nanoplastics should be reported in future studies. The same was previously assessed for engineered nanomaterials, and thus building on these existing criteria is reasonable. As we suggest, once further refined, the existing study quality assessment originally developed for nano(eco)toxicity studies can now be applied to nanoplastics studies, with a further outlook for microplastics. This will enable better comparisons across different studies, with the more relevant parameters being reported. Overall, this will help decrease uncertainties in risk assessment, as the relevant parameters for nanoplastics exposure, fate and hazard assessment will be better defined in the future.

 

Jemec Kokalj, A., et al., Quality of nanoplastics and microplastics ecotoxicity studies: Refining quality criteria for nanomaterial studies. Journal of Hazardous Materials, 2021. 415: p. 125751.

DaNa Knowledgebase, https://nanopartikel.info/en/basics/cross-cutting/nanoplastic-in-the-environment/. Last accessed September 2021


 

Author biographies

Anita Jemec Kokalj is an Assistant Professor in the Biotechnical Faculty of the University of Ljubljana, Slovenia. She has more than 15 years of experience in the field of ecotoxicology, with the major part of her research focused on nanoecotoxicity, in terms of both fate and environmental hazard. In recent years, she has been actively involved in microplastics and nanoplastics research. Her research is focused on the potential effects of materials using different test systems, from cell lines, to aquatic and terrestrial invertebrates, to vertebrate embryos. As well as the classical toxicological endpoints, her interest is to investigate changes at the cellular and subcellular levels, including stress responses, detoxification, and immune responses. She is involved in the development of the assessment criteria for the quality of ecotoxicity data for regulatory purposes. Within this line of research, she cooperates with Dana Kühnel, supported by the joint Slovenian Research Agency (ARRS) and German Academic Exchange Service (DAAD) bilateral exchange funding.

ORCID: https://orcid.org/0000-0003-3143-4461

LOOP: https://loop.frontiersin.org/people/1433975/overview


 Dana Kühnel (left) and Anita Jemec Kokalj

Dana Kühnel is a Toxicologist who is focusing on the effects of anthropogenic particles on environmental organisms. She has a PhD in Biology from the University of Potsdam (2006), and works at the Helmholz Centre for Environmental Research - UFZ. Her research covers engineered nanoparticles, smart and advanced materials, and microplastics, with the aim to unravel their specific mechanisms of toxicity in aquatic organisms. A further focus of her research includes investigations into mixture effects with other pollutants, and the grouping principles to facilitate their risk assessment. For many years she has been actively involved in science communication in terms of the environmental safety of anthropogenic particles, via the web-based knowledge base www.nanoobjects.info.

ORCID: https://orcid.org/0000-0003-1712-6278.

Research ID: https://publons.com/researcher/AAA-7072-2020.

 

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